Design Ideas: February 1, 1996

The circuits in Figures 1 and 2 use a current mirror and a power MOSFET to increase the output-current capability of an IC-voltage regulator. The circuits retain all the protection features of the IC regulator and provide protection for the MOSFET as well. The regulators' dropout voltage increases by only 0.1V at 10A, as compared with the dropout voltage of the IC regulator operating alone.

In both Figures 1 and 2, R₁ and R₂ are current-sensing resistors. Current flowing through the IC regulator flows through R₁; current flowing through the MOSFET flows through R₂. The op amp modulates the on-resistance of the MOSFET, forcing the voltage drop across R₁ to equal the drop across R₂. This modulation causes the current flowing through the MOSFET to mirror the current flowing through the IC regulator, in the ratio of R₁ to R₂. The op amp can operate with input common-mode voltages equal to its supply voltages; it can thus sense current in the positive supply rail. The gain of the op amp makes it possible to use small values for R₁ and R₂ to minimize increases in the regulator's dropout voltage.

The regulator and the MOSFET both experience the same voltage drop. Therefore, the power dissipation in each device is directly proportional to the current flowing through it. Because the dissipation in each device is a known, well-controlled quantity, you can control the junction temperatures of both devices by paying attention to thermal impedances and heat-sink selection. The designs shown here use a Thermalloy 6159B heat sink for the MOSFET and a Thermalloy 6236B clip-type heat sink for the IC regulator. The regulator's thermal-protection circuit also protects the MOSFET. Because the current through the MOSFET is a fixed multiple of the regulator current, the regulator's current-limiting feature also protects the MOSFET against excessive currents.

R₃ introduces a small offset at the op amp's inputs and thus ensures that the MOSFET is turned off under no-load conditions. You can eliminate R₃ if the circuit always operates with a minimal load. The circuits in Figures 1 and 2 deliver currents as high as 15.4A with a maximum dropout voltage of 1.64V. Thus, V_IN can drop as low as 6.64V. The V_GS (maximum) of the MOSFET (15V) limits V_IN (maximum) at light loads, and the power-dissipation limits of the IC regulator and MOSFET limit the input voltage at heavy loads.

These circuits apply to all types of linear regulators: conventional and low-dropout and positive and negative. In fact, it's easier and less expensive to boost negative regulators with this method, because you can use an n-channel MOSFET and a conventional, single-supply op amp, such as the LM324. However, it's advantageous to use a rail-to-rail op amp, to ensure full enhancement of the MOSFET gate, especially with low input voltages, and also to ensure that the MOSFET can turn off completely.

In Figure 2, a pair of pnp transistors in a current-mirror configuration replaces the op amp. This configuration works satisfactorily, except that differences in the base-emitter voltages of the two transistors can degrade the accuracy of the current mirroring. This degradation presents a problem only under light-load conditions; you can ignore it if you specify a minimum load. Both circuits exhibit good stability and fast transient response.

The p-channel MOSFET used here is a low-threshold, logic-level type. It exhibits low values of on-resistance within low values of gate-source voltage. Additionally, the Lm6142 op amp can operate with a 1.8V minimum supply voltage. These features allow the circuit in Figure 1 to deliver the output voltages and currents shown in Table 1. To obtain these low voltage levels, you replace the fixed-voltage regulator with an adjustable-output version of the same IC, as shown in the inset in Figure 1. The circuit still maintains low dropout voltage. Thus, the circuit in Figure 1 could function as a high-current, 5 to 3V supply. (DI #1824)