LT8607 datasheet(9/24 Pages) AD

9 / 24 page

LT8609S

Rev. D

For more information www.analog.com

PIN FUNCTIONS

RT (Pin 1): A resistor is tied between RT and ground to

set the switching frequency.

INTVCC (Pin 2): Internal 3.5V Regulator Bypass Pin. The

internal power drivers and control circuits are powered

from this voltage. INTVCC max output current is 20mA.

Voltage on INTVCC will vary between 2.8V and 3.5V.

Decouple this pin to power ground with at least a 1μF

low ESR ceramic capacitor. Do not load the INTVCC pin

with external circuitry.

GND (Pins 3, 4, 8, 14, 17): Exposed Pad Pin. These pads

must be connected to the negative terminal of the input

capacitor and soldered to the PCB in order to lower the

thermal resistance.

SW (Pins 5, 6): The SW pin is the output of the inter-

nal power switches. Connect this pin to the inductor and

boost capacitor. This node should be kept small on the

PCB for good performance.

N/C (Corner Pins, Pin 7): Connect these pins to the

ground plane for improved mechanical performance while

temperature cycling.

VIN (Pins 9, 10): The VIN pin supplies current to the

LT8609S internal circuitry and to the internal topside

power switch. This pin must be locally bypassed. Be

sure to place the positive terminal of the input capacitor

as close as possible to the VIN pins, and the negative

capacitor terminal as close as possible to the GND pins.

EN/UV (Pin 11): The LT8609S is shut down when this

pin is low and active when this pin is high. The hyster-

etic threshold voltage is 1.05V going up and 1.00V going

down. Tie to VIN if the shutdown feature is not used. An

external resistor divider from VIN can be used to program

a VIN threshold below which the LT8609S will shut down.

PG (Pin 12): The PG pin is the open-drain output of an

internal comparator. PG remains low until the FB pin is

within ±8.5% of the final regulation voltage, and there are

no fault conditions. PG is valid when VIN is above 3.2V and

EN/UV is high. PG will pull low when VIN is above 3.2V and

EN/UV is low. PG will be high impedance when VIN is low.

FB (Pin 13): The LT8609S regulates the FB pin to 0.774V.

Connect the feedback resistor divider tap to this pin.

TR/SS (Pin 15): Output Tracking and Soft-Start Pin.

This pin allows user control of output voltage ramp rate

during start-up. A TR/SS voltage below 0.774V forces the

LT8609S to regulate the FB pin to equal the TR/SS pin

voltage. When TR/SS is above 0.774V, the tracking func-

tion is disabled and the internal reference resumes control

of the error amplifier. An internal 2μA pull-up current from

INTVCC on this pin allows a capacitor to program out-

put voltage slew rate. This pin is pulled to ground with a

300Ω MOSFET during shutdown and fault conditions; use

a series resistor if driving from a low impedance output.

SYNC (Pin 16): External Clock Synchronization Input.

Ground this pin for low ripple Burst Mode operation at

low output loads. Tie to a clock source for synchronization

to an external frequency. Leave floating for pulse-skipping

mode with no spread spectrum modulation. Tie to INTVCC

or tie to a voltage between 3.2V and 5.0V for pulse-skip-

ping mode with spread spectrum modulation. When in

pulse-skipping mode, the IQ will increase to several mA.

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