LT8640 datasheet(13/32 Pages) AD

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LT8640S/LT8643S/

LT8640SA/LT8643SA

Rev. D

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TR/SS (Pin 21): Output Tracking and Soft-Start Pin. This

pin allows user control of output voltage ramp rate dur-

ing start-up. For the LT8640S/LT8640SA, a TR/SS volt-

age below 0.97V forces it to regulate the FB pin to equal

the TR/SS pin voltage. When TR/SS is above 0.97V, the

tracking function is disabled and the internal reference

resumes control of the error amplifier. For the LT8643S/

LT8643SA, a TR/SS voltage below 1.6V forces it to regu-

late the FB pin to a function of the TR/SS pin voltage.

See plot in the Typical Performance Characteristics sec-

tion. When TR/SS is above 1.6V, the tracking function is

disabled and the internal reference resumes control of

the error amplifier. An internal 1.9µA pull-up current from

INTVCC on this pin allows a capacitor to program output

voltage slew rate. This pin is pulled to ground with an

internal 200Ω MOSFET during shutdown and fault condi-

tions; use a series resistor if driving from a low impedance

output. This pin may be left floating if the tracking function

is not needed.

GND (Pin 22, LT8640S/LT8640SA Only): Ground. Connect

this pin to system ground and to the ground plane.

VC (Pin 22, LT8643S/LT8643SA Only): The VC pin is the

output of the internal error amplifier. The voltage on this

pin controls the peak switch current. Tie an RC network

from this pin to ground to compensate the control loop.

PG (Pin 23): The PG pin is the open-drain output of an

internal comparator. PG remains low until the FB pin is

within ±8% of the final regulation voltage, and there are

no fault conditions. PG is also pulled low when EN/UV is

below 1V, INTVCC has fallen too low, VIN is too low, or

thermal shutdown. PG is valid when VIN is above 3.4V.

FB (Pin 24): The LT8640S/LT8643S/LT8640SA/LT8643SA

regulates the FB pin to 0.970V. Connect the feedback

resistor divider tap to this pin. Also, connect a phase lead

capacitor between FB and VOUT. Typically, this capacitor

is 4.7pF to 22pF.

Corner Pins: These pins are for mechanical support only

and can be tied anywhere on the PCB, typically ground.

PIN FUNCTIONS

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