LT3504 Datasheet by Analog Devices Inc.

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LTLII‘IEAQ mm TECHNOLOGY L L7 LJUW 1
LT3504
1
3504fa
For more information www.linear.com/LT3504
Typical applicaTion
FeaTures
applicaTions
DescripTion
Quad 40V/1A Step-Down
Switching Regulator with
100% Duty Cycle Operation
The LT
®
3504 consists of four 1A output current buck
regulators. The LT3504 has a wide operating input range
of 3.2V to 40V. An on-chip boost regulator allows each
channel to operate up to 100% duty cycle and eliminates
the need for four external charge pump circuits. The LT3504
is designed to minimize external component count and
results in a simple and small application circuit.
The LT3504 operates robustly in fault conditions. Cycle-
by-cycle peak current limit and catch diode current limit
sensing protect the part during overload conditions. Ther-
mal shutdown protects the power switches at elevated
temperatures. Soft-start helps keep the peak inductor
current under control during startup.
The LT3504 also features output voltage tracking and
sequencing, programmable frequency, programmable
undervoltage lockout, and a power good pin to indicate
when all outputs are in regulation.
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
Quad Buck Regulator in 4 × 5 QFN
n Wide Input Range: 3.2V to 40V
n Four 1A Outputs
n 100% Duty Cycle Operation
n Resistor-Programmed Constant Frequency
n Short-Circuit Robust
n Wide SYNC Range: 350kHz to 2.2MHz
n Anti-Phase Switching Reduces Ripple
n 800mV FB Voltage
n Independent Run/Soft-Start Pins
n Shutdown with UVLO
n Internal Compensation
n Thermal Shutdown
n Tiny 28-Lead (4mm × 5mm) Thermally Enhanced
QFN Package
n Automotive Battery Regulation
n Industrial Control Supplies
n Wall Transformer Regulation
n Distributed Supply Regulation
LT3504 Start-Up and Shutdown
Waveform. VIN (Top Trace) Is Ramped
from 0V Up to 8V and Then Back Down
to 0V. The Other Four Traces Are the
Output Voltages of All Four Channels
3504 TA01a
SW4
DA4
FB4
SW3
DA3
FB3
SW2
DA2
FB2
SW1
DA1
FB1
SKY
SW5
EN/UVLO
VIN
VIN
VIN
VIN
RUN/SS1
RUN/SS2
RUN/SS3
RUN/SS4
RT/SYNC
5V/1A
10µH
52.3k
10k
10µF
3.3V/1A
6.8µH10µH
31.6k
10k
10µF
1µF
VIN
5.4V to 20V
TRANSIENT TO 40V
50nF
F
F
2.5V/1A
4.7µH
22.1k
10k
22µF
1.8V/1A
3.3µH
24.9k
20k
18.2k
fSW = 1MHz
22µF
GND
LT3504
100ms/DIV 3504 TA01b
5V CHANNEL BEGINS
100% DC OPERATION
UVLO = ~2.9V
PARTS SHUTS
OFF
3.3V CHANNEL BEGINS
100% DC OPERATION
VIN 1V/DIV
CH4 1V/DIV
CH3 1V/DIV
CH2 1V/DIV
CH1 1V/DIV
LT3504 TUPV‘EW ‘zm‘zn‘zm‘zm‘zax‘za L7 LINE/“2 v toerch
LT3504
2
3504fa
For more information www.linear.com/LT3504
pin conFiguraTionabsoluTe MaxiMuM raTings
EN/UVLO Pin .............................................................40V
EN/UVLO Pin Above VIN Pin ........................................ 5V
VIN Pin ......................................................................40V
SKY Pin ..................................................................... 46V
SW5 Pin ....................................................................47V
RUN/SS Pins ...............................................................6V
FB Pins ........................................................................6V
RT/SYNC Pin ...............................................................6V
PG Pin ....................................................................... 25V
Operating Junction Temperature Range (Notes 2, 8)
LT3504EUFD .......................................... 40°C to 125°C
LT3504IUFD ........................................... 40°C to 125°C
Storage Temperature Range .................. 65°C to 150°C
(Note 1)
9 10
TOP VIEW
UFD PACKAGE
28-LEAD (4mm × 5mm) PLASTIC QFN
29
GND
11 12 13
28 27 26 25 24
14
23
6
5
4
3
2
1
DA2
SW2
DA3
SW3
SW1
DA1
SW4
DA4
FB2
FB3
FB1
FB4
GND
RT/SYNC
EN/UVLO
RUN/SS3
VIN
GND
VIN
SKY
SW5
PG
VIN
GND
VIN
RUN/SS4
RUN/SS1
RUN/SS2
7
17
18
19
20
21
22
16
815
θJA = 43°C/W
EXPOSED PAD (PIN 29) IS GND, MUST BE SOLDERED TO PCB
orDer inForMaTion
LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE
LT3504EUFD#PBF LT3504EUFD#TRPBF 3504 28-Lead (4mm × 5mm) Plastic QFN –40°C to 125°C
LT3504IUFD#PBF LT3504IUFD#TRPBF 3504 28-Lead (4mm × 5mm) Plastic QFN –40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
elecTrical characTerisTics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V unless otherwise noted.
SYMBOL CONDITIONS MIN TYP MAX UNITS
EN/UVLO Threshold Voltage Rising l1.2 1.44 1.6 V
EN/UVLO Threshold Voltage Hysteresis 110 mV
EN/UVLO Threshold Current Hysteresis VEN/UVLO = Measured Rising Threshold – 50mV
(Note 3)
1.3 µA
Internal VIN Undervoltage Lockout 2.4 2.9 3.2 V
Quiescent Current (VIN) in Shutdown VEN/UVLO = 0V 0.01 2 µA
Quiescent Current (VIN) VEN/UVLO = 1V (Note 4) 4 10 µA
Quiescent Current (VIN) VEN/UVLO = 1.5V, VRUN/SS(1,2,3,4) = Open,
VFB(1,2,3,4) = 0.9V, VSKY = 17V
2.7 mA
Quiescent Current (SKY) VEN/UVLO = 1.5V, VRUN/SS(1,2,3,4) = Open,
VFB(1,2,3,4) = 0.9V, VSKY = 17V
4.4 mA
LT3504 L7 LJUW 3
LT3504
3
3504fa
For more information www.linear.com/LT3504
elecTrical characTerisTics
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LT3504EUF is guaranteed to meet performance specifications
from 0°C to 125°C junction temperature. Specifications over the –40°C
to 125°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls. The
LT3504IUF is guaranteed over the full –40°C to 125°C operating junction
temperature range.
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V unless otherwise noted.
Note 3: Current flows into pin.
Note 4: Quiescent current (VIN) is measured at VEN/UVLO = 1V
Note 5: Current flows out of pin.
Note 6: Current limit is guaranteed by design and/or correlation to static
test. Slope compensation reduces current limit at higher duty cycles.
