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LT1681I

Dual Transistor Synchronous Forward Controller

Linear Technology 

LT1681

TYPICAL PERFOR A CE CHARACTERISTICS

Soft-Start Output Current

vs Soft-Start Pin Voltage

60

TA = 25°C

40

20

0

0

1

2

3

4

5

SOFT-START PIN VOLTAGE (V)

1681 G13

Current Sense Amplifier

Bandwidth vs Temperature

8

7

6

5

4

3

2

–55 –35 –15 5 25 45 65 85 105 125

TEMPERATURE (°C)

1681 G14

PI FU CTIO S

SHDN (Pin 1): Shutdown Pin. Pin voltages exceeding

positive-going threshold of 1.25V enables the LT1681.

150mV of input hysteresis resists mode switching insta-

bility.

The SHDN pin can be controlled by either a logic-level

input or with an analog signal. This shutdown feature is

typically used for input supply undervoltage protection. A

resistor divider from the converter input supply to the

SHDN pin monitors that supply for control of system

power-up sequencing, etc. All internal functions are dis-

abled during shutdown.

OVLO (Pin 2): Overvoltage Shutdown Sense. Typically

connected to input supply through a resistor divider. If pin

voltage exceeds 1.25V, the LT1681 switching function is

disabled to protect boosted circuitry from exceeding ab-

solute maximum voltage. 40mV of input hysteresis resists

mode switching instability. Exceeding the OVLO threshold

also triggers soft-start reset, resulting in a graceful recov-

ery from an input transient event.

THERM (Pin 3): System Thermal Shutdown. Auxiliary

shutdown pin that is typically used for system thermal

protection. If pin voltage exceeds 1.25V, the LT1681

switching function is disabled. 40mV of input hysteresis

6

resists mode switching instability. Exceeding the THERM

threshold also triggers soft-start reset, resulting in a

graceful recovery.

SGND (Pin 4): Signal Ground Reference. Careful board

layout techniques must be used to prevent corruption of

the signal ground reference. High current switching paths

must be oriented on the converter ground plane such that

currents to/from the switches do not affect the integrity of

the LT1681 signal ground reference.

5VREF (Pin 5): 5V Local Reference. Allows connection of

external loads up to 20mA DC. Typically bypassed with

1µF ceramic capacitor to SGND. Reference output is

current limit protected to a typical value of 45mA. If the

load on the 5V reference exceeds the current limit value,

LT1681 switching function is disabled and the soft-start

function is reset.

FSET (Pin 6): Oscillator Timing Pin. Connect a resistor

(RFSET) from the 5VREF pin to this pin and a capacitor

(CFSET) from this pin to ground.

The LT1681 oscillator operates by monitoring the voltage

on CFSET as it is charged via RFSET. When the voltage on the

FSET pin reaches 2.5V, the oscillator rapidly discharges

the capacitor with an average current of 0.8mA. Once the

1681f

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