LT8645S datasheet(13/30 Pages) LINER

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LT8645S/LT8646S

8645Sfa

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PIN FUNCTIONS

TR/SS (Pin 29): Output Tracking and Soft-Start Pin. This

pin allows user control of output voltage ramp rate during

start-up. For the LT8645S, a TR/SS voltage below 0.97V

forces it to regulate the FB pin to equal the TR/SS pin volt-

age. When TR/SS is above 0.97V, the tracking function is

disabled and the internal reference resumes control of the

error amplifier. For the LT8646S, a TR/SS voltage below

1.6V forces it to regulate the FB pin to a function of the

TR/SS pin voltage. See plot in the Typical Performance

Characteristics section. When TR/SS is above 1.6V, the

tracking function is disabled and the internal reference

resumes control of the error amplifier. An internal 1.9A

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 200 MOSFET during shutdown

and fault conditions; 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 30 LT8645S Only): Ground. Connect this pin to

system ground and to the ground plane. This pin is also

connected to ground internally, and can be left floating on

PCB to be pin compatible with the LT8646S.

VC (Pin 30, LT8646S 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 31): 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 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 32): The LT8645S/LT8646S regulates the FB pin

to 0.97V. Connect the feedback resistor divider tap to this

pin. Also, connect a phase lead capacitor between FB and

VOUT. Typically, this capacitor is 1pF to 10pF.

Corner Pins: These pins are for mechanical support only

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

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