LTC1735CS datasheet(7/32 Pages) LINER

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LTC1735

COSC (Pin 1): External capacitor COSC from this pin to

ground sets the operating frequency.

RUN/SS (Pin 2): Combination of Soft-Start and Run

Control Inputs. A capacitor to ground at this pin sets the

ramp time to full output current. The time is approximately

1.25s/

µF. Forcing this pin below 1.5V causes the device to

be shutdown. In shutdown all functions are disabled.

Latchoff overcurrent protection is also invoked via this pin

as described in the Applications Information section.

ITH (Pin 3): Error Amplifier Compensation Point. The

current comparator threshold increases with this control

voltage. Nominal voltage range for this pin is 0V to 2.4V.

FCB (Pin 4): Forced Continuous/Synchronization Input.

Tie this pin to ground for continuous synchronous opera-

tion, to a resistive divider from the secondary output when

using a secondary winding or to INTVCC to enable Burst

Mode operation at low load currents. Clocking this pin with

a signal above 1.5VP–P disables Burst Mode operation but

allows cycle-skipping at low load currents and synchro-

nizes the internal oscillator with the external clock.

SGND (Pin 5): Small-Signal Ground. All small-signal

components such as COSC, CSS, the feedback divider plus

the loop compensation resistors and capacitor(s) should

single-point tie to this pin. This pin should, in turn, connect

to PGND.

VOSENSE (Pin 6): Receives the feedback voltage from an

external resistive divider across the output.

SENSE – (Pin 7): The (–) Input to the Current Comparator.

SENSE + (Pin 8): The (+) Input to the Current Comparator.

Built-in offsets between SENSE – and SENSE + pins in

conjunction with RSENSE set the current trip threshold.

EXTVCC (Pin 9): Input to the Internal Switch Connected to

INTVCC. This switch closes and supplies VCC power when-

ever EXTVCC is higher than 4.7V. See EXTVCC connection

in the Applications Information section. Do not exceed 7V

on this pin and ensure EXTVCC ≤ VIN.

PGND (Pin 10): Driver Power Ground. Connects to the

source of bottom N-channel MOSFET, the anode of the

Schottky diode, and the (–) terminal of CIN.

BG (Pin 11): High Current Gate Drive for Bottom

N-Channel MOSFET. Voltage swing at this pin is from

ground to INTVCC.

INTVCC (Pin 12): Output of the Internal 5.2V Regulator and

EXTVCC Switch. The driver and control circuits are pow-

ered from this voltage. Decouple to power ground with a

µF ceramic capacitor placed directly adjacent to the IC

together with a minimum of 4.7

µF tantalum or other low

ESR capacitor.

VIN (Pin 13): Main Supply Pin. Must be closely decoupled

to power ground.

SW (Pin 14): Switch Node Connection to Inductor and

Bootstrap Capacitor. Voltage swing at this pin is from a

Schottky diode (external) voltage drop below ground to

VIN.

BOOST (Pin 15): Supply to Topside Floating Driver. The

bootstrap capacitor is returned to this pin. Voltage swing

at this pin is from a diode drop below INTVCC to (VIN +

INTVCC).

TG (Pin 16): High Current Gate Drive for Top N-Channel

MOSFET. This is the output of a floating driver with a

voltage swing equal to INTVCC superimposed on the

switch node voltage SW.

PI FU CTIO S

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