Note 7: This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
SYMBOL CONDITIONS MIN TYP MAX UNITS
RUN/SS Pin Source Current VRUN/SS = 0V 1.3 µA
RUN/SS Pin Threshold for Switching VFB = 0V 50 100 mV
Feedback Voltage
l
790
784
800
800
810
816
mV
mV
FB Pin Current VFB = Measured VFB (Note 5) l15 150 nA
Reference Line Regulation VIN = 5V to 40V –0.015 %/V
SKY Pin Current ISW = 1A 27 40 mA
SKY Voltage above VIN Voltage VSKY – VIN 4.85 V
Switching Frequency RT = 6.34k
RT = 18.2k
RT = 100k
l
l
l
1.8
0.85
200
2.1
1
250
2.4
1.15
300
MHz
MHz
kHz
Switching Phase RT = 18.2k 150 180 210 Deg
SYNC Threshold Voltage 1.25 V
SYNC Input Frequency 0.35 2.2 MHz
Switch Current Limit (SW1,2,3,4) (Note 6) 1.45 1.75 2.1 A
Switch VCESAT (SW1,2,3,4) ISW = 1A 400 mV
Switch Leakage Current (SW1,2,3,4) 0.1 2 µA
Catch Diode Current Limit (SW1,2,3,4) FB = 0V
FB = 0.7V
0.75
1.0
1.15
1.45
1.33
1.67
A
A
Switch Current Limit (SW5) (Note 6) 220 320 mA
Switch VCESAT (SW5) ISW = 200mA 230 mV
Switch Leakage Current (SW5) 0.1 2 µA
Boost Diode Current Limit (SW5) VIN = 5V 350 450 mA
PG Threshold Offset VFB Rising 65 90 125 mV
PG Hysteresis VFB Rising – VFB Falling 35 mV
PG Voltage Output Low IPG = 250µA 180 300 mV
PG Pin Leakage VPG = 2V 0.01 1 µA
LT3504 EV 555mb g momEEwDEm g 55.2% L7HCU§QB
LT3504
4
3504fa
For more information www.linear.com/LT3504
Typical perForMance characTerisTics
Efficiency, f = 1MHz
Efficiency, f = 1MHz
Efficiency, f = 1MHz
Efficiency, f = 1MHz
Load Regulation
EN/UVLO Threshold
EN/UVLO Pin Current
TA = 25°C, unless otherwise noted.
LOAD CURRENT (A)
0
EFFICIENCY (%)
40
60
1
3504 G01
20
00.2
0.1 0.3 0.5
0.4 0.6 0.7 0.9
0.8
80
30
50
10
70
VIN = 6V
VIN = 12V
VIN = 24V
VIN = 36V
VOUT = 1.8V
LOAD CURRENT (A)
0
EFFICIENCY (%)
40
60
1
3504 G02
20
00.2
0.1 0.3 0.5
0.4 0.6 0.7 0.9
0.8
90
80
30
50
10
70
VIN = 6V
VIN = 12V
VIN = 24V
VIN = 36V
VOUT = 2.5V
LOAD CURRENT (A)
0
EFFICIENCY (%)
40
60
1
3504 G03
20
00.2
0.1 0.3 0.5
0.4 0.6 0.7 0.9
0.8
90
80
30
50
10
70
VIN = 6V
VIN = 12V
VIN = 24V
VIN = 36V
VOUT = 3.3V
LOAD CURRENT (A)
0
EFFICIENCY (%)
40
60
1
3504 G04
20
00.2
0.1 0.3 0.5
0.4 0.6 0.7 0.9
0.8
100
90
80
30
50
10
70
VIN = 6V
VIN = 12V
VIN = 24V
VIN = 36V
VOUT = 5V
LOAD CURRENT (A)
0
PERCENT ERROR (%)
–3.0
–2.0
1
3504 G05
–4.0
–5.0 0.2
0.1 0.3 0.5
0.4 0.6 0.7 0.9
0.8
0
–0.5
–1.0
–3.5
–2.5
–4.5
–1.5
VOUT = 1.8V
VOUT = 2.5V
VOUT = 3.3V
VOUT = 5V
VIN = 12V
Efficiency 5V/3.3V/2.5V/1.8V,
f = 1MHz
Efficiency, f = 1MHz
LOAD CURRENT EACH CHANNEL (A)
0
OVERALL APPLICATION EFFICIENCY (%)
40
60
1
3504 G06
20
00.2
0.1 0.3 0.5
0.4 0.6 0.7 0.9
0.8
100
90
80
30
50
10
70
VIN = 6V
VIN = 12V
VIN = 24V
VIN = 36V
VOUT1,2,3,4 = 5V
LOAD CURRENT EACH CHANNEL (A)
0
OVERALL APPLICATION EFFICIENCY (%)
40
60
1
3504 G07
20
00.2
0.1 0.3 0.5
0.4 0.6 0.7 0.9
0.8
100
90
80
30
50
10
70
VIN = 6V
VIN = 12V
VIN = 24V
VIN = 36V
TEMPERATURE (°C)
–50
THRESHOLD (V)
1.30
1.40
125
3504 G08
1.20 0
–25 25 75
50 100
1.60
1.55
1.50
1.25
1.35
1.45
FALLING
RISING
VEN/UVLO (V)
0
IEN/UVLO (µA)
0.8
1.2
2.0
3504 G09
00.4
0.2 0.6 1.0
0.8 1.2 1.6
1.4 1.8
2.0
1.8
1.6
0.4
0.6
0.2
1.0
1.4
25°C
–45°C
150°C
LT3504 L7 HEW 5
LT3504
5
3504fa
For more information www.linear.com/LT3504
Input Voltage Undervoltage
Lockout
VIN Pin Current
Typical perForMance characTerisTics
RUN/SS vs FB Voltage
Soft Start Current
Switching Frequency
vs Temperature
Switch Voltage Drop
Switch Voltage Drop,
ISW = 500mA
Switch and Diode Current Limit
TA = 25°C, unless otherwise noted.
TEMPERATURE (°C)
–50
UVLO (V)
2.2
2.8
125
3504 G10
2.0 0
–25 25 75
50 100
3.6
3.4
3.2
2.4
2.6
3.0
VEN/UVLO (V)
0
IVIN (µA)
4
6
2.0
3504 G11
00.4
0.2 0.6 1.0
0.8 1.2 1.6
1.4 1.8
10
9
8
2
3
1
5
7
RUN/SS VOLTAGE (mV)
0
FB VOLTAGE (mV)
400
1200
0400
200 600 1000
800
900
800
700
200
300
100
500
600
TEMPERATURE (°C)
–50
IRUN/SS (µA)
–1.8
–1.2
125
3504 G14
–2.0 0
–25 25 75
50 100
0
–0.6
–0.8
–0.2
–0.4
–1.6
–1.4
–1.0
TEMPERATURE (°C)
–50
FREQUENCY (MHz)
0.90
150
3504 G15
0.80 50
250–25 75 125
100
1.20
1.10
1.15
1.05
0.85
0.95
1.00
SWITCH CURRENT (mA)
0
SWITCH VOLTAGE DROP (mV)
100
1000
3504 G16
0200 400 800
600
600
500
400
200
300
TEMPERATURE (°C)
–50
SWITCH VOLTAGE DROP (mV)
240
125
3504 G17
200 0
–25 25 75
50 100
320
300
280
220
260
TEMPERATURE (°C)
–50
CURRENT LIMIT (A)
1.2
125
3504 G18
1.0 0
–25 25 75
50 100
2.0
1.9
1.4
1.1
1.3
1.5
1.7
1.8
1.6
Input Quiescent Current
vs Input Voltage
INPUT VOLTAGE (V)
0
INPUT QUIESCENT CURRENT (mA)
25
45
3504 G12
10
010
5 15 25
20 30 40
35
40
35
15
20
5
30
ALL SS = 2V
ALL SS = 0V
LT3504 6 L7LJ1‘JW
LT3504
6
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For more information www.linear.com/LT3504
Typical perForMance characTerisTics
Feedback Voltage
Power Good Threshold
Operating Waveforms,
Discontinuous Mode
Operating Waveforms,
Continuous Mode
Switch Beta
Minimum On-Time
TEMPERATURE (°C)
–50
BETA
50
125
3504 G20
40 0
–25 25 75
50 100
70
60
45
55
65
0.5A
1A
TEMPERATURE (°C)
–50
ON-TIME (ns)
70
125
3504 G21
50 0
–25 25 75
50 100
120
90
60
80
100
110
TEMPERATURE (°C)
–50
FEEDBACK VOLTAGE (mV)
797
125
3504 G22
795 0
–25 25 75
50 100
805
799
796
798
800
801
802
803
804
TEMPERATURE (°C)
–50
THRESHOLD (mV)
640
125
3504 G23
600 0
–25 25 75
50 100
740
680
620
660
700
720
FALLING
RISING
500ns/DIV 3504 G24
SW1
10V/DIV
SW2
10V/DIV
SW3
10V/DIV
SW4
10V/DIV
IOUT1,2,3,4 = 40mA
VOUT1,2,3,4 = 5V
500ns/DIV 3504 G25
SW1
10V/DIV
SW2
10V/DIV
SW3
10V/DIV
SW4
10V/DIV
IOUT1,2,3,4 = 0.5A
VOUT1,2,3,4 = 5V
Switch Current Limit
DUTY CYCLE (%)
0
SWITCH CURRENT LIMIT (A)
1.1
100
3504 G19
1.0 20 40 80
60
2.0
1.9
1.4
1.2
1.3
1.5
1.8
1.6
1.7
LT3504 VOUT L7 LJUW 7
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7
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pin FuncTions
DA (Pins 1, 3, 6, 8): Return the Schottky catch diode
anode to the diode anode (DA) pin. An internal compara-
tor senses the diode current and prevents switching when
the diode current is higher than the DA pin current limit.
