LTC3851-1_15 Datasheet, PDF(8/28 Page) Linear Technology – Synchronous Step-Down Switching Regulator Controller

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LTC3851-1_15 Datasheet, PDF (8/28 Pages) Linear Technology – Synchronous Step-Down Switching Regulator Controller

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LTC3851-1

PIN FUNCTIONS (MSE/UD)

MODE/PLLIN (Pin 1/Pin 15): Forced Continuous Mode,

Burst Mode or Pulse skipping Mode Selection Pin and

External Synchronization Input to Phase Detector Pin.

Connect this pin to INTVCC to force continuous conduction

mode of operation. Connect to GND to enable pulse skipping

mode of operation. To select Burst Mode operation, tie

this pin to INTVCC through a resistor no less than 50k,

but no greater than 250k. A clock on the pin will force the

controller into forced continuous mode of operation and

synchronize the internal oscillator.

FREQ/PLLFLTR (Pin 2/Pin 16): The phase-locked loopâs

lowpass ï¬lter is tied to this pin. Alternatively, a resistor

can be connected between this pin and GND to vary the

frequency of the internal oscillator.

RUN (Pin 3/Pin 1): Run Control Input. A voltage above

1.25V on this pin turns on the IC. However, forcing this

pin below 1.1V causes the IC to shut down the IC. There

is a 2Î¼A pull-up current on this pin.

TK/SS (Pin 4/Pin 2): Output Voltage Tracking and Soft-Start

Input. A capacitor to ground at this pin sets the ramp rate

for the output voltage. An internal soft-start current of of

1Î¼A charges this capacitor.

ITH (Pin 5/Pin 3): Current Control Threshold and Error

Ampliï¬er Compensation Point. The current comparator

tripping threshold increases with its ITH control voltage.

FB (Pin 6/Pin 4): Error Ampliï¬er Feedback Input. This pin

receives the remotely sensed feedback voltage from an

external resistive divider across the output.

SENSEâ (Pin 7/Pin 5): Current Sense Comparator Inverting

Input. The (â) input to the current comparator is connected

to the output.

SENSE+ (Pin 8/Pin 6): Current Sense Comparator

Noninverting Input. The (+) input to the current comparator

is normally connected to the DCR sensing network or

current sensing resistor.

PGOOD (Pin 9/Pin 7): Power Good Indicator Output. Open-

drain logic out that is pulled to ground when the output

voltage exceeds the Â±10% regulation window, after the

internal 17Î¼s power bad mask timer expires.

GND (Pin 10/Pin 8): Ground. All small-signal components

and compensation components should be Kelvin connected

to this ground. The (â) terminal of CVCC and the (â) terminal

of CIN should be closely connected to this pin.

BG (Pin 11/Pin 9): Bottom Gate Driver Output. This pin

drives the gate of the bottom N-channel MOSFET between

GND and INTVCC.

INTVCC (Pin 12/Pin 10): Internal 5V Regulator Output. The

control circuit is powered from this voltage. Decouple this

pin to GND with a minimum 2.2Î¼F low ESR tantalum or

ceramic capacitor.

VIN (Pin 13/Pin 11): Main Input Supply. Decouple this pin

to GND with a capacitor.

BOOST (Pin 14/Pin 12): Boosted Floating Driver Supply.

The (+) terminal of the boost-strap capacitor is connected

to this pin. This pin swings from a diode voltage drop

below INTVCC up to VIN + INTVCC.

TG (Pin 15/Pin 13): Top Gate Driver Output. This is the

output of a ï¬oating driver with a voltage swing equal to

INTVCC superimposed on the switch node voltage.

SW (Pin 16/Pin 14): Switch Node Connection to the

Inductor. Voltage swing at this pin is from a Schottky diode

(external) voltage drop below ground to VIN.

Exposed Pad (Pin 17): Ground. Must be soldered to PCB,

providing a local ground for the IC.

38511fa

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