APPLICATION NOTE 38

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


Abstract: Application showing how a high-side switch can be used to connect/disconnect a battery from a load to reduce power consumption. By using an op amp in the circuit, short-circuit protection is provided.

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 (VBATT + 10V) generated by the regulated charge-pump IC1.

Turn on the circuit by applying VBATT to the ON/OFF input. VOUT (pins 9 and 10) then pumps up, reaching VCC + 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-VBATT transition at PR (pin 6) when the rising output equals VCC +8V.

The appearance of VBATT at the PR terminal produces 0.75(VBATT) at the low-power op amp's inverting input and a 100msec pulse at the non-inverting input. The pulse (of amplitude VBATT 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 VBATT.

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

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APP 38:
APPLICATION NOTE 38,AN38, AN 38, APP38, Appnote38, Appnote 38