An LDO regulator with discrete components

10^th December 2016

Earlier this year I wrote an article about the design and performance of the 1-transistor regulator. This article demonstrated that this simple design is quite useful if configured appropriately.

However, one inherent limitation of that design is it's high dropout voltage. It's difficult to make it work well with less than about 2.5volt of headroom.

So here I present an LDO regulator made from discrete components. I designed it myself during insomnia in the middle of the night.

The concept

The current source switches on the darlington Q2, which in turn switches on the output transistor, Q3.

Resistor divider R1 & R2 creates 0.6V when the desired output voltage is acquired. This activates Q1 to eat some of the current from the current source, throttling Q2.

R3 is essential to stop Q2 blowing itself up if the input voltage is too low to produce the desired output voltage.

Note that the above is a simplified concept drawing and it might need an output capacitor to be stable.

Actually making it

Here's one I made and tested. It produces a USB2 compliant output. This means it produces 5V +0.25 −0.60, at up-to 500mA. It has a current limiter at about 1Amps. My tests indicated low output impedance and good supply rejection down to about 1V of headroom.

I power this off a traditional, old-fashioned transformer+rectifier. It's for powering a pair of USB speakers which buzz when plugged into every other mains-power supply I have (including the computer itself). You get smoother power output with this old-fashioned, big, heavy and expensive tech than with small, powerful and cheap modern switch-mode PSUs.

If you build a mains power-supply please be very mind-full about safety!