The simple current-limiting load switch shown in **Figure 1** will be familiar to most readers. In this circuit, a high level signal applied to the input switches on MOSFET Q2, which energizes the load. The load current is limited by negative feedback applied via Q1.

In most applications, the current flowing via R2 into Q1’s base will be very small compared to the load current I_{L}, so the voltage V_{CS} developed across current-sensing resistor R_{CS} is roughly equal to Q1’s base-emitter voltage, V_{BE}. Therefore, V_{BE} ≈ V_{CS}, and since V_{CS} = I_{L}R_{CS}, it follows that V_{BE} ≈ I_{L}R_{CS}, or I_{L} ≈ V_{BE} / R_{CS}.

The value of R_{CS} is chosen to limit the load current to a maximum value determined by I_{L(max)} ≈ V_{BE(on)} / R_{CS}, where V_{BE(on)} is the base-emitter voltage needed to bias Q1 into conduction. At room temperature, V_{BE(on)} ≈ 650mV. So, for example, a value of R_{CS} = 3.3Ω would set I_{L(max)} to around 200mA.

Under normal, ‘no-fault’ conditions, where I_{L} is within normal limits, V_{BE} is too small to bias Q1 on, such that MOSFET Q2 remains fully enhanced by the high level signal applied to its gate via R1. Under these conditions, the load current is determined mainly by the load resistance and the supply voltage, V_{S}. However, if a fault causes I_{L} to approach I_{L(max)}, Q1 starts to conduct, and reduces Q2’s gate-source voltage to a level that holds the load current roughly constant, at a value given by I_{L(max)} ≈ V_{BE(on)} / R_{CS}.

This linear current limiter is very effective for applications where I_{L(max)} and the supply voltage are not too large. However, the circuit’s ability to limit the load current safely is determined by the power dissipated in Q2. For example, if R_{CS} is selected to set I_{L(max)} to 400mA and if V_{S} = 12V, a short circuit across the load would dissipate almost 5W in Q2. Not only must Q2 be capable of handling this power with adequate margin, but additional heat-sinking may be required to keep its junction temperature at a safe level. Larger values of I_{L(max)} and/or V_{S} would simply exacerbate this problem.

For more detail: Load switch with self-resetting circuit breaker