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LT4256-3CGN Datasheet(PDF) 7 Page - Linear Technology |
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LT4256-3CGN Datasheet(HTML) 7 Page - Linear Technology |
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7 / 20 page  LT4256-3 7 42563f PI FU CTIO S GATE (Pin 13): High Side Gate Drive for the External N-Channel MOSFET. An internal charge pump guarantees at least 10V of gate drive for VCC supply voltages above 20V and 4.5V of gate drive for VCC supply voltages between 10.8V and 20V. The rising slope of the voltage on GATE is set by an external capacitor connected from GATE to GND and an internal 30 µA pull-up current source from the charge pump output. If the current limit is reached, the GATE voltage is adjusted to maintain a constant voltage across the sense resistor while the timing capacitor starts to charge. If the TIMER voltage ever exceeds 4.65V, GATE is pulled low. GATE is also pulled to GND whenever UV is pulled low; the VCC supplyvoltagedropsbelowtheexternallyprogrammed undervoltage threshold, above the overvoltage threshold or below the internal UVLO threshold (9.8V). GATE is clamped internally to a maximum voltage of 11.6V (typ) above VOUT under normal operating conditions. Driving this pin beyond the clamp voltage may damage the part. GATE is also clamped to 2V (typ) below VOUT. When the gate is commanded off due to a fault condition, it is discharged quickly by a 62mA (typ) capable switch until GATE is 2V (typ) below VOUT. When GATE is below VOUT by 2V, the 62mA is reduced to 130 µA to protect the LT4256-3 against damage if VOUT has large capacitance. A Zener diode is needed between the gate and source of the external MOSFET to protect its gate oxide under instanta- neous short-circuit conditions. See Applications Informa- tion. SENSE (Pin 15): Current Limit Sense Input. A sense resistor is placed in the supply path between VCC and SENSE. The current limit circuit regulates the voltage across the sense resistor (VCC – SENSE) to 55mV while in current limit when FB is 2V or higher. If FB drops below 2V, the regulated voltage across the sense resistor de- creases linearly and stops at 14mV when FB is 0V. The OPEN output also uses SENSE to detect when the output current is less than (3mV)/R5. To defeat current limit, connect SENSE to VCC. VCC (Pin 16): Input Supply Voltage. The positive supply input ranges from 10.8V to 80V for normal operation. ICC is typically 1.8mA. An internal circuit disables the LT4256-3 for inputs less than 9.8V (typ). |
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