LT8685SRVPBF datasheet(11/24 Pages) AD

11 / 24 page

LT8685S

Rev. 0

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PIN FUNCTIONS

a low ESR ceramic capacitor of value 0.1µF or greater.

When driven by another LT8685S channel, the required

decoupling capacitance may be satisfied with the output

capacitance of the driving channel. When not used, tie

this pin to ground.

SW1, SW2, SW3, SW4 (Pin 29, Pin 28, Pins 19 - 20,

Pins 17 - 18): Channel Switch Pins. These pins are the

outputs of each corresponding channel’s internal power

switches. When channels are operating independently,

connect each SW pin to the corresponding channel’s

inductor and boost capacitor. When combining channels,

tie the combined channel’s SW pins together with a low

impedance connection. SW traces on the PCB should be

kept short for best efficiency and EMI performance.

BST1, BST2, BST3, BST4 (Pin 30, Pin 27, Pin 21, Pin

16): Channel Boost Pins. These pins provide a drive volt-

age, higher than the supply voltage, to the gate of each

channel’s top power switch.

PG1, PG2, PG3, PG4 (Pin 31, Pin 26, Pin 22, Pin 15):

Open-Drain Power Good Output Pins. Each channel’s PG

pin is pulled to ground when the voltage at the corre-

sponding FB pin is not within ±7.5% of the internal refer-

ence. PG becomes high impedance once the voltage at the

corresponding FB pin returns to within ±6% of the internal

reference. PG outputs for enabled channels are valid for

VIN1 voltages greater than 3V. PG outputs are pulled low

for channels which are not enabled. When channels are

combined, the dependent (slave) channel’s PG pin should

be open.

TRK/SS1, TRK/SS2, TRK/SS3, TRK/SS4 (Pin 32, Pin 3,

Pin 23, Pin 6): Output Tracking and Soft-Start Pins. These

pins allow user control of output voltage ramp rate during

startup. A TRK/SSx voltage below the internal reference

forces the channel to regulate the FBx pin to equal the

TRK/SSx voltage. When TRK/SSx is above internal ref-

erence, the tracking function is disabled and the internal

reference resumes control of the error amplifier. An inter-

nal 2μA pull-up current from INTVCC on this pin allows

a capacitor to program output voltage slew rate. When

channels are combined, the dependent (slave) channel’s

TRK/SS pin should be open.

EN/UVLO (Pin 33): Channel 1 Enable and Undervoltage

Lockout Pin. The LT8685S is in low power shutdown

when this pin is below 0.4V regardless of the state of the

remaining LT8685S enable pins. A voltage above 0.9V

(rising) enables LT8685S operation and channel 1. A pre-

cision threshold at 0.8V (falling) allows this pin to be used

as an input undervoltage lockout by connecting a resistor

divider between VIN1 and ground.

FB1 (Pin 34): Channel 1 Output Voltage Feedback Pin.

Channel 1 regulates this pin to a precision, internal, 0.8V

reference voltage. Connect this pin to the appropriate

resistor divider network to program the desired output

voltage.

EN2, EN3, EN4 (Pin 35, Pin 12, Pin 14): Channel Enable

Pins. Each LT8685S channel is active when the voltage

on its corresponding enable pin is above 0.9V (rising). A

precision threshold at 0.81V (falling) allows each enable

pin to act as a programmable undervoltage lockout by

connecting a resistor divider between the correspond-

ing input supply and ground. When channels are com-

bined, the dependent (slave) channel’s EN pin should be

connected to the enable pin of the controlling (master)

channel.

FB2 (Pin 36): Channel 2 Output Voltage Feedback Pin.

Channel 2 regulates this pin to a precision, internal, 0.8V

reference voltage. Connect this pin to the appropriate

resistor divider network to program the desired output

voltage. When this pin is tied to INTVCC, channel 2 is com-

bined with channel 1 to create a single output channel with

increased output current capability. See the Application

Section for more detail.

GND (Exposed Pad Pins 37 - 40): Ground Pins. These

pins must be soldered to the PCB to provide low imped-

ance electrical contact to ground and good thermal con-

tact to the PCB. See the Applications Information section

for more detail.

CORNER PINS: These pins are for mechanical support

only and can be tied anywhere on the PCB, typically

ground.

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