SW (Pins 2, 4, 5, 7): The SW pins are the output of the
internal power switches. Connect each SW pin to an in-
ductor and Schottky catch diode cathode.
VIN (Pins 9, 11, 26, 28): The VIN pins supply current to
the LT3504’s internal regulator and to the internal power
switches. The VIN pins should be tied together and locally
bypassed with a capacitor to ground, preferably to pins
10 and 27.
GND (Pins 10, 18, 27, Exposed Pad Pin 29): Tie the GND
pins to a local ground plane below the LT3504 and the
circuit components. The exposed pad must be soldered
to the PCB and electrically connected to ground. Use a
large ground plane and thermal vias to optimize thermal
performance.
RUN/SS (Pins 12, 13, 14, 15): The RUN/SS pins are used
to soft start each channel and to allow each channel to
track other outputs. Output tracking is implemented by
connecting a resistor divider to this pin from the tracked
output. For soft start, tie a capacitor from this pin to ground.
An internal 1.3µA soft-start current charges the capacitor
to create a voltage ramp at the pin. Each channel can be
individually shut down by pulling RUN/SS below 0.1V.
EN/UVLO (Pin 16): The EN/UVLO pin is used to start up
the internal regulator to power the reference and oscilla-
tor. It also starts up the internal boost regulator. Pull the
EN/UVLO pin below 1.44V to shut down the LT3504. The
L
T3504 will draw less than 10µA of current from the VIN
pin when EN/UVLO is less than 1.44V. Pull EN/UVLO pin
below 0.7V to put the LT3504 in a state where the part
draws 0µA from the VIN pin. The threshold can function
as an accurate undervoltage lockout (UVLO), preventing
the regulator from operating until the input voltage has
reached the programmed level. Do not drive the EN/UVLO
pin more than 5V above VIN.
RT/SYNC (Pin 17): Set the switching frequency of the
LT3504 by tying an external resistor from this pin to
ground. Select the value of the programming resistor
(RT) according to Table 1 in the Applications Information
section. The RT/SYNC pin is also used to synchronize the
internal oscillator of the LT3504 to an external signal. The
synchronization (sync) signal is directly logical compatible
and can be driven by any signal with pulse width greater
than 50ns. The synchronization range is from 250kHz to
2.2MHz.
FB (Pins 19, 20, 21, 22): Each feedback pin is regulated
to 800mV. Connect the feedback resistor divider to this
pin. The output voltage is programmed according to the
following equation:
R1=R2 VOUT
0.8V 1
where R1 connects between OUT and FB, and R2 connects
between FB and GND. A good value for R2 is 10kΩ.
PG (Pin 23): The Power Good pin is the open collector
output of an internal comparator. PG remains low until
all FB pins are greater than 710mV. If not in use, this pin
can be left unconnected. The PG comparator is disabled
in shutdown.
SW5 (Pin 24): The SW5 pin is an open collector of an
internal boost regulator power switch. This power switch
generates the drive voltage 4.85V above the input voltage
(VIN), to drive the internal buck regulator power switches.
Connect an inductor from this pin to the VIN pin.
SKY (Pin 25): The SKY pin is the output of an integrated
power Schottky diode and is the source of drive voltage
to the internal buck regulator power switches. Connect a
1µF capacitor from this pin to the VIN pin. Do not drive this
pin with an external voltage source. Do not draw current
from this pin with an external component.
LT3504 8 L7LJ1‘JW
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block DiagraM
3504 BD
VIN
Σ
S
R NQ
S
R NQ
Σ
SYNC
DETECT
CLK1
CLK2
OSC
FREQUENCY
TO CURRENT
TO CH2, CH4
STARTUP/SHUTDOWN
THERMAL SHUTDOWN
0.4V
1.3µA
1.44V ON REF
PRECISION UVLO
V
IN
EN/UVLO
LOCK 1SHOT
SLOPE
SLOPE 1SHOT
TO CH3
SKY
4.85V
BOOST ERROR AMP
0.7V
BOOST SWITCH AND DRIVE
SW5
SW1
DA1
PG
SKY
VIN
PGOOD
OUT1
COMPARATORS FROM OTHER CHANNELS
FB1
0.72VCURRENT LIMIT FOLDBACK
0.8V
0.7V
SKYBAD
0.8V
BOOST REGULATOR
Q5
D5
Q1
SWITCH AND DRIVE
ONE OF THE FOUR BUCK REGULATORS SHOWN
POWER GOOD LOGIC
RT/SYNC RUN/SS1 FB1 GND
2.2V
0.1V
4.5V
RAMP
A
SKYBAD
SKY
VIN
SKYBAD
0
1
+
LT3504 L7 LJUW 9
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operaTion
A comparator starts the reference when the EN/UVLO pin
rises above the 1.44V rising threshold. Other comparators
prevent switching when the input voltage is below 2.9V or
the die temperature is above 175°C. When the EN/UVLO
is above 1.44V, the input voltage is above 3.2V, and the
temperature is below 175°C, the boost regulator begins
switching and charges the SKY capacitor to 4.85V above
VIN. When the SKY voltage is less than 4.5V above VIN,
the RUN/SS pins and VC nodes are actively pulled low to
prevent the buck regulators from switching.
The boost regulator (Channel 5) consists of an internal
0.4A power switch (Q5), an internal power Schottky diode
(D5), and the necessary logic and other control circuitry
to drive the switch. The switch current is monitored to
enforce cycle-by-cycle current limit. The diode current
is monitored to prevent inductor current runaway during
transient conditions. An error amplifier servos the SKY
voltage to 4.85V above VIN. A comparator detects when
the SKY voltage is 4.5V above VIN and allows the buck
regulators to begin switching.
The oscillator produces two antiphase clock signals running
at 50% duty cycle. Channels 1, 3 and 5 run antiphase to
Channels 2 and 4. The oscillator can be programmed by
connecting a single resistor from RT/SYNC to ground, or
by applying an external clock signal to RT/SYNC. A sync
detect circuit distinguishes between the type of input.
Tying a resistor to GND directly sets the bias current of
the oscillator. The sync signal is converted to a current to
set the bias current of the oscillator.
