Circuit adds latch-off current limit to regulator

-March 30, 2000

In many applications, forcing a high-current power supply to latch off if a sustained fault condition exists can minimize the likelihood of damage to the pc-board traces and the power devices in the supply. Pulse-width-modulation (PWM)-control circuits provide no latch-off feature, but the circuit in Figure 1 does. The circuit is based on IC1, an LTC1430 PWM controller. The current-limiting feature of the IC operates by sensing the voltage across the high-side MOSFET and compares it with a threshold voltage developed across R3. If an overcurrent condition exists, an internal current source starts to discharge the soft-start capacitor, C9. At this point, the latch-off circuit begins to take over. When the voltage across C9 decreases to a couple of volts below VCC, Q2 turns on and begins to source current, charging C12.

After a time interval depending on the values of R5 and C12, Q5 turns off and pulls the shutdown pin low, thereby turning off the controller. Because this action internally grounds the soft-start pin (to allow a normal soft-start cycle at turn-on), the circuit remains latched in the off state. You can initiate a reset by applying a fast logic high to the reset line. C6 and C7 provide a differential pulse to the base of Q4, which discharges C12, allowing the regulator to restart. If you don't need the reset function, you can eliminate C6, R7, R10, and Q4. You can then initiate a restart by recycling the 5V input power. The only timing requirement is that the latch-off delay be greater than the soft-start rise time at turn-on. Otherwise, the regulator can never start. You can modify the circuit to work with any other controller, such as the LTC1553, having the soft-start function. (DI #2503)

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