Op-Amp Adds Short-Circuit Protection to High-Side Switch

High-side switches provide a basic method for extending battery life. They eliminate unnecessary power consumption by simply removing supply voltage from peripherals and subsystems when circuits are not in use.

The logic-controlled switch circuit of Figure 1 provides output short-circuit protection in addition to low-impedance switching and low quiescent current. The actual switch (Q₁) is an n-channel MOSFET with a gate drive (V_BATT + 10V) generated by the regulated charge-pump IC₁.

Turn on the circuit by applying V_BATT to the ON/OFF input. V_OUT (pins 9 and 10) then pumps up, reaching V_CC + 10V within a millisecond or so and providing power to the IC₂ op amp. To ensure that Q₁ remains off until sufficient gate drive is available, a threshold detector internal to IC₁ triggers a 0V-to-V_BATT transition at PR (pin 6) when the rising output equals V_CC +8V.

The appearance of V_BATT at the PR terminal produces 0.75(V_BATT) at the low-power op amp's inverting input and a 100msec pulse at the non-inverting input. The pulse (of amplitude V_BATT minus one diode drop) kickstarts Q₁ into conduction and connects the battery to the load. The amplifier (configured as a comparator) then compares Q₁'s source voltage with the inverting-input voltage. As long as the source voltage is more positive, Q₁ remains on.

Feedback through R₄ provides short-circuit protection. If excessive load current pulls the source voltage below the reference level at the inverting input, the gate drive goes low and turns off Q₁. Collapsing load voltage then latches the switch off. To rest, pull the ON/OFF input to ground (for at least 100msec) and back to V_BATT.

Figure 1. The regulated IC₁ charge-pump generates a gate drive of V_BATT + 10V for the high-side power switch Q₁ (an n-channel power MOSFET) Feedback via R₄ provides short-circuit protection.