The oscillator enables an RS flip-flop, turning on the
internal 1.7A power switch Q1. An amplifier and compara-
tor monitor the current flowing between the VIN and SW
pins, turning the switch off when this current reaches a
level determined by the voltage at the VC node. A second
comparator enforces a catch diode current limit to prevent
inductor current runaway during transient conditions. An
error amplifier measures the output voltage through an
external resistor tied to the FB pin and servos the VC node.
If the error amplifiers output increases, more current is
delivered to the output; if it decreases, less current is
delivered. A clamp on the VC pin provides switch current
limit. Each buck regulator switch driver operates by drawing
current from the SKY pin. Regulating the SKY pin to 4.85V
above the VIN pin voltage is necessary to fully saturate the
bipolar power switch for efficient operation.
Soft-start is implemented by generating a voltage ramp at
the RUN/SS pin. An internal 1.3µA current source pulls the
RUN/SS pin up to 2.1V. Connecting a capacitor from the
RUN/SS pin to ground programs the rate of the voltage
ramp on the RUN/SS pin. A voltage follower circuit with a
0.1V offset connected from the RUN/SS pin to the RAMP
node prevents switching until the voltage at the RUN/SS
pin increases above 0.1V. When the voltage at the RAMP
node is less than 0.9V, the error amplifier servos the FB
voltage to the RAMP node voltage. When the RAMP node
voltage increases above 0.9V, then the error amplifier ser-
vos the FB voltage to 0.8V. Additionally, a current amplifier
reduces the catch diode current limit when the FB voltage
is below 0.8V to limit the inductor current during startup.
Each individual buck regulator can be placed in shutdown
by pulling the respective RUN/SS pin below 0.1V. The EN/
UVLO pin can be pulled low (below a VBE) to place the
entire part in shutdown, disconnecting the outputs and
reducing the input current to less than 2µA.
The LT3504 is pin compatible with the 28-lead QFN package
LT3514. The LT3514 is a three channel step-down converter
and has one channel (CH3) that outputs 2A instead of 1A.
LT3504 Vom
LT3504
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FB Resistor Network
The output voltage is programmed with a resistor divider
connected from the output and the FB pin. Choose the 1%
resistor according to:
R1=R2 VOUT
0.8V 1
A good value for R2 is 10kΩ, R2 should not exceed 20kΩ
to avoid bias current error.
Input Voltage Range
The input voltage range for LT3504 applications depends
on the output voltage and on the absolute maximum rat-
ing of the VIN pin.
The minimum input voltage to regulate the output gener-
ally has to be at least 400mV greater than the greatest
programmed output voltage. The only exception is when
the largest programmed output voltage is less than 2.8V.
In this case the minimum input voltage is 3.2V
.
The absolute maximum input voltage of the LT3504 is
40V and the part will regulate output voltages as long
as the input voltage remains less than or equal to 40V.
However for constant-frequency operation (no pulse-
skipping) the maximum input voltage is determined by
the minimum on-time of the LT3504 and the programmed
switching frequency. The minimum on-time is the shortest
period of time that it takes the switch to turn on and off.
Therefore the maximum input voltage to operate without
pulse-skipping is:
VIN(PS) = [ (VOUT + VD)/(fSW • tON(MIN)) ] + VSW – VD
where:
VIN(PS) is the maximum input voltage to operate in
constant frequency operation without skipping pulses.
VOUT is the programmed output voltage
VSW is the switch voltage drop, at IOUT = 1A, VSW = 0.4V
VD is the catch diode forward voltage drop, for an ap-
propriately sized diode, VD = 0.4V
fSW is the programmed switching frequency
tON(MIN) is the minimum on-time, worst-case over
temperature = 110ns (at T = 125°C)
applicaTions inForMaTion
At input voltages that exceed VIN(PS) the part will continue
to regulate the output voltage up to 40V. However the
part will skip pulses (see Figure 1) resulting in unwanted
harmonics, increased output voltage ripple, and increased
peak inductor current. Provided that the inductor does not
saturate and that the switch current remains below 2A,
operation above VIN(PS) is safe and will not damage the
part. For a more detailed discussion on minimum on-time
and pulse-skipping, refer to the Applications Information
section of the LT3505 data sheet.
Avoid starting up the LT3504 at input voltages greater
than 36V, as the LT3504 must simultaneously conduct
maximum currents at high VIN. The maximum operating
junction temperature of 125°C may be exceeded due to
the high instantaneous power dissipation.
Figure 1a: The LT3504 Operating in Constant-Frequency
Operation (Below VIN(PS)), VIN = 26.5V, VOUT = 3.3V,
fSW = 2MHz, tON(MIN) = 74ns at T = 25°C
Figure 1b.The LT3504 Operating in Pulse-Skipping Mode
(Above VIN(PS)), VIN = 27V, VOUT = 3.3V, fSW = 2MHz,
tON(MIN) = 74ns at T = 25°C
2µs/DIV 3504 F01a
IL
0.5A/DIV
VSW
10V/DIV
2µs/DIV
3504 F01b
IL
0.5A/DIV
VSW
10V/DIV
LT3504 20.5uH L7 LJUW 1 1
LT3504
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Frequency Selection
The maximum frequency that the LT3504 can be pro-
grammed to is 2.5MHz. The minimum frequency is 250kHz.
The switching frequency can be programmed in two ways.
The first method is by tying a 1% resistor (RT) from the
RT/SYNC pin to ground. Table 1 can be used to select the
value of RT. The second method is to synchronize (sync)
the internal oscillator to an external clock. The external
clock must have a minimum amplitude from 0V to 1.6V
and a minimum pulse-width of 50ns.
Table 1. RT/SYNC Pin Resistance to Program Oscillator
Frequency
FREQUENCY (MHz) RT/SYNC PIN RESISTANCE (kΩ)
0.20 140
0.3 82.5
0.4 56.2
0.5 43.2
0.6 34.8
0.7 28.0
0.8 23.7
0.9 20.5
1.0 18.2
1.1 16.9
1.2 14.7
1.3 13.0
1.4 11.5
1.5 10.7
1.6 9.76
1.7 8.66
1.8 8.06
1.9 7.32
2.0 6.81
2.1 6.34
2.2 6.04
2.3 5.62
2.4 5.36
2.5 4.99
In certain applications, the LT3504 may be required to be
alive and switching for a period of time before it begins
to receive a sync signal. If the sync signal is in a high
impedance state when it is inactive then the solution is to
Figure 2. Driving the RT/SYNC Pin From
a Port That Is in a High Impedance State
When it Is Inactive
Figure 3. Driving the RT/SYNC Pin from
a Port That Is in a Low Impedance State
When it Is Inactive
simply tie an RT resistor from the RT/SYNC pin to ground
(Figure 2). The sync signal should be capable of driving the
RT resistor. If the sync signal is in a low impedance state
or an unknown state when it is inactive, then the solution
is to tie the RT resistor from the RT/SYNC pin to ground
and then to drive the RT/SYNC pin with the sync signal
through a 1nF capacitor as shown in Figure 3.
3504 F02
PORT
GND
LT3504
RT/SYNC
RT
3504 F03
PORT
1nF
GND
LT3504
RT/SYNC
RT
BOOST Regulator and SKY Pin Considerations
The on-chip boost regulator generates the SKY voltage
to be 4.85V above VIN. The SKY voltage is the source of
drive current for the buck regulators which is used to fully
saturate the power switch. The boost regulator requires
two external components: an inductor and a capacitor.
A good first choice for an inductor is given by:
L=
20.5µH
f
where f is in MHz.
Thus, for a 250kHz programmed switching frequency,
a good first choice for an inductor value is 82µH. For a
2.5MHz programmed switching frequency, a good first
LT3504 V °DCS 1 2 L7HCUEQB
LT3504
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applicaTions inForMaTion
choice for an inductor value is 8.2µH. These values will
ensure that each buck regulator will have sufficient drive
current to saturate the power switch in all applications
and under all operating conditions.
