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LT3511

3511fa

pin FuncTions

EN/UVLO (Pin 1): Enable/Undervoltage Lockout. The EN/

UVLO pin is used to start up the LT3511. Pull the pin to 0V

to shut down the LT3511. This pin has an accurate 1.21V

threshold and can be used to program an undervoltage

lockout (UVLO) threshold using a resistor divider from

supply to ground. A 2.6μA pin current hysteresis allows

the programming of undervoltage lockout (UVLO) hys-

teresis. EN/UVLO can be directly connected to VIN. If left

open circuit the part will not power up.

VIN (Pin 3): Input Supply Pin. This pin supplies current to

the internal start-up circuitry, and serves as a reference

voltage for the DCM comparator and feedback circuitry.

Must be locally bypassed with a capacitor.

GND (Pin 5, 8, 9): Ground Pins. All three pins should be

tied directly to the local ground plane.

BIAS (Pin 6): Bias Voltage. This pin supplies current to

the switch driver and internal circuitry of the LT3511.

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

is not used and if VIN < 20V. The part can operate down

to 4.5V when BIAS and VIN are connected together. If a

third winding is used, the BIAS voltage should be lower

than the input voltage and greater than 3.3V for proper

operation. BIAS must be bypassed with a 4.7μF capacitor

placed close to the pin.

VC(Pin10):CompensationPinforInternalErrorAmplifier.

ConnectaseriesRCfromthispintogroundtocompensate

theswitchingregulator.Anadditional100pFcapacitorfrom

this pin to ground helps eliminate noise.

TC (Pin 11): Output Voltage Temperature Compensa-

tion. Connect a resistor to ground to produce a current

proportional to absolute temperature to be sourced into

the RREF node.

ITC = 0.55V/RTC.

RREF (Pin 12): Input Pin for External Ground-Referred

Reference Resistor. The resistor at this pin should be 10k.

For nonisolated applications, a traditional resistor voltage

divider from VOUT may be connected to this pin.

RFB (Pin 14): 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).

For nonisolated applications, this pin should be connected

to GND with a 1M resistor.

SW (Pin 16): Switch Pin. Collector of the internal power

switch. Minimize trace area at this pin to minimize EMI

and voltage spikes.

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