In a previous post to our Power Tips Blog I shared a little circuit for regulating isolated supplies with low output voltages. But, what if you are working on a non-isolated supply with a very low output voltage? The first thing that you will probably do is spend a day scouring the internet for a controller that has a reference voltage that is lower than your required output voltage. If your output voltage is 0.8V or higher, you probably will not have too many problems finding a controller. However, below 0.8V, your choices will become very limited. With two extra components, you can get around this limitation.
As shown in Figure 1, all that you need is an external reference and a resistor to provide bias current to that reference. For this technique to work, the external reference voltage must be higher than the internal reference of the controller. By connecting the feedback divider from the external reference and output voltage, the controller is now able to regulate the output to voltages that are lower than the internal reference.
Figure 1. This 0.5V buck regulator circuit shows how a controller can be tricked into regulating to voltages that are lower than the internal reference.
The external reference will affect the accuracy of the output voltage. If this is a concern in your application, you may need to use a 0.1% accurate reference which will cost a little more than commonplace 1% and 2% references. A few controllers even pin out auxiliary bias rails that could be used instead of the external reference. However, be careful, as these auxiliary rails tend to be very loosely regulated.
Also, don’t forget to check the minimum controllable on-time of the controller, especially if you are designing for a high switching frequency. As the input to output ratio gets very small, this can become a limiting factor. To get around this limitation, you can always reduce your switching frequency.
For an example of a supply that uses this technique, check out the PMP9166 design in PowerLab…
- PMP9166 – 3.3V Input, -0.5V/0.9A Output, Synchronous Buck-Boost
What is the lowest voltage you have had to regulate in a power supply?
