negative power supply

[bmandl]

Hello!
I am designing **broken link removed** and I noticed one problem in circuit. I already mailed author, but I didn't get answer. My thoughts was, how it is possible to generate -9V from +9V with MAX660 to be than regulated to -5V with 7905 voltage regulator? MAX660 can only generate -5.5 V from +5.5 V and nothing more. I've been searching but I didn't find any sufficient IC with output current able to go 100mA and can generate at least -9 V. Is there an option to connect two MAX660 in series? What are side effects of this solution?

 

[FoxyRick]

That does look wrong.

I guess the MAX660 is operating a well above its maximum rating, but it might not function for long like that.

A quick fix would be to jumper the V+ for the MAX660 to the output of the +5V regulator instead of the 9V, and leave the 7905 out altogether. The -5V rail is only powering the op-amp and the AC output. Perhaps the 7805 would need uprating from a 78L05 then, to provide a little extra current (since it is now seeing its own current and the -5V rails), perhaps not, depending how much current you want from the AC output.

 

[bmandl]

Yes, but If I generate -5V with MAX660, then this voltage will not be regulated and it will cause troubles in high frequencies. I need really fixed -5V and for that, I need -7 or higher negative voltage to be than regulated with 7905. Isn't really there any other solution?

 

[FoxyRick]

You're going to have digital noise from the MCU logic anyway, but on page 8 of the pdf it shows how to cascade the MAX660s to increase the voltage.

**broken link removed**

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Alternatively, just use a higher voltage device:

The ICL7660S converts a +1.5 to +12V into an equal magnitude negative output. It only needs two capacitors for a basic circuit:

**broken link removed**

The LT1054 is similar and gives a bit more current:

https://www.linear.com/product/LT1054

 

[schmitt trigger]

I was going to suggest the ICL7660...it is higher voltage, and since it is an older component, it is widely available at low cost.

You can even do the same with a 555, just google "555 negative charge pump" and you get a ton of hits, one such circuit is below:

**broken link removed**

 

[bmandl]

@FoxyRick thank you for this recommendation. I couldn't find any proper chip, but seems like LT1054 will do the job. Thank you again.