A user desiring a lower inductor current value can calculate
their optimum inductor size based on their output cur-
rent requirements. Each buck regulator instantaneously
requires 20mA from the SKY pin per 1A of switch current.
The average current that each buck regulator draws from
the SKY pin is 20mA multiplied by the duty cycle. So if
all four buck regulators run at 100% duty cycle with each
channel supplying 1A of output current, then the SKY pin
should be able to source 80mA. However if each chan-
nel runs at 50% duty cycle then the SKY pin only has to
source 40mA. Alternatively if each channel runs at 100%
duty cycle but the output current requirement is 0.5A per
channel instead of 1A, then again the SKY pin only has to
source 40mA. To summarize, the SKY pin output current
requirement is calculated from the following equation:
ISKY =
I
OUT1
V
OUT1 +
I
OUT2
V
OUT2 +
IOUT3 VOUT3 +IOUT4 VOUT4
50 VIN
where IOUTX is the desired output current from Channel
X, VOUTX is the programmed output voltage of Channel X,
and VIN is input voltage.
Once the SKY pin output current requirement is deter-
mined, the inductor value can be calculated based on
the maximum tolerable inductor current ripple from the
following equation:
L=
V
IN
DC5
2fSW 0.3 10.25 DC5
( )
ISKY
where fSW is the programmed switching frequency and
DC5 is the boost regulator duty cycle, given by: DC5 =
5V/(VIN + 5V).
For a 1MHz application, with VIN = 12V, VOUT1 = 5V, VOUT2
= 3.3V, VOUT3 = 2.5V, VOUT4 = 1.8V, and all channels
supplying 1A of output current, the required SKY pin cur-
rent is 21mA and the inductor value is 6µH.
Soft-Start/T
racking
The RUN/SS pin can be used to soft-start the correspond-
ing channel, reducing the maximum input current during
start-up. The RUN/SS pin is pulled up through a 1µA current
source to about 2.1V. A capacitor can be tied to the pin to
create a voltage ramp at this pin. The buck regulator will
not switch while the RUN/SS pin voltage is less than 0.1V.
As the RUN/SS pin voltage increases above 0.1V, the chan-
nel will begin switching and the FB pin voltage will track
the RUN/SS pin voltage (offset by 0.1V), until the RUN/SS
pin voltage is greater than 0.8V + 0.1V. At this point the
output voltage will be at 100% of it’s programmed value
and the FB pin voltage will cease to track the RUN/SS
pin voltage and remain at 0.8V (the RUN/SS pin will
continue ramping up to about 2.1V with no effect on the
output voltage). The ramp rate can be tailored so that the
peak start up current can be reduced to the current that
is required to regulate the output, with little overshoot.
Figure 4 shows the start-up waveforms with and without
a soft-start capacitor (CSS) on the RUN/SS pin.
Figure 4a. Inductor Current Waveform During
Start-Up without a Soft-Start Capacitor
Figure 4b. Inductor Current Waveform During
Start-Up with a 1nF Soft-Start Capacitor (CSS)
100µs/DIV 3504 F04a
IL
0.5A/DIV
VOUT
2V/DIV
100µs/DIV 3504 F04b
IL
0.5A/DIV
VOUT
2V/DIV
L7 LJUW 1.33 LT3504 e catch diod h this value pplications RMS cur m load C ut 30% satu h 10 13
LT3504
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Figure 5. Circuit to Prevent Switching When VIN < 10V, with 700mV of Hysteresis
R2
20.5k
R1
133k
GND
LT3504
EN/UVLO
VIN
VIN
3504 F05
NOT
SWITCHING
SWITCHING
VIN, FALLING = 10V
VIN (V)VIN (V)
9 10 11 12
VIN, RISING = 11V
applicaTions inForMaTion
Undervoltage Lockout
The LT3504 prevents switching when the input voltage
decreases below 3.2V. Alternatively, the EN/UVLO pin
can be used to program an undervoltage lockout at input
voltages exceeding 3.2V by tapping a resistor divider from
VIN to EN/UVLO as shown in Figure 5.
The rising threshold on the EN/UVLO pin is 1.44V. The
falling threshold on the EN/UVLO pin is 1.33V. When EN/
UVLO is rising and less than 1.44V then the EN/UVLO pin
sinks 1.3µA of current. This 1.3µA current can be used to
program additional hysteresis on the EN/UVLO pin. For the
circuit in Figure 5, R1 can be determined from:
R1=
VIN,HYSTERESIS
0.11
1.33 VIN,FALLING
( )
1.3µA
where VIN,HYSTERESIS is the desired amount of hysteresis
on the input voltage and VIN,FALLING is the desired input
voltage threshold at which the part will shut down. Notice
that for a given falling threshold (VIN,FALLING), the amount
of hysteresis (VIN,HYSTERESIS) must be at least:
VIN, HYSTERESIS >
0.11
1.33
VIN,FALLING
( )
For a falling threshold of 10V, the minimum hysteresis
is 0.827V. For a falling threshold of 30V, the minimum
hysteresis is 2.48V.
R2 can be calculated once R1 is known:
R2 =R1
1.33
VIN, FALLING 1.33
The circuit shown in Figure 5 will start when the input
voltage rises above 11V and will shutdown when the input
voltage falls below 10V.
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
L = 2 • (VOUT + VD)/fSW
where VD is the voltage drop of the catch diode (~0.4V),
L is in µH and fSW is in MHz. With this value there will
be no subharmonic oscillation for applications with 50%
or greater duty cycle. The inductors RMS current rating
must be greater than your maximum load current and
its saturation current should be about 30% higher. For
robust operation in fault conditions, the saturation current
should be above 2A. To keep efficiency high, the series
resistance (DCR) should be less than 0.1 . Table 2 lists
several vendors and types that are suitable.
Of course, such a simple design guide will not always
result in the optimum inductor for your application. A
larger value provides a higher maximum load current and
reduces output voltage ripple at the expense of slower
transient response. If your load is lower than 1A, then you
can decrease the value of the inductor and operate with
higher ripple current. This allows you to use a physically
smaller inductor, or one with a lower DCR resulting in
higher efficiency. Low inductance may result in discontinu-
ous mode operation, which is okay, but further reduces
maximum load current. For details on maximum output
current and discontinuous mode operation, see Linear
Technology Application Note 44.
LT3504
LT3504
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Catch Diode
Use a 1A Schottky diode. The diode must have a reverse
voltage rating equal to or greater than the maximum input
voltage. The ON Semiconductor MBRM140 is a good
choice; it is rated for 1A continuous forward current and
a maximum reverse voltage of 40V.
Input Capacitor
The input of the LT3504 circuit must be bypassed with a
X7R or X5R type ceramic capacitor. Y5V types have poor
performance over temperature and amplified voltage
and should not be used. There are four VIN pins. Each
VIN pin should be bypassed to the nearest ground pin.
However it is not necessary to use a dedicated capaci-
tor for each VIN pin. Pins 9 and 11 may be tied together
on the board layout so that both pins can share a single
bypass capacitor. Since the channels running on Pins 9
and 11 are 180 degrees out-of-phase, it is not necessary
to double the capacitor value either. Similarly, Pins 26
and 28 may be tied together on the board layout to save
a bypass capacitor. For switching frequencies greater than
750kHz, a 1µF capacitor or higher value ceramic capacitor
should be used to bypass each group of two VIN pins. For
switching frequencies less than 750kHz, a 2.2µF or higher
value ceramic capacitor should be used to bypass each
group of two VIN pins. The ceramic bypass capacitors
should be located as close to the VIN pins as possible.
