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LTC1435I

High Efficiency Low Noise Synchronous Step-Down Switching Regulator

Linear Technology 

TYPICAL PERFORMANCE CHARACTERISTICS

LTC1435

Burst Mode Operation

Soft Start: Load Current vs Time

VOUT

20mV/DIV

VITH

200mV/DIV

ILOAD = 50mA

1435 G16

RUN/SS

5V/DIV

INDUCTOR

CURRENT

1A/DIV

1435 G17

PIN FUNCTIONS

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 current output. The time is approximately

0.5s/µF. Forcing this pin below 1.3V causes the device to

be shut down. In shutdown all functions are disabled.

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 2.5V.

SFB (Pin 4): Secondary Winding Feedback Input. Nor-

mally connected to a feedback resistive divider from the

secondary winding. This pin should be tied to: ground to

force continuous operation; INTVCC in applications that

don’t use a secondary winding; and a resistive divider from

the output in applications using a secondary winding.

SGND (Pin 5): Small-Signal Ground. Must be routed

separately from other grounds to the (–) terminal of COUT.

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 thresholds.

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 Applications Information section. Do not exceed 10V on

this pin. Connect to VOUT if VOUT ≥ 5V.

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

of bottom N-channel MOSFET 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 5V Regulator and

EXTVCC Switch. The driver and control circuits are pow-

ered from this voltage. Must be closely decoupled to power

ground with a minimum of 2.2µF tantalum or electrolytic

capacitor.

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

to the IC’s signal ground pin.

SW (Pin 14): Switch Node Connection to Inductor. Volt-

age 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 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.

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