LT3748IMSPBF datasheet(6/28 Pages) LINER

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LT3748

3748f

pin Functions

VIN (Pin 1) Input Voltage. This pin supplies current to the

internal start-up circuitry and is the reference voltage for

the feedback circuitry connected to the RFB pin. This pin

must be locally bypassed with a capacitor.

EN/UVLO(Pin3):Enable/UndervoltageLockout.Aresistor

divider connected to VIN is tied to this pin to program the

minimum input voltage at which the LT3748 will operate.

At a voltage below ~0.5V, the part draws less than 1µA

quiescent current. When below 1.223V but above ~0.5V,

the part will draw quiescent current but will not regulate

the INTVCC supply or power the gate drive circuitry. Above

1.223V, all internal circuitry will start and the SS pin will

source 5μA. When EN/UVLO falls below 1.223V, 2.4μA is

sunk from the pin to provide programmable hysteresis

for undervoltage lockout.

INTVCC(Pin 5): Gate Driver Bias Voltage. This pin supplies

current to the internal gate driver circuitry of the LT3748.

The INTVCC pin must be locally bypassed with a capacitor.

This pin may also be connected to VIN if a third winding

is not used and if VIN ≤ 20V. If a third winding is used, the

INTVCC voltage should be lower than the input voltage for

proper operation.

GATE (Pin 6): N-Channel MOSFET Gate Driver Output.

Switches between INTVCC and GND.

SENSE (Pin 7): The Current Sense Input for the Control

Loop. Kelvin connect this pin to the positive terminal of

the switch current sense resistor, RSENSE, in the source

of the N-channel MOSFET. The negative terminal of the

current sense resistor should be connected to the GND

plane close to the IC.

GND (Pins 8, 9): Ground.

SS (Pin 10): Soft-Start Pin. This pin delays start-up and

clamps VC pin voltage. Soft-start timing is set by the size

of the external capacitor at the pin. Switching starts when

VSS reaches ~0.65V.

VC (Pin 11): Compensation Pin for the Internal Error

Amplifier. Connect a series RC from this pin to ground to

compensate the switching regulator. A 100pF capacitor in

parallel helps eliminate noise.

TC (Pin 12): Output Voltage Temperature Compensation.

Connect a resistor to ground to produce a current pro-

portional to absolute temperature to be sourced into the

RREF node. ITC = 0.55V/RTC.

RREF (Pin 14): Input Pin for the External Ground-Referred

ReferenceResistor.Theresistoratthispinshouldbe6.04k,

but for convenience in selecting a resistor divider ratio,

the value may range from 5.76k to 6.34k. The resistor

should be as close to the LT3748 as possible.

RFB (Pin 16): Input Pin for the External Feedback Resistor.

This pin is connected to the transformer primary at the

external MOSFET power switch. The ratio of this resistor

to the RREF resistor, times the internal bandgap reference,

determines the output voltage (plus the effect of any

non-unity transformer turns ratio). The average current

through this resistor during the flyback period should be

approximately 200μA. The resistor should be as close

to the LT3748 as possible.

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