LT1768IGN datasheet(8/20 Pages) LINER

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LT1768

PIN FUNCTIONS

RMAX (Pin 10): The RMAX pin outputs a regulated voltage

of 1.25V that is to be loaded with an external resistor. The

current through the external resistor sets the maximum

lamp current. Maximum lamp current in a dual lamp

application will be approximately equal to 100 times IRMAX

when the voltage on the PROG pin is greater than 4V. The

value of RRMAX must be greater than 5K and less than

[RRMIN • 2.5 • (VPWM–1/3)] for proper PWM operation.

RMIN (Pin 11): The RMIN pin outputs a regulated voltage of

1.26V that is to be loaded with an external resistor. The

current through the external resistor sets the minimum

lamp current. Minimum lamp current in a dual lamp

application will be approximately 10 times the value of

IRMIN when the voltage on the PROG pin is between 0.5V

and 1V. To set the minimum current to zero (IRMIN = 0µA)

for maximum dimming range, connect the RMIN pin to the

VREG pin. The value of RRMIN (RRMIN = ∞ when RMIN is

connected to VREG) must be greater than the value of

RRMAX/[0.4 • (VPWM–1)/3] for proper PWM operation.

SHDN (Pin 12): The SHDN pin controls the operation of

the LT1768. Pulling the SHDN pin above 1.26V or leaving

the pin open will result in normal operation of the LT1768.

Pulling the SHDN pin below 1V causes a complete shut-

down of the LT1768 which results in a typical quiescent

current of 65

µA. The SHDN pin has an internal 7µA pull-up

source to VIN and 200mV of voltage hysteresis.

FAULT (Pin 13): The FAULT pin is an open collector output

with a sink capability of 1mA that is activated when lamp

current falls below 125

µA in either DIO1 or DIO2 for at least

1 full PWM cycle.

VREF (Pin 14): The VREF pin is a regulated 5V output that is

derived from the VIN pin. The regulated voltage provides up

to 10mA of current to power external circuitry. During

undervoltage lockout, shutdown mode or thermal

shutdown, drive to the VREF pin will be disabled.

VIN (Pin 15): The VIN pin is the voltage supply pin for the

LT1768. For normal operation, the VIN pin must be above an

undervoltage lockout of 7.9V and below a maximum of 24V.

GATE (Pin 16): The GATE pin is the output of a NPN high

current output stage used to drive the gate of an external

MOSFET. It has a dynamic source and sink capability of

1.5A. During normal operation, the GATE pin is driven high

at the beginning of each oscillator period and then low

when the appropriate current in the switch is reached. The

GATE pin has a minimum on time of 125ns and a maximum

duty cycle of 93% at a frequency of 350kHz. For input

voltages less than 13V the gate will be driven to within 2V

of VIN. For input voltages greater than 13V the gate pin high

level will be clamped at a typical voltage of 12.5V.

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