Design Ideas January 16, 1997

Circuit breaker has programmable delay

David Bell, Linear Technology Corp, Milpitas, CA

The circuit in Figure 1 provides overload and short-circuit protection to a 5V power-supply output. The heart of the circuit is the LT1620, a charge-current controller designed for high-efficiency, switch-mode battery chargers. The IC's versatility allows you to use it as an electronic circuit breaker. The circuit amplifies the voltage across current-sense resistor R₁ by a factor of 10 and averages it with capacitor C₂. An error amplifier within the LT1620 compares the voltage on the AVG pin to the programming voltage on the PROG pin. Both amplifier inputs use V_CC as a reference.

The output of the amplifier, which is capable only of sinking current, is present at I_OUT. Whenever the AVG voltage is more positive than the PROG voltage (with normal load current), the I_OUT pin is in a high-impedance state, allowing current to flow through R₃ into the base of Q₂. The saturated Q₂ pulls the gate of p-channel MOSFET Q₁ low, thereby holding Q₁ in the fully on state. Diode D₁ has approximately 135-mA bias current, producing typically 525-mV forward drop at 258C. (The voltage is inversely proportional to temperature.) This voltage serves as the reference for the circuit-breaker trip point.

Because the voltage across C₂ is 10 times the voltage across sense resistor R₁, the circuit reaches the overcurrent trip point when the voltage across R₁ is approximately 53 mV. This drop corresponds to a 1.6A trip current, using a 33-mV sense resistor. Once the current exceeds the trip value, the voltage at AVG drops below the reference voltage on PROG, and I_OUT pulls the base of Q₂ to ground. Q₂ turns off, allowing R₂ to pull the gate of Q₁ to 5V, thereby turning the MOSFET off. The gate voltage of Q₁ also turns Q₃ on, holding AVG below PROG, even after the load current drops to zero, thereby latching the circuit breaker in the open state.

You can reset the breaker by cycling the 5V power or by momentarily pulling the base of Q₃ to ground with an open-drain driver. The signal at the gate of Q₁ is a clean 0 to 5V logic signal that you can use to indicate that a fault condition has occurred. The LT1620 amplifier driving the AVG pin has a 2.5-kV output impedance that forms a time constant with C₂. By selecting different values for C₂, you can tune the response time of the breaker for specific applications. For example, the circuit breaker should be insensitive to momentary inrush currents that charge bypass capacitors at the load, but it should trip quickly enough to prevent damage in the event of a short circuit.

With C₂=1 mF, the breaker trips in approximately 2 msec with a 3A fault current. The time to trip becomes progressively shorter with higher overload currents. Resistor R₄ limits the current pulled from the AVG pin if the 5V input suddenly drops to 0V. The 33-mV current-sense resistor, along with Q₁'s low R_DS(ON) (typically 25 mV), drop only 58 mV with a 1A load current. (DI #1971)

FIGURE 1

A battery-charger controller forms the heart of an electronic circuit breaker, whose trip current and response time are fully programmable.

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