Low-drop voltage regulators get wide application soon after they appear on the semiconductor market, mainly because of power savings in ac-dc supplies. Proceeding in the same direction to minimize the losses, we developed a "zero-drop" linear regulator based on logic-level MOSFETs with low RON (Fig. 1). Our objective was a target price of $1 for all components.
The component values are for VOUT = 5 V. For input voltages under 5 V, the output follows the input. Input voltages greater than 5 V are held at 5 V. The voltage drop through the regulator depends on the chosen MOSFET. With an IRF7416, the voltage drop is 15 mV for an input-voltage range of 3.5 to 5 V at 100-mA output current. With an IRL5602S, it's 25 mV at 500 mA for the same input-voltage range.
Figure 2 shows the behavior of the output voltage during power-on, when the input voltage rises up to 10 V within 1 ms. In a normal stabilization mode (VIN = 10 V dc), ripple rejection is better than 50 dB if the input voltage is overlaid by a 1-V, 120-Hz ac waveform (Fig. 3). That simulation shows that for lower frequencies, this value strongly depends on the hFE of Q2.
The accuracy of VOUT depends on the zener diode accuracy, on the hFE of Q2, and on the accuracy of the R6/R11 ratio. Q1 and Q2 should be of the same type (i.e., 2N3904). They were chosen as different types only for a more convenient simulation in PSpice.
For most applications, the VOUT accuracy is good enough. Higher accuracy is achievable by replacing the Q1-Q2 differential stage with an ultra-low-voltage op amp. In such a case, special attention should be paid to its "power-on" behavior.