Best practice to get a -1V supply from 3.3V/5V? - Page 1
Forum section: electronics / beginners

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: February 19, 2015, 12:09:18 pm
content: For an OpAmp circuit, I want to create a small negative voltage to be able to reach 0V on the output.While it's easy to invert e.g. a 3.3V supply with an ICL7660, MAX660 or some discrete charge pump circuit, this will end in e.g. -3.3V, where I just want -1V.Problem is that most OPs are specified for 6V (Vdd-Vss), so if the positive supply is 5V, the negative supply most not be smaller -1V.Using an OpAmp with a wider supply range limits the choices quite a bit and in my case would about twice the cost for slightly worse parameters (like offset voltage).So I would prefer to get something like -0.5 to -1V from the negative supply. I yes, I know about phantom grounds and virtual grounds, but for the moment, I'd rather use a negative supply.Now I could use e.g. a LM337 to get a voltage closer to-1V, but it's limited -1.25V. Just slightly out of nominal operation condition for e.g. an MCP607x , but still out.Also e.g. the MAX660 doesn't work for input voltages smaller than 1.5V, so assuming no losses in the inverter, I can't get -1V by using a voltage regulator before the charge pump either.I guess I could go on further by using a diode or a voltage divider with a voltage follower, but this seems like adding a hell of complexity for something as simply as a low current -1V supply.So what would be the best practice approach to create -1V from e.g. a 5V supply (while keeping the 6V difference between 5V and -1V)?

author: Richard Head
author_rank: Frequent Contributor
timestamp: February 19, 2015, 12:28:57 pm
content: I'd use a capacitive charge pump with a flying capacitor and clamp the voltage to -1v with a BAV99 or similar. You will need a simple oscillator that can source and sink current. A 555 would work but you may well be able to one of the digital outputs of a micro if you have one available. I'd use an operating frequency of about 10khz.

author: bktemp
author_rank: Super Contributor
timestamp: February 19, 2015, 12:48:18 pm
content: Use a LM7705:www.ti.com/product/lm7705

author: Zero999
author_rank: Super Contributor
timestamp: February 19, 2015, 12:53:32 pm
content: Why not use a different op-amp which is rated to more than 6V? Very few op-amps have such a low voltage rating.http://nonlinear.eecs.berkeley.edu/chaos/LMC6482.pdfhttp://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/DM00108580.pdfhttp://www.st.com/web/en/catalog/sense_power/FM123/SC61/SS1613http://www.onsemi.com/PowerSolutions/product.do?id=TLV271http://ww1.microchip.com/downloads/en/DeviceDoc/22243c.pdfThen there's the MAX44267 which has a built-in capacitive voltage doubler.http://www.maximintegrated.com/en/products/analog/amplifiers/MAX44267.html

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: February 19, 2015, 05:22:28 pm
content: Thanks for all the suggestions. Then again, being a lazy sloth, I think I'll try the LM7705.Sounds like the most elegant solution. One part plus three caps. And the fixed ?0.232V is just fine.

author: Andreas
author_rank: Super Contributor
timestamp: February 19, 2015, 08:34:20 pm
content: Hello,I would try a LT1054 (regulated voltage inverter).With best regardsAndreas

author: Cliff Matthews
author_rank: Unknown
timestamp: February 20, 2015, 03:11:49 am
content: Won't most cheap maxim negative solutions work with a zener-resistor combo output to get the job done?