For an opAmp rails I need positive and negative voltages i.e. Vcc, Vee. I look for a good IC which is commonly used for creating split power from a single power supply. Many recommends ICL7660. I currently have a MAX232. Can it be used instead? I need +5V -5V or +9V -9V or +12V -12V all would work.
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Which pin are you going to take the negative voltage from? – Greg d'Eon Apr 01 '15 at 14:06
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...and how are you going to figure out how much current it can sink? – Greg d'Eon Apr 01 '15 at 14:07
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for +-12V not more than 10mA – user16307 Apr 01 '15 at 14:10
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Good idea but keep in mind that V+ and V- are not regulated – Alexxx Apr 01 '15 at 14:19
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not regulated in MAX232 case? or also not regulated in ICL7660 case as well? – user16307 Apr 01 '15 at 14:50
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Read MAX232 datsheets: http://datasheets.maximintegrated.com/en/ds/MAX220-MAX249.pdf page 14. The more current you draw the lower the voltage. – Alexxx Apr 01 '15 at 15:44
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BTW MAX865 could be a better choice than ICL7660 as it's just one IC – Alexxx Apr 01 '15 at 16:07
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A charge pump can be appropriate for making power supplies for low-power opamps. A MAX232 contains two charge pump supplies, which can be used for your purpose, maybe.
The MAX232 will create a positive and negative voltage, but since those supplies are designed to power its internal circuitry, extra available current will be limited. If you don't load the RS-232 lines of the MAX232, then it should in theory have some excess current capacity you can use.
Check the datasheet to see if there is any spec on how much current the charge pumps generate and what, if any, is left over for the external circuit to use. You are probably on thin ice spec-wise, since the MAX232 isn't intended for how you plan to use it. You can probably get some guidance on the current capacity by looking at the maximum current the MAX232 can source and sink from/to the RS-232 lines. Then leave those lines unconnected and assume that the extra current is probably available for you to use.
However, dedicated charge pump chips are available and no more expensive than MAX232, so it makes more sense to use one. They will then come with proper specs so that you can know up front whether your design is violating maximum capability or not.
Regardless of whether you use a MAX232 or a dedicated charge pump chip to make your opamp power supplies, be sure to put a little filtering between the supply and the opamp. A ferrite "chip inductor" followed by 20 µF or so cap to ground on each power feed of each opamp would be a good idea. That kills the high frequency noise from the supply, leaving the lower frequency variations that the active circuit in the opamp should be able to deal with.
Olin Lathrop
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@Alex says that in MAX232 case V+ and V- are not regulated. do i have the same problem in ICL7660 case? – user16307 Apr 01 '15 at 14:52
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i just checked the data sheet of ICL7660. it seems like this chip has buiit-in voltage regulator. now i understand why MAX232 is not the right one. – user16307 Apr 01 '15 at 15:02
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Yes, you can, but the MAX232 is really not designed to create supply voltages. The output voltage is not regulated very well, and there is a lot of noise on the outputs, especially when the load is high, so you will probably have to use some filter capacitors.
Oh, and you absolutely need to pay attention to max output current because the output voltage from the MAX232 may drop if you overload it.
So yeah, go for it. I'd give it a shot.
fuzzyhair2
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