See the sample layout shown in the PCB Layout section.
All four VIN pins should be tied together on the board and
bypassing with a low performance electrolytic capacitor
is recommended especially if the input power source has
high impedance, or there is significant inductance due to
long wires or cables.
Step-down regulators draw current from the input sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT3504 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
To accomplish this task, the input bypass capacitor must
be placed close to the LT3504 and the catch diode; see
the PCB Layout section. A second precaution regarding
the ceramic input capacitor concerns the maximum input
voltage rating of the LT3504. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (underdamped) tank circuit. If the LT3504 circuit
is plugged into a live supply, the input voltage can ring to
Table 2. Inductor Vendors
VENDOR URL PART SERIES INDUCTANCE (µH) SIZE (mm)
Sumida www.sumida.com CDRH4D28
CDRH5D28
CDRH5D28
1.2 TO 4.7
2.5 TO 10
2.5 TO 33
4.5 × 4.5
5.5 × 5.5
8.3 × 8.3
Toko www.toko.com A916CY
D585LC
2 TO 12
1.1 TO 39
6.3 × 6.2
8.1 × 8
Würth Elektronik www.we-online.com WE-TPC(M)
WE-PD2(M)
WE-PD(S)
1 TO 10
2.2 TO 22
1 TO 27
4.8 × 4.8
5.2 × 5.8
7.3 × 7.3
Table 3. Capacitor Vendors
VENDOR PHONE URL PART SERIES COMMENTS
Panasonic (714) 373-7366 www.panasonic.com Ceramic, Polymer, Tantalum EEF Series
Kemet (864) 963-6300 www.kemet.com Ceramic, Tantalum T494, T495
Sanyo (408) 749-9714 www.sanyovideo.com Ceramic, Polymer, Tantalum POSCAP
Murata (404) 436-1300 www.murata.com Ceramic
AVX www.avxcorp.com Ceramic, Tantalum TPS Series
Taiyo Yuden (864) 963-6300 www.taiyo-yuden.com Ceramic
LT3504 L7HEJWEGR 1 5
LT3504
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applicaTions inForMaTion
twice its nominal value, possibly exceeding the LT3504’s
voltage rating. This situation can be easily avoided by add-
ing an electrolytic capacitor in parallel with the ceramic
input capacitors. See Application Note 88.
Output Capacitor
The output capacitor has two essential functions. Along
with the inductor
, it filters the square wave generated by
the L
T3504 to produce the DC output. In this role it deter-
mines the output ripple so low impedance at the switching
frequency is important. The second function is to store
energy in order to satisfy transient loads and stabilize the
LT3504’s control loop.
Ceramic capacitors have very low equivalent series re-
sistance (ESR) and provide the best ripple performance.
A good value is:
COUT = 33/(VOUT • fSW)
where COUT is in µF and fSW is in MHz. Use X5R or X7R
types and keep in mind that a ceramic capacitor biased
with VOUT will have less than its nominal capacitance. This
choice will provide low output ripple and good transient
response. Transient performance can be improved with a
high value capacitor, if the compensation network is also
adjusted to maintain the loop bandwidth.
A lower value of output capacitor can be used, but transient
performance will suffer.
High performance electrolytic capacitors can be used for
the output capacitor. Low ESR is important, so choose one
that is intended for use in switching regulators. The ESR
should be specified by the supplier and should be 0.1Ω
or less. Such a capacitor will be larger than a ceramic
capacitor and will have a larger capacitance, because the
capacitor must be large to achieve low ESR. Table 3 lists
several capacitor vendors.
Figure 6 shows the transient response of the LT3504 with
several output capacitor choices. The output is 3.3V. The
load current is stepped from 500mA to 1A and back to
500mA and the oscilloscope traces show the output volt-
age. The upper photo shows the recommended value. The
second photo shows the improved response (less voltage
drop) resulting from a larger output capacitor and a larger
phase lead capacitor. The last photo shows the response
to a high performance electrolytic capacitor. Transient per-
formance is improved due to the large output capacitance.
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, an LT3504 buck regulator will tolerate a shorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
L
T3504 is absent. This may occur in battery charging ap-
plications or in battery backup systems where a battery
or some other supply is diode OR-ed with the LT3504’s
output. If the VIN pin is allowed to float and the EN/UVLO
pin is held high (either by a logic signal or because it is
tied to VIN), then the LT3504’s internal circuitry will pull
its quiescent current through its SW pin. This is fine if
your system can tolerate a few mA in this state. If you
ground the EN/UVLO pin, the SW pin current will drop to
essentially zero. However, if the VIN pin is grounded while
the output is held high, then parasitic diodes inside the
LT3504 can pull large currents from the output through
the SW pin and the VIN pin. Figure 7 shows a circuit that
will run only when the input voltage is present and that
protects against a shorted or reversed input.
LT3504 E §:§===ii§:
LT3504
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Figure 6. Transient Load Response of the LT3504 with Different Output Capacitors as the
Load Current Is Stepped from 500mA to 1A. VIN = 12V, VOUT = 3.3V, L = 10µH, RT = 18.2k
10µF
31.6k
10k
IOUT
1A/DIV
VOUT
20mV/DIV
IOUT
1A/DIV
VOUT
20mV/DIV
20µs/DIV
20µs/DIV
IOUT
1A/DIV
VOUT
20mV/DIV
20µs/DIV
VOUT
3504 F06a
3504 F06b
3504 F06c
VOUT
31.6k
10k
10µF
×2
100pF
31.6k
10k
VOUT
+
22µF
FB
LT3504
FB
LT3504
FB
LT3504
applicaTions inForMaTion
Figure 7. Diode D4 Prevents a Shorted Input from Discharging a Backup Battery Tied to the Output;
It Also Protects the Circuit from a Reversed Input. The LT3504 Runs Only When the Input Is Present
3504 F07
D4
VIN
VOUT
BACKUP
GND
LT3504
SW1
DA1
FB1
EN/UVLO
SKY
SW5
VIN
VIN
RUN/SS1
RT/SYNC
LT3504 L7HEJWEGR 1 7
LT3504
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applicaTions inForMaTion
PCB Layout
For proper operation and minimum EMI, care must be taken
during printed circuit board layout. Figure 8 shows the
recommended component placement with trace, ground
plane, and via locations.
Note that large, switched currents flow in the LT3504’s VIN,
SW and DA pins, the catch diodes (D1, D2, D3, D4) and
the input capacitors (C5, C6). The loop formed by these
components should be as small as possible and tied to
system ground in only one place. These components,
along with the inductors (L1, L2, L3, L4, L5) and output
capacitors (C1, C2, C3, C4, C7), should be placed on the
same side of the circuit board, and their connections
should be made on that layer. Place a local, unbroken
ground plane below these components, and tie this
ground plane to system ground at one location (ideally
at the ground terminal of the output capacitors). Ground
pins (Pins 10, 27) are provided near the VIN pins so that
the VIN pins can be bypassed to these ground pins. The
SW nodes should be kept as small as possible and kept
far away from the RT/SYNC and FB nodes. Keep the RT/
SYNC node and FB nodes small so that the ground pin
and ground traces will shield them from the SW nodes. If
the user plans on using a SYNC signal to set the oscillator
frequency then the RT/SYNC node should be kept away
from the FB nodes. Include vias near the exposed pad of
the LT3504 to help transfer heat from the LT3504 to the
ground plane. Keep the SW5 pad/trace as far away from
the FB pads as possible.
High Temperature Considerations
While the LT3504 is capable of delivering total output
current up to 4A, total power dissipation for an applica-
tion circuit and the resulting temperature rise must be
considered, especially if all four channels are operating
at high duty cycle.
