LT8686SJVPBF datasheet(11/26 Pages) AD

11 / 26 page

LT8686S

Rev. 0

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PIN FUNCTIONS

this pin to INTVCC to enable pulse-skipping mode with

spread-spectrum modulation. When this pin is driven by

an external clock source, the LT8686S will synchronize its

switching frequency to that of the external clock and oper-

ate in pulse-skipping mode. Channels 1–4 clock phases

default to 0°, 180°, 270°, and 90°, respectively when

operating independently. When combining channels, the

master channel determines the operating phase. See the

Applications Information section for more detail.

RT (Pin 13): Switching Frequency Program Pin. Connect

an external resistor from this pin to ground to program

the LT8686S switching frequency from 350kHz to 3MHz.

When using external clock synchronization, an RT resistor

must be selected to match the nominal incoming clock

frequency. See the Applications Information section for

more detail.

VIN3 (Pin 15): Channel 3 Power Supply Input. When driven

from another LT8686S channel, the required decoupling

capacitance for this pin is largely satisfied by the output

capacitance of the driving channel. In this scenario, a low

ESR capacitor of value 1μF or greater located close to the

VIN3 pin is generally sufficient. If driven from an external

source, this pin should be closely decoupled with a low

ESR capacitor of value 4.7μF or greater.

VIN4 (Pin 16): Channel 4 Power Supply Input. When driven

from another LT8686S channel, the required decoupling

capacitance for this pin is largely satisfied by the output

capacitance of the driving channel. In this scenario, a low

ESR capacitor of value 1μF or greater located close to the

VIN4 pin is generally sufficient. If driven from an external

source, this pin should be closely decoupled with a low

ESR capacitor of value 4.7μF or greater.

FB3 (Pin 17): Channel 3 Output Voltage Feedback Pin.

Channel 3 regulates this pin to a precision, internal,

0.8V reference voltage. Connect this pin to the appro-

priate resistor divider network to program the desired

output voltage.

FB4 (Pin 19): Channel 4 Output Voltage Feedback Pin.

Channel 4 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 4 is com-

bined with channel 3 to create a single output channel with

increased output current capability. See the Applications

Information section for more detail.

INTVCC (Pin 28): Internal Regulator Output Pin. This reg-

ulator provides the supply current for the power MOSFET

driver circuits and internal control circuitry. This pin

should be decoupled to ground with a low ESR ceramic

capacitor of value 4.7μF. This capacitor should be placed

close to the INTVCC pin with a low impedance connection

to the exposed pad ground. This supply is not intended as

a power supply output. Do not connect external circuitry

to this pin.

BIAS (Pin 29): External Regulator Input Pin. The inter-

nal regulator, INTVCC, will draw current from this supply

instead of VIN1 when BIAS is tied to a voltage higher than

4.5V and VVIN1 is greater than VBIAS + 1V. Connecting

this pin to a high efficiency supply, such as an LT8686S

output channel regulating to 5V, improves overall effi-

ciency by reducing the on-chip power consumption that

would normally result when drawing current from VIN1.

When used, this pin should be decoupled to ground with

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

When driven by another LT8686S channel, the required

decoupling capacitance may be satisfied with the output

capacitance of the driving channel. When not used, tie

this pin to ground.

GND (Exposed Pad Pins 33–36): Ground Pins. These pins

must be soldered to the PCB to provide low impedance

electrical contact to ground and good thermal contact

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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