High-Side Current Amplifier Forms 28-V Circuit Breaker

For the PDF version of this article, click here.

The latching-comparator output (COUT1) on the MAX4373, a high-side current amplifier with comparator and voltage reference, makes possible a circuit-breaker application for that device. This application is shown in the device's data sheet.[1] While the circuit shown there is suitable only for supply voltages in the range of 2.7 V to 5.5 V, that range can be extended to 28 V. This is achieved by using a general-purpose npn bipolar transistor (Q1) as shown in Fig. 1.

The open-drain comparator output is driven low during normal operation, allowing a nominal current of 430 µA to flow from Q1's emitter via R2 to COUT1. Approximately the same current then flows through the collector of Q1 and R1, producing a negative V_GS drive voltage having a magnitude greater than 6.4 V for the p-channel MOSFET Q2.

If, for example, a circuit-breaker threshold of 900 mA is desired, the MAX4373FESA — whose gain from the differential sense-resistor inputs to the single-ended output (OUT) is 50 — would be a good choice. Thus, 900 mA through the 100-mΩ sense-resistor R6 shown in Fig. 1 drops 90 mV, which is multiplied by the gain of 50 to yield 4.5 V at the OUT terminal.

The comparator's input threshold at CIN1 is 600 mV, so a resistor divider with a ratio of 6.5-to-1 is the target value. Choosing R4 at 68 kΩ and R5 at 10 kΩ yields an actual load-current threshold of 936 mA, which is close enough for this application.

If the load current exceeds this threshold, the comparator output is latched into a high-impedance state. The transistor goes into cutoff and causes the MOSFET to turn off. Actuating the reset push button restores the circuit to normal operation, provided the fault has been removed.

This circuit topology works for applications in which the higher voltage (28 V in this example) is at least double the V_CC voltage. Also, V_CC is limited to a maximum value of 5.5 V to stay within the limits of the comparator output COUT1. The unequal values for R1 and R2 in this circuit give a slight gain in the voltage developed across R2 that can generally be approximated as:

This increases Q2's gate-drive voltage, minimizing losses due to MOSFET on-resistance.

The circuit responds to an overcurrent condition in about 100 µs (Fig. 2). Its response, unlike that of a thermomechanical circuit breaker, is virtually unchanged when overcurrents remain within 10% above the circuit-breaker threshold.