LT3750 datasheet(4/16 Pages) LINER

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LT3750

3750fa

PI FU CTIO S

VTRANS (Pin 1): Transformer Supply Pin. Powers the

primary coil of the transformer as well as internal circuitry

that performs boundary mode detection. Bypass at the pin

with a 1

µF to 10µF capacitor. Bypass the primary winding

of the transformer with a large capacitor.

DONE (Pin 2): Open Collector Indication Pin. When target

output voltage is reached, an NPN transistor turns on.

Requires a pull-up resistor or current source. Any fault

conditions such as thermal shutdown or undervoltage

lockout will also turn on the NPN.

CHARGE (Pin 3): Charge Pin. Initiates a new charge cycle

when brought high or discontinues charging and puts part

into shutdown when low. To properly enable the device, a

step input with a minimum ramp rate of 1V/

µs is required.

Drive to 1.1V or higher to enable the device; drive below

0.2V to disable the device.

VCC (Pin 4): Input Supply Pin. Bypass locally with a

ceramic capacitor. A 1

µFto10µFceramiccapacitorshould

be sufficient for most applications.

GND (Pin 5): Ground Pin. Connect directly to local ground

plane.

SOURCE (Pin 6): Source Pin. Senses NMOS drain current.

Connect NMOS source terminal and current sense resistor

to this pin. The current limit is 78mV/RSENSE.

GATE (Pin 7): Gate Pin. Connect NMOS gate terminal to

this pin. Internal gate driver will drive voltage to within

VCC – 2V during each switching cycle.

RDCM (Pin 8): Discontinuous Mode Sense Pin. Senses

when current in transformer has decayed to zero and ini-

tiates a new charge cycle if output voltage target has not

been reached. Place a resistor between this pin and the drain

of the NMOS. A good choice is a 43k, 5% resistor.

RVOUT (Pin 9): Output Voltage VI Converter Pin. Develops

a current proportional to output capacitor voltage. Con-

nect a resistor between this pin and the drain of the NMOS.

RBG (Pin 10): Output Voltage Sense Pin. Senses the

voltage across the RBG resistor, which is proportional to

the current flowing into the RVOUT pin. When voltage

equals 1.24V, charging is disabled and DONE pin goes

low. Connect a resistor (2.5k or less is recommended)

from this pin to GND. A 2.49k, 1% resistor is a good

choice.

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