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pin for turning off the controller. If the pin is left open or
decoupled with a capacitor to ground, the SS pin is internally
provided with a starting current, permitting external control
for turning on the controller. If the pin is connected to a
voltage greater than 1.25V, the internal regulator (INTVCC)
will be on even when the controller is shut down (RUN
pin voltage <1.6V). In this mode, the onboard 6V output
linear regulator can provide power to keep-alive functions
such as a keyboard controller.
INTVCC and EXTVCC
An internal P-channel low dropout regulator produces 6V
at the INTVCC pin from the VIN supply pin. INTVCC powers
the control chip and internal circuitry within the module.
The LTM4609 also provides the external supply voltage pin
EXTVCC. When the voltage applied to EXTVCC rises above
5.7V, the internal regulator is turned off and an internal
switch connects the EXTVCC pin to the INTVCC pin thereby
supplying internal power. The switch remains closed as long
as the voltage applied to EXTVCC remains above 5.4V. This
allows the MOSFET driver and control power to be derived
from the output when (5.7V < VOUT < 7V) and from the
internal regulator when the output is out of regulation (start-
up, short-circuit). If more current is required through the
EXTVCC switch than is specified, an external Schottky diode
can be interposed between the EXTVCC and INTVCC pins.
Ensure that EXTVCC ≤ VIN.
The following list summarizes the three possible connec-
tions for EXTVCC:
1. EXTVCC left open (or grounded). This will cause INTVCC
to be powered from the internal 6V regulator at the cost
of a small efficiency penalty.
2. EXTVCC connected directly to VOUT (5.7V < VOUT < 7V).
This is the normal connection for a 6V regulator and
provides the highest efficiency.
3. EXTVCC connected to an external supply. If an external
supply is available in the 5.5V to 7V range, it may be
used to power EXTVCC provided it is compatible with
the MOSFET gate drive requirements.
Thermal Considerations and Output Current Derating
In different applications, LTM4609 operates in a variety
of thermal environments. The maximum output current is
limited by the environmental thermal condition. Sufficient
cooling should be provided to ensure reliable operation.
When the cooling is limited, proper output current de-
rating is necessary, considering ambient temperature,
airflow, input/output condition, and the need for increased
The power loss curves in Figures 5 and 6 can be used
in coordination with the load current derating curves in
Figures 7 to 14 for calculating an approximate θJA for
the module. Column designation delineates between no
heat sink, and a BGA heat sink. Each of the load current
derating curves will lower the maximum load current as
a function of the increased ambient temperature to keep
the maximum junction temperature of the power module
at 115°C allowing a safe margin for the maximum operat-
ing temperature below 125°C. Each of the derating curves
and the power loss curve that corresponds to the correct
output voltage can be used to solve for the approximate
θJA of the condition.
Buck Mode Operation
As a design example, use input voltage VIN = 12V to 36V,
VOUT = 12V and ƒ = 400kHz.
Set the PLLFLTR pin at 2.4V or more for 400kHz frequency
and connect FCB to ground for continuous current mode
operation. If a divider is used to set the frequency as shown
in Figure 16, the bottom resistor R3 is recommended not
to exceed 1kΩ.
To set the output voltage at 12V, the resistor RFB from VFB
pin to ground should be chosen as:
VOUT – 0.8V ≈ 7.15k