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LT1738

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PI FU CTIO S

Part Supply

V5 (Pin 5): This pin provides a 5V output that can sink or

source 10mA for use by external components. V5 source

current comes from VIN . Sink current goes to GND. VIN

must be greater than 6.5V in order for this voltage to be in

regulation. If this pin is used, a small capacitor (<1

µF) may

be placed on this pin to reduce noise. This pin can be left

open if not used.

GND (Pin 11): Signal Ground. The internal error amplifier,

negative feedback amplifier, oscillator, slew control cir-

cuitry, V5 regulator, current sense and the bandgap refer-

ence are referred to this ground. Keep the connection to

this pin, the feedback divider and VC compensation net-

work free of large ground currents.

SHDN (Pin 14): The shutdown pin can disable the switcher.

Grounding this pin will disable all internal circuitry.

Increasing SHDN voltage will initially turn on the internal

bandgap regulator. This provides a precision threshold for

the turn on of the rest of the IC. As SHDN increases past

1.39V the internal LDO regulator turns on, enabling the

control and logic circuitry.

µA of current is sourced out of the pin above the turn on

threshold. This can be used to provide hysteresis for the

shutdown function. The hysteresis voltage will be set by

the Thevenin resistance of the resistor divider driving this

pin times the current sourced out. Above approximately

2.1V the hysteresis current is removed. There is approxi-

mately 0.1V of voltage hysteresis on this pin as well.

The pin can be tied high (to VIN for instance).

VIN (Pin 17): Input Supply. All supply current for the part

comes from this pin including gate drive and V5 regulator.

Charge current for gate drive can produce current pulses

of hundreds of milliamperes. Bypass this pin with a low

ESR capacitor.

When VIN is below 2.55V the part will go into supply

undervoltage lockout where the gate driver is driven low.

This, along with gate drive undervoltage lockout, prevents

unpredictable behavior during power up.

PGND (Pin 20): Power Driver Ground. This ground comes

from the MOSFET gate driver. This pin can have several

hundred milliamperes of current on it when the external

MOSFET is being turned off.

Oscillator

SYNC (Pin 6): The SYNC pin can be used to synchronize

the part to an external clock. The oscillator frequency

should be set close to the external clock frequency.

Synchronizing the clock to an external reference is useful

for creating more stable positioning of the switcher volt-

age and current harmonics. This pin can be left open or

tied to ground if not used.

CT (Pin 7): The oscillator capacitor pin is used in conjunc-

tion with RT to set the oscillator frequency. For RT = 16.9k:

COSC(nf) = 129/fOSC(kHz)

RT (Pin 8): A resistor to ground sets the charge and

discharge currents of the oscillator capacitor. The nomi-

nal value is 16.9k. It is possible to adjust this resistance

±25% to set oscillator frequency more accurately.

Gate Drive

GATE (Pin 1): This pin connects to the gate of an external

N-channel MOSFET. This driver is capable of sinking and

sourcing at least 300mA.

The GCL pin sets the upper voltage of the gate drive. The

GATE pin will not be activated until VIN reaches a minimum

voltage as defined by the GCL pin (gate undervoltage

lockout).

The gate drive output has current limit protection to safe

guard against accidental shorts.

GCL (Pin 3): This pin sets the maximum gate voltage to the

GATE pin to the MOSFET gate drive. This pin should be

either tied to a zener, a voltage source, or VIN.

If the pin is tied to a zener or a voltage source, the

maximum gate drive voltage will be approximately VGCL

– 0.2V. If it is tied to VIN, the maximum gate voltage is

approximately VIN – 1.6.

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