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3575FE

Isolated Flyback Converter without an Opto-Coupler

Linear Technology 

LT3575

PIN FUNCTIONS

NC (Pins 1, 15, 16): No Connect Pins. Can be left open

or connected to any ground plane.

VIN (Pin 2): 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.

SW (Pins 3, 4): 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.

BIAS (Pin 5): Bias Voltage. This pin supplies current to the

switch driver and internal circuitry of the LT3575. 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.

SHDN/UVLO (Pin 6): Shutdown/Undervoltage Lockout.

A resistor divider connected to VIN is tied to this pin to

program the minimum input voltage at which the LT3575

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 7μA 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, 2.8μA will be pulled from the pin to

provide programmable hysteresis for UVLO.

SS (Pin 7): 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.

RILIM (Pin 8): 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.

VC (Pin 9): 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.

RFB (Pin 10): 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.

RREF (Pin 11): 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.

TC (Pin 12): 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 (Pin 13): This pin is used for testing purposes only

and must be connected to ground for the part to operate

properly.

GND (Pin 14, Exposed Pad Pin 17): Ground. The exposed

pad of the package provides both electrical contact to

ground and good thermal contact to the printed circuit

board. The exposed pad must be soldered to the circuit

board for proper operation and should be well connected

with many vias to an internal ground plane.

3575f

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