The die temperature of the LT3504 must be lower than the
maximum rating of 125°C. This is generally not a concern
unless the ambient temperature is above 85°C. For higher
temperatures, extra care should be taken in the layout of
the circuit to ensure good heat sinking of the LT3504. The
maximum load current should be derated as the ambient
temperature approaches 125°C. Programming the LT3504
to a lower switching frequency will improve efficiency and
reduce the dependence of efficiency on input voltage. The
die temperature is calculated by multiplying the LT3504
power dissipation by the thermal resistance from junc-
tion to ambient. Power dissipation within the LT3504 can
be estimated by calculating the total power loss from an
efficiency measurement and subtracting the catch diode
losses. Thermal resistance depends on the layout of the
circuit board, but 43°C/W is typical for the QFN package.
Thermal shutdown will turn off the buck regulators and
the boost regulator when the die temperature exceeds
175°C, but this is not a warrant to allow operation at die
temperatures exceeding 125°C.
Outputs Greater Than 9V
For outputs greater than 9V, add a 1k resistor in series with
a 1nF capacitor across the inductor to damp the discon-
tinuous ringing of the SW node, preventing unintended
SW current. An application with a 15V output (back page)
shows the location of this damping network.
Other Linear Technology Publications
Application Notes 19, 35, 44 contain more detailed descrip-
tions and design information for step-down regulators and
other switching regulators. Design Note 318 shows how
to generate a bipolar output supply using a step-down
regulator
.
LT3504 L7H11N§AQ 18
LT3504
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applicaTions inForMaTion
Figure 8
VIA TO LOCAL GROUND PLANE
OUTLINE OF LOCAL GROUND PLANE
VIA TO VIN 3504 F08
+
L5
D3
D1
D2
D4
C7
SW5
VIN
VIN
GND
GND
GND
GND
RT/SYNC
GND GND
FB2 FB3 FB1 FB4
SKY
SW2
SW3
SW4
C4
SW1
OUT2
OUT3
OUT4
OUT1
GND
C3
GND
C1
C2 C8
L3
L2
L1
L4
C5
C6
RUN/SS4
RUN/SS1
RUN/SS2
RUN/SS3
EN/UVLO
R9
GND
PG
R2 R5 R3 R6 R1 R7 R4 R8
LT3504 P H1— é —|F —’W"‘—I E L7HEJWEGR 1 9
LT3504
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Overvoltage Transient Protection
Figure 9 shows the complete application circuit for a
4-output step-down regulator with 100% duty cycle
operation that withstands 180V surges. Under normal
operating conditions (VIN < 33V), the VSKY rail supplies
gate drive to MOSFET Q1, providing the LT3504 with a low
applicaTions inForMaTion
resistance path to VSUPPLY. In the event that a supply surge
occurs, Zener diode D1 clamps Q1’s gate voltage to 36V.
The source-follower configuration prevents VIN from rising
any further than about 33V (a VGS below the Zener clamp
voltage ). Figure 10 shows the LT3504 regulating all four
channels through a 180V surge event without interruption.
SW4
DA4
FB4
SW3
DA3
FB3
SW2
DA2
FB2
SW1
DA1
FB1
SW5
EN/UVLO
VIN
VIN
VIN
VIN
RUN/SS1
RUN/SS2
RUN/SS3
RUN/SS4
RT/SYNC
5V/1A
L4
8.2µH
D3
D4
D5
D6
D7
53.6k
R2
100k
10.2k
10µF
3.3V/1A
L3
8.2µH
L5
10µH
31.6k
10.2k
R3
1k
10Ω
10µF
2.2µF
C1
0.1µF
V
SUPPLY
3.2V to 30V
SURGE PROTECTION TO 180V
0.1µF
C2
22µF
F
×4
2.5V/1A
L2
4.2µH
21.5k
10.2k
22µF
22pF
D2
6.8V
D1
36V
Q1
43pF
82pF
100pF
1.8V/1A
L1
4.2µH
12.7k
10.2k
18.2k
f
SW
= 1MHz
22µF
GND
LT3504
+
SKY
VIN
C1: Sanyo 50CE22BS
D1: BZT52C36-7-F
D2: BZT52C6V8-7-F
D3: BAT54-7-F
D4–D7: ON SEMI MBRM140T3
L3, L4: SUMIDA CDRH5D28-8R2 (8.2µH)
L1, L2: CDRH5D28-4R2 (4.2µH)
L5: TAIYO YUDEN CBC2016T100M (10µH)
Q1: FQB34N20L
3504 F09
3504 F10
100ms/DIV
VOUT1,2,3,4
2V/DIV
VSUPPLY
50V/DIV
VIN
50V/DIV
Figure 9. Complete Quad Buck Regulator with 180V Surge Protection
Figure 10. Overvoltage Protection Withstands 180V Surge
LT3504 vxfl—t 20 L7ELUEN2
LT3504
20
3504fa
For more information www.linear.com/LT3504
applicaTions inForMaTion
Bear in mind that significant power dissipation occurs in
Q1 during an overvoltage event. The MOSFET junction
temperature must be kept below its absolute maximum
rating. For the overvoltage transient shown in Figure 10,
MOSFET Q1 conducts 0.5A (full load on all buck channels)
while withstanding the voltage difference between VSUPPLY
(180V peak) and VIN (33V). This results in a peak power of
74W. Since the overvoltage pulse in Figure 10 is roughly
triangular, average power dissipation during the transient
event (about 400ms) is approximately half the peak power.
As such, the average power is given by:
PAVG(W) =
1
2
PPEAK(W) =37W
In order to approximate the MOSFET junction temperature
rise from an overvoltage transient, one must determine
the MOSFET transient thermal response as well as the
MOSFET power dissipation. Fortunately, most MOSFET
transient thermal response curves are provided by the
manufacturer (as shown in Figure 11). For a 400ms pulse
duration, the FQB34N20L MOSFET thermal response
ZΘJC(t) is 0.65°C/W. The MOSFET junction temperature
rise is given by:
T
RISE(
°
C)
=
ZθJC(t) PAVG(W)
=
24
°
C
Note that, by properly selecting MOSFET Q1, it is possible
to withstand even higher input voltage surges. Consult
manufacturer data sheets to ensure that the MOSFET
operates within its Maximum Safe Operating Area.
The application circuit start-up behavior is shown in
Figure 12. Resistor R2 pulls up on the gate of Q1, forcing
source-connected VIN to follow approximately 3V below
VSUPPLY. Once VIN reaches the LT3504’s 3.2V minimum
start-up voltage, the on-chip boost converter immedi-
ately regulates the VSKY rail 4.85V above VIN. Diode D3
and resistor R3 bootstrap Q1’s gate voltage to the VSKY,
fully enhancing Q1. This connects VIN directly to VSUPPLY
through Q1’s low resistance drain-source path. It should
be noted that, prior to VSKY being present, the minimum
input voltage is about 6.2V. However, with VSKY in regulation
and Q1 enhanced, the minimum run voltage drops to 3.2V,
permitting the LT3504 to maintain regulation through deep
input voltage dips Figure 13 shows all channels operating
down to the LT3504’s 3.2V minimum input voltage.
