Shunt regulator improves power amplifier's current-limit accuracy

Adding current-limiting circuitry to a power amplifier's or a linear voltage regulator's emitter-follower output stage protects both the output transistor and the downstream circuitry from excessive-current damage. Figure 1 shows the classic current-limiter circuit: Transistor Q₂ senses the output-current-induced voltage drop across ballast resistor R₂ and diverts base current from Darlington-connected transistors Q₁ and Q₃. Transistor Q₂'s base-emitter voltage, V_BE, sets the circuit's current-limit threshold. Unfortunately, a small-signal transistor's V_BE exhibits a temperature coefficient of -2 mV/°C, which causes a substantial variation in the current-limiting threshold over the circuit's operating-temperature range.

You can improve the circuit's performance by replacing Q₂ with IC₁, an adjustable shunt regulator (Figure 2). With an input threshold voltage of 0.6V, the MAX8515 allows use of a lower value for current-sense resistor R₂ and thus helps minimize R₂'s power and thermal losses. Alternative commonly available shunt regulators present input voltages of 1.25 to 2.5V. In addition, a separate power-supply input connection allows the MAX8515 to maintain accuracy when its internal output transistor approaches saturation.

Figure 3 compares current-limit accuracy for the circuits of Figure 1 and Figure 2 over an operating-temperature range of -40 to +85°C. Neglecting the temperature coefficient of sense resistor R₂, the shunt-regulator version maintains its output current to an accuracy of better than 2%, and the small-signal-transistor version exhibits a 25% current variation over the operating-temperature range.