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Help with charge-pump converter LMC7660

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Androxys

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Hello everyone, first time poster, and in dire need for assistance.

So I have an instrumentation op-amp, the INA111 Instrumentation Amplifier - Dual Supply - INA111 - TI.com

I am supplying it with +-9Vdc, and am using an LMC7660 converter LMC7660 - Switched Capacitor Voltage Converter to switch the 9V to -9V. However I am having two issues with it.

I am using two 10uF electrolytic capacitors like the datasheet says to, and every pin is connected properly, according to the sheet. However at the output I am having only -6.5V, rather than -9V as was supposed to. And even worse than that, when I connect the LMC7660's output to the V- pin of the INA111, the voltage drops to -0.6V, which isn't near enough for my application.

Some reading of the datasheet of the LMC7660 leads me to believe that this probably has something to do with the load current, but that is as far as I can go with solving the problem myself. I investigated a little but no luck yet.

Anyone has an idea of how to solve this?

Thank you very much in advance.
 

dick_freebird

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You may use electrolytics for the reservoir / filter caps
but you should be using nonpolar, ceramic fly-caps.

I've built a pretty usable charge pump to power an
instrumentation amplifier, making ~+/-11V from +12
automotive. I used a LM555 at 50% duty, 1N914
diodes and 0.1uF ceramic caps forming a pair of
charge pumps positive and negative. The capacitor
blocking lets the instrumentation amp float relative
to the sheet metal.

Single stage capacitive charge pumps will only give
you close to VIN, when load is zero. Maybe you need
bigger fly caps, higher pump frequency or a lower
current instrumentation amplifier (I use AD620).
 

Androxys

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Thank you for your input.

Unfortunately the AD620 won't do it for me, I'm trying to make a pH meter and 1 nA of input bias current is just too much for such a high impedance signal from the pH sensor.

I'm looking at the possibility of using a transformer to get the reference from the center tap, and positive and negative voltages from the other two.

In any case, would you mind to post a diagram for your charge pump build? I'm not entirely sure I understood.

Thank you very much.
 

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