LT3504 L7 HEW 2 1
LT3504
21
3504fa
For more information www.linear.com/LT3504
applicaTions inForMaTion
Figure 11. FQB34N20L Transient Thermal Response
Figure 12. Figure 9’s Start-Up Behavior Figure 13. Figure 9’s Dropout Performance
3504 F11
ZθJC(t), THERMAL RESPONSE (°C/W)
ZθJC(t) = 0.7°C/W MAX
DUTY FACTOR = D = t1/t2
TJM – TC = PDM • ZθJC(t)
t1, SQUARE WAVE PULSE DURATION (s)
1010–5
1
10–3
10–4 10–3 0.01 0.1 1
0.01
0.1
D = 0.5
D = 0.2
D = 0.1
D = 0.05
D = 0.02
D = 0.01
SINGLE PULSE
PDM
t1
t2
3504 F12
20ms/DIV
SKY
2V/DIV
VSUPPLY
2V/DIV
VIN
2V/DIV
3504 F13
100ms/DIV
VOUT2
1V/DIV
VOUT3
1V/DIV
VOUT4
1V/DIV
VOUT1
1V/DIV
VIN
50V/DIV
LT3504 ¢ E m i E m fl Ccccaccc / , 7 a m H W e 33:53: ‘ j‘ L 4i4444i \ 7L7, \ \ 3*"fiBfiEhBfiBfi’j , A O L7LJCUEN2 22
LT3504
22
3504fa
For more information www.linear.com/LT3504
package DescripTion
UFD Package
28-Lead Plastic QFN (4mm × 5mm)
(Reference LTC DWG # 05-08-1712 Rev B)
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
4.00 ±0.10
(2 SIDES)
2.50 REF
5.00 ±0.10
(2 SIDES)
NOTE:
1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WXXX-X).
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
PIN 1
TOP MARK
(NOTE 6)
0.40 ±0.10
27 28
1
2
BOTTOM VIEW—EXPOSED PAD
3.50 REF
0.75 ±0.05 R = 0.115
TYP
R = 0.05
TYP
PIN 1 NOTCH
R = 0.20 OR 0.35
× 45° CHAMFER
0.25 ±0.05
0.50 BSC
0.200 REF
0.00 – 0.05
(UFD28) QFN 0506 REV B
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
0.70 ±0.05
0.25 ±0.05
0.50 BSC
2.50 REF
3.50 REF
4.10 ±0.05
5.50 ±0.05
2.65 ±0.05
3.10 ±0.05
4.50 ±0.05
PACKAGE OUTLINE
2.65 ±0.10
3.65 ±0.10
3.65 ±0.05
UFD Package
28-Lead Plastic QFN (4mm × 5mm)
(Reference LTC DWG # 05-08-1712 Rev B)
LT3504 L7HEJWEGR 23
LT3504
23
3504fa
For more information www.linear.com/LT3504
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
revision hisTory
REV DATE DESCRIPTION PAGE NUMBER
A 7/13 Clarified the minimum SYNC range.
Clarified the parameters in the Electrical Characteristics section.
Added the Input Quiescent Current vs Input Voltage graph.
Clarified SKY capacitor maximum voltage.
Clarified the Applications Information section.
1
3
5
9
10, 11, 12, 13, 17, 18
LT3504 _A J:— E _A ._—I: E L _ —+ E
LT3504
24
3504fa
For more information www.linear.com/LT3504
LINEAR TECHNOLOGY CORPORATION 2012
LT 0713 REV A • PRINTED IN USA
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507 www.linear.com/LT3504
relaTeD parTs
Typical applicaTion
40V Quad Output Application at 500kHz
3504 TA02
SW4
DA4
FB4
SW3
DA3
FB3
SW2
DA2
FB2
SW1
DA1
FB1
SKY
SW5
EN/UVLO
VIN
VIN
VIN
VIN
RUN/SS1
RUN/SS2
RUN/SS3
RUN/SS4
RT/SYNC
1.2V/200mA
4.7µH
16.9k
33.2k
100µF
7V/200mA
10µH15µH
39.2k
4.99k
1.02k
10µF
1µF
VIN
15.4V to 40V
50nF
2.2µF
15V/200mA
10µH
1nF
45.3k
2.55k
4.7µF
3.3V/200mA
4.7µH
43.2k
13.7k
43.2k 22µF
GND
LT3504
2.2µF
PART DESCRIPTION COMMENTS
LT3507/
LT3507A
36V 2.5MHz, Triple [2.4A + 1.5A + 1.5A (IOUT)] with LDO Controller
High Efficiency Step-Down DC/DC Converter
VIN(MIN) = 4V, VIN(MAX) = 36V, VOUT(MIN) = 0.8V, IQ = 7mA,
ISD = 1µA, 5mm x 7mm QFN-38 Package
LT8610 42V 2.2MHz, Synchronous, Low IQ = 2.5µA, Step-Down DC/DC
Converter
VIN(MIN) = 3.4V, VIN(MAX) = 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA,
ISD = 1µA, MSOP-16E Package
LT3988 60V with Transient Protection to 80V, 2.5MHz, Dual 1A High
Efficiency Step-Down DC/DC Converter
VIN(MIN) = 4.0V, VIN(MAX) = 60V, VOUT(MIN) = 0.75V, IQ = 2mA,
ISD = 1µA, MSOP-16E Package
LT3509 36V with Transient Protection to 60V, Dual 0.70(IOUT), 2.2MHz,
High Efficiency Step-Down DC/DC Converter
VIN(MIN) = 3.6V, VIN(MAX) = 36V, VOUT(MIN) = 0.8V, IQ = 1.9mA,
ISD = 1µA, 3mm × 4mm DFN-14, MSOP-16E Packages
LT3500 36V, 40VMAX, 2A, 2.5MHz High Efficiency Step-Down DC/DC
Converter and LDO Controller
VIN(MIN) = 3.6V, VIN(MAX) = 36V, VOUT(MIN) = 0.8V, IQ = 2.5mA,
ISD <10µA, 3mm × 3mm DFN-10 Package
LT3508 36V with Transient Protection to 40V, Dual 1.4A (IOUT), 3MHz,
High Efficiency Step-Down DC/DC Converter
VIN(MIN) = 3.7V, VIN(MAX) = 37V, VOUT(MIN) = 0.8V, IQ = 4.6mA,
ISD = 1µA, 4mm × 4mm QFN-24, TSSOP-16E Packages
LT3980 58V with Transient Protection to 80V, 2A (IOUT), 2.4MHz, High
Efficiency Step-Down DC/DC Converter with Burst Mode
®
Operation
VIN(MIN) = 3.6V, VIN(MAX) = 58V, Transient to 80V, VOUT(MIN) = 0.8V,
IQ = 85µA, ISD <1µA, 3mm × 4mm DFN-16 and MSOP-16E Packages
LT3480 36V with Transient Protection to 60V, 2A (IOUT), 2.4MHz, High
Efficiency Step-Down DC/DC Converter with Burst Mode Operation
VIN(MIN) = 3.6V, VIN(MAX) = 38V, VOUT(MIN) = 0.78V, IQ = 70µA,
ISD <1µA, 3mm × 3mm DFN-10, MSOP-10E Packages
LT3689 36V, 60V Transient Protection, 800mA, 2.2MHz High Efficiency
Micropower Step-Down DC/DC Converter with POR Reset and
Watchdog Timer
VIN(MIN) = 3.6V, VIN(MAX) = 36V, Transient to 60V, VOUT(MIN) = 0.8V,
IQ = 75µA, ISD <1µA. 3mm × 3mm QFN-16 Package
LT3970 40V, 350mA, 2MHz High Efficiency Micropower Step-Down
DC/DC Converter
VIN(MIN) = 4V, VIN(MAX) = 40V, Transient to 60V, VOUT(MIN) = 1.21V,
IQ = 2µA, ISD <1µA, 3mm × 2mm DFN-10 and MSOP-10 Packages
LT3682 36V, 60VMAX, 1A, 2.2MHz High Efficiency Micropower Step-Down
DC/DC Converter
VIN(MIN) = 3.6V, VIN(MAX) = 36V, VOUT(MIN) = 0.8V, IQ = 75µA,
ISD < 1µA, 3mm × 3mm DFN-12 Package

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