I'm pleased to welcome Mark Pieper to the Power House blog. Mark is the mind behind the article "Designing a negative boost converter from a standard positive buck converter" featured in TI's Analog Applications Journal newsletter.
What is the problem/challenge you saw that spurred you to write, “Designing a negative boost converter from a standard positive buck converter?” As a power applications specialist at Texas Instruments, I was working with a customer to address a yield issue. A particular (and very expensive) ASIC required a power supply voltage of negative 2.0V, which was made available in the system chassis. A percentage of the ASIC’s were failing at 2.0V and it was determined that increasing the power rail to 2.2V solved the yield problem. It was not practical to change the chassis, so a negative boost circuit was added to the circuit card.
What was the most difficult aspect of the topic you wrote about and why? The more difficult part of this design was to understand the transfer function of the converter and implementing an adequate error amplifier compensation design.
Who and what application do you think this article is best suited for? The basic idea is suitable for any application where a negative voltage is available and a larger negative voltage is needed. This will result in the smallest, most efficient implementation of that design.
Did you learn anything as you researched and wrote this article that you did not know before? Up until working with this customer, I hadn’t thought about boost converters in a while so it was nice to revisit the control theory aspects.
Do you have an interesting tidbit of information associated with the development of this article that you can share? The Devil is in the details! Often, I am in a rush to implement an idea. In this case, I used an evaluation board, modified it to accommodate the various power inputs, put a big feedback capacitor on the error amp (dominant pole - just to get it working) and powered it on. It worked quite well, that is until until I increased the load beyond 5A or so. At higher loads it would turn off and not start again until I powered down and reapplied power. After scratching my head for a while, I figured out that at my input voltage the pre-existing under-voltage lockout setting (which I had not changed) happened to land right at my input voltage. At higher loads, I was tripping this threshold and the hysteresis would stop it from restarting. It was just dumb luck that it turned on in the first place. So:
Lesson #1: Do a thorough design up front and you will save time in the long run.
Lesson #2: Let the laws of physics teach you how your circuit is really working - and listen to the lesson.
An excerpt from his article:
“There are very few options for the designer when it comes to creating negative voltage rails in point-of-load applications. Integrated devices that are specifically designed for this are uncommon, and other available options typically have significant drawbacks, such as being too large, noisy, inefficient, etc. If a negative voltage is available, it is advantageous to use that as the input for the converter. This article describes a method using a standard positive buck converter to form a negative boost converter, which takes an existing negative voltage and creates an output voltage with a larger (more negative) amplitude. Using TI's SWIFTTM TPS54020 boost regulator results in a smaller, more efficient, and more cost-effective design. A complete design example using an integrated FET buck converter is provided. The basic theory of operation, high-level design trade-offs, and closed-loop compensation design of the resulting converter are discussed.”