Circuit adds latch-off current limit to regulator

Craig Varga, Linear Technology Corp, Milpitas, CA -- EDN, 3/30/2000

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 V_CC, 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)