Power Supply Voltage Control Using Current DAC

Introduction

In this post, I am analyzing the feedback circuit of power supply with an output voltage that is controlled using a current-output Digital-to-Analog Converter (DAC). I analyzed a related situation (voltage DAC) in this earlier post.

This exercise started when an engineer grabbed me and said he was having issues with using a current DAC to control the output voltage of power supply. He was using the device exactly has stated in the application note, but it was not producing the proper voltages. He needed to find the problem immediately because he was on a critical project and was beginning to run late. This post goes through the analysis I prepared for him. As it turns out, the datasheet from the power supply vendor had a number of errors in it. This is not unusual -- I have spent many hours trying to work my way through datasheet errors.

Using the analysis below, resistor settings were computed that had the power supply working as predicted in just a few minutes. I worked the exercise at the desk of an engineer who I am training in Mathcad. This proved to be an excellent demonstration of how a mathematical tool can speed an engineer's work.

Background

I described the operation of this type of voltage-variable power supply in my previous post, but I will list the key operational points here:

• The power supply's control system will adjust its output voltage so that the voltage at the feedback pin (V_FB) is 0.8 V.
• In this situation, the engineer had an unused current DAC available on the Printed Circuit Board (PCB). He wanted to use this device to control the output voltage of the power supply. To understand the function of a PCB, and the various types that can be used for devices like this, people can visit mktpcb.com to gain more information.
• V_FB is the sum of the power supply's output voltage (V_Out) through a voltage divider and the voltage generated by the DAC's output current fed into the center node of the voltage divider.
• Superposition is used to determine V_FB.

Nothing too sophisticated -- this analysis represents the kind of work we do everyday.

Analysis

Figure 1 shows the circuit and my analysis. I need to compute two resistor values: R_0A and R_0B.

These two resistor values were then substituted into the circuit and everything worked!

Conclusion

Just a quick example. The engineer grabbed me at 4:00 PM and we had a working circuit by 4:30 PM. I was home before 5:00 PM.