LT8551 datasheet(11/32 Pages) AD

11 / 32 page

LT8551

Rev 0

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PIN FUNCTIONS

RT/MS (Pin 26): Timing Resistor Pin and Master Slave

Selection Pin. This pin, combined with PHS1, PHS2 and

PHS3, sets the switching frequency and the phase of

each channel. Connecting a resistor to ground sets the

chip as master LT8551. Connecting this pin to the REG

pin sets the chip as slave LT8551. See the Applications

Information section for more information.

ILIM (Pin 27): Maximum Current Sense Voltage

Programming Pin. This pin is used to set the maximum

sense voltage in the primary channel current sense ampli-

fier and expanded channel current sense amplifier. It is a

three level input pin. Connecting this pin to ground, REG

or leaving it floating sets the maximum current sense

voltage to 30mV, 60mV or 90mV, respectively. When the

ILIM is floating, add a 1nF cap from the ILIM to ground.

ENOUT (Pin 28): For a master LT8551, this pin is an open-

drain logic output pin. For a slave LT8551, it is an input

pin. See more details in ENOUT Connection Section.

VCC (Pin 29): Power Supply for Control Circuits. Decouple

this pin to ground with a minimum 1µF low ESR ceramic

capacitor. VCC and REG need to be connected through a

1Ω resistor.

MODE (Pin 30):Stage Shedding Selection Pin. Connecting

this pin to GND disables stage shedding feature. See the

Operation section for more information.

SHDN (Pin 31): Shutdown Pin. This pin is used to enable/

disable the chip. Drive below 0.3V to disable the chip.

Drive above 1.2V (typical) to activate the chip. Do not

float this pin.

IAMPN (Pin 32): For a master LT8551, connect this pin

to local ground. For a slave LT8551, connect this pin

to the master LT8551’s IAMPN. See the Applications

Information section for more information.

IAMPP (Pin 33): For a master LT8551, this is an out-

put pin. It is the buffered signal of the Primary Channel

Current Sense Amplifier output. For a slave LT8551, this

is an input pin. When multiple LT8551 devices are used,

connect all IAMPP pins together. See the Applications

Information section for more information.

ISP (Pin 35): Primary Channel Current Sense Amplifier

Input. The (+) input to the current sense amplifier is nor-

mally connected to DCR sensing network or current sens-

ing resistor. This pin is only used for a master LT8551.

Ground this pin for a slave LT8551.

ISN (Pin 34): Primary Channel Current Sense Amplifier

Input. The (–) input to the current sense amplifier is

normally connected to DCR sensing networks or cur-

rent sensing resistors. This pin is only used for a master

LT8551. Ground this pin for a slave LT8551.

REGSNS (Pin 40): REG LDO Voltage Sense Pin. Connect

this pin to the primary channel gate driver power supply

pin.

PHS3 (Pin 41): Phase Select Pin. This pin, combined with

PHS1, PHS2 and RT/MS, set the switching frequency and

the phase of each channel. PHS3 connects to REG or

ground. See the Operation section for more information.

NC (Pin 42): No Connection. Leave this pin floating or

connect to any adjacent pin.

ISN1, ISN2, ISN3, ISN4 (Pins 44, 36, 45, 38): Expanded

Channel Current Sense Amplifier (–) Input. The (–) input

to the current sense amplifier is normally connected to

DCR sensing network or current sensing resistor.

ISP1, ISP2, ISP3, ISP4 (Pins 43, 37, 46, 39): Expanded

Channel Current Sense Amplifier (+) Input. The (+) input

to the current sense amplifier is normally connected to

DCR sensing network or current sensing resistor.

REGIS (Pin 47): REG LDO Current Sense Pin. Connect

this pin to the external PMOS source side.

REGDRV (Pin 48): Gate Driver Output for REG LDO.

Connect this pin to the external PMOS gate.

VIN (Pin 49): Input Supply Pin. Must be locally bypassed

to ground.

GND (Exposed Pad Pin 53/Pin 27): Ground. Tie directly

to local ground plane.

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