Originally, the venerable three-terminal linear regulator featured a bipolar junction transistor emitter-follower output that exhibited very low output impedance. Many of today's applications demand dropout voltages lower than that of the first-generation regulators (1.5 V). Most modern regulators, therefore, include a common-source MOSFET as the pass element.
One compromise that's associated with the common-source (or common emitter) output is an intrinsically high output impedance. Feedback reduces the dc output impedance by the loop gain of the regulator. However, loop gain decreases rapidly as frequencyincreases. This effect causes an undesirable rise in the output impedance.
Yet many complex systems feature multiple voltage rails. This makes it possible to create low-dropout (LDO) linear regulators with good transient response by connecting a higher-voltage supply rail to bias the regulator's internal circuitry. In an example of this approach, the circuit uses a 5-V supply for internal bias while regulating an input voltage of 1.25 V down to 0.9 V (Fig. 1).
Resistor R1 supplies a bias current, modulated by the shunt regulator (U1) to drive a Darlington pair comprising Q1 and Q2. The circuit output is taken from the emitter of Q2 and fed back to the shunt regulator through the R3-R4 divider. Capacitors C1 and C5 bypass the input supplies, and capacitors C2, C3, and C4 compensate the circuit for stable operation.
To illustrate circuit performance, as shown in Figure 2, a 0- to 4-A load transient (lower trace) causes less than 5-mV change in the output voltage (upper trace). You must exercise some care in component selection to obtain this performance. The shunt regulator must have a low feedback threshold and a separate supply terminal as shown. Also, C1 and C4 must have ripplecurrent ratings comparable to the desired output current (MLCC ceramic chip capacitors work well). The FZT849 (by Zetex) is a good choice for transistor Q2, which should have adequate gain near saturation.