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LT3574IMS

Isolated Flyback Converter Without an Opto-Coupler

Linear Technology 

LT3574

Pin Functions

BIAS: Bias Voltage. This pin supplies current to the switch

driver and internal circuitry of the LT3574. This 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

≤ 15V. If a third winding is used, the BIAS voltage should

be lower than the input voltage for proper operation.

GND: Ground.

RFB: Input Pin for External Feedback Resistor. This pin is

connected to the transformer primary (VSW). 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. For nonisolated

applications, this pin should be connected to VIN.

RILIM: Maximum Current Limit Adjust Pin. A resistor

should be tied to this pin to ground to set the current

limit. Use a 10k resistor for the full current capabilities

of the switch.

RREF: Input Pin for External Ground-Referred Reference

Resistor. This resistor should be in the range of 6k, but

for convenience, need not be precisely this value. For

nonisolated applications, a traditional resistor voltage

divider may be connected to this pin.

SHDN/UVLO: Shutdown/Undervoltage Lockout. A resistor

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

minimum input voltage at which the LT3574 will operate.

At a voltage below ~0.7V, the part draws no quiescent

current. When below 1.22V and above ~0.7V, the part will

draw 10µA of current, but internal circuitry will remain

off. Above 1.22V, the internal circuitry will start and a 7µA

current will be fed into the SS pin. When this pin falls

below 1.22V, 2.5µA will be pulled from the pin to provide

programmable hysteresis for UVLO.

SS: Soft-Start Pin. Place a soft-start capacitor here to

limit start-up inrush current and output voltage ramp

rate. Switching starts when the voltage at this pin reaches

~ 0.7V.

SW: Collector Node of the Output Switch. This pin has

large currents flowing through it. Keep the traces to the

switching components as short as possible to minimize

electromagnetic radiation and voltage spikes.

TC: Output Voltage Temperature Compensation. Connect

a resistor to ground to produce a current proportional to

absolute temperature to be sourced into the RREF node.

ITC = 0.55V/RTC.

TEST: This pin is used for testing purposes only and must

be connected to ground for the part to operate properly.

VC: Compensation Pin for 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.

VIN: Input Voltage. This pin supplies current to the internal

start-up circuitry and as a reference voltage for the DCM

comparator and feedback circuitry. This pin must be locally

bypassed with a capacitor.

3574f



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