# Converting -3.7V to -32V (both negative), Is it a boost or a buck?

I need to get create a split supply at +/-32V, but all I have to work with are LiPo batteries. I was thinking something like the block diagram below. The positive part is simple, but the negative part is not so straight-forward.

simulate this circuit – Schematic created using CircuitLab

I tired googling this, but there is not much literature on dc-dc conversion with negative voltage. Main questions I have and need help with are:

1. When you convert -3.7V to -32V, is that considered a boost or buck? The output voltage is lower, but the absolute value is greater.

2. Judging by the lack of information, it seems this is not a typical problem, but is it practical?

3. Can the negative "boost" be done using the same positive boost IC? Ideally, I'd like to use the same chip if possible.

4. Any recommendation on reading material or schematics? Or any other method of getting +/-32V?

• It doesn't answer your question, but why two lipos? Why not generate -32V from +3.7V? (for info, in that case, it is called an inverter) – dim Dec 24 '16 at 1:27
• It's boost. Boost or buck isn't about polarity but about increasing or decreasing the absolute voltage. Of course, for boosting a negative voltage you have to use inverse-polarity components – at least for the output stage. Usually datasheets of controller ICs have several converter layouts as examples, so you could easily adapt the same IC type for both the positive and the negative part. – Janka Dec 24 '16 at 1:28
• What's your application? If you try to build a mobile amplifier: simply use a single-supply one and don't add the complexity of needing two supplies. – Marcus Müller Dec 24 '16 at 1:44
• @dim Two lipos only because of the idea above I wanted to get others' opinion on. Your suggestion was my initial solution, but I've had trouble finding a chip that can invert that much and still supply 200mA. If you have come across any, it would be very helpful! – Shubham Dec 24 '16 at 1:50
• @MarcusMüller The application is to provide high voltage split supply to an opamp that drives a special device. – Shubham Dec 24 '16 at 1:51

Going from -3.7V to -32V to is a boost, a negative boost.

To make things more less confusing, here's are application notes Designing a negative boost converter from a standard positive buck converter and Positive Buck Regulator Makes Negative Boost DC/DC Converter. (The regulators in these app notes are intended to be positive bucks, but repurposed as negative boosts.)

For a simple inductive boost converter, it's advisable not to have a boost factor greater than 6. This applies to both positive and negative boost.

p.s. We’re situated roughly in the same neck of the woods, interestingly.

• Found the first one earlier, but did not stumble across second one. It's a good read, thanks, pretty much what I was looking for. Going to give it a try. – Shubham Dec 27 '16 at 23:03

It would still be called a boost converter even though the output is lower absolute potential than the the input. I would recommend developing both the +32V and the -32V from a positive +3.7V input. You will have more luck finding devices that will do that. Many of the datasheets for voltage converter ICs will have an example for generating negative voltages.

An alternative is that you could use a transformer coupled isolating converter to provide the -32V or both outputs, then it doesn't care where ground is. There are off the shelf devices from manufacturers such as PICO who may have something that meets your needs. Some of those have dual outputs.

• I'd even go as far as saying that op should put the batteries in series to create 7.4 V instead - smaller factor – Marcus Müller Dec 24 '16 at 1:45
• @marcus, Good point, I agree, especially since the required output is about 13W. – Kevin White Dec 24 '16 at 17:59
• That was my initial thought as well, but I had troubling finding a dual (positive and negative) supply regulator that could meet the power requirement. But going off of @MarcusMüller comment, I will try putting 3 Lipos in series to see if I can get get +/-32V @ 200mA. – Shubham Dec 27 '16 at 23:05

You need a BIPOLAR BOOST circuit. You can take +7.4V from your two LIPO cells and convert directly to ±32V There are many examples of DC-DC boost converters that will produce bipolar output voltages.

"Boost" refers to the output voltage being greater than the input voltage. The polarity is not a factor in the name/function. It is only a detail of implementation.