Hello,
Is there a trick that I can use to generate a negative supply from a LM7805 (positive +5V voltage regulator)? I'd like to avoid the noise of the switched cap and SMPS solutions. I only have a +9V wall-wart power line, so I can't use a LM7905 negatve voltage regulator.
I noticed the following:
http://www.schmartboard.com/index.asp?page=products_populated&id=198
They suggest getting a negative voltage by reversing output and ground.
Any better ideas?
Thanks,
Alex
Hello Alex,
The wall wart is an isolated supply that outputs 9V. You can use it as either a +9V or -9V supply by reversing the output leads.
Once you go through a LM7805 (for +9V) or LM7905 (for -9V) regulator you will have 5V potential.
You can use either as +5V or -5V by reversing the leads.
The only catch is that you can only "ground" one point as your circuit reference. So you can't get +5V and -5V at the same time.
Regards,Ron M.
Regards,Ronald MichallickLinear Applications
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Ron,
Thanks for the quick reply. I figured this would be the case.
The reason I ask is because I need to power six OPA365's with dual supplies. I have a +5V 100mA source.
The OPA 365 quotes its quiscent current at 5mA(max). My assumption is that the current is drawn from *both* the positive and negative supply. With six amplifiers, I would need 30 mA of current to be sourcable from the negative supply. I have been using the TL7660 switched cap voltage inverter. The issue is that it can't source anywhere near those kinds of currents.
My application is instrumentation, so noise is a big issue. I am debating trying another switched cap voltage inverter with more current supply capabilities, or potentially an inverting SMPS.
Do you have any suggestions regarding this?
Hi Alex,
The OPA365 wouldn't be able to run at +/-5V - the best you could do is +/-2.5V. The current would remain 5mA max. At the expense of wasting power, you could turn your 9V wall wort into say +/-4.5V supply by using a resistor divider and calling the center point GND. With the proper resistors, you could do +7/-2 and still use the LM7905 to get a clean 5V rail.
Regards,
Tom
Tom,
Thanks for the info. My power is coming from USB. I do have another IC on the board that produces +3.3V. My latest thoughts are to take that and an inverting SMPS to generate +/- 3.3V supplies. The issue is finding a SMPS that can generate -3.3V at a low level of ripple.
Is there an equivalent OpAmp to the OPA365 that could do a +/- 3.3V supply (and work well with the above power configuration)? I selected the OPA365 for its ability to have two amplifiers in one package, cost range, and very-low offset (my application requires precise DC gain).
Hi Alex,If your bus-voltage is 5V-USB:for generating the negative voltage rails to your current requirements you could use either the charge pump TPS6040x family, or the DC-DC Inverter TPS63700. In addition to get a very clean DC supply I would recommend using LDOs with high PSRR over a wide frequency range. For the positive rail I would recommend TPS791xx family if price matters take TLV700xx for highest performance you could go for TPS7A49xx. For the negative rail I would recommend TPS723xx, (for highest performance) TPS7A30xx.
If your bus-voltage is 9V:you could derive your negative rail from your 3.3V.
Be aware OPA365 could not do +/- 3.3V supply. You could do as Tom already stated +/-2.5V with this device. In your case probably +3.3V / -1.7V. (asymetric supply possible)
For a proper alternative proposal we should know if the total rail of max 5V (V+)-(V-) fits to your requirements. What is your required bandwidth.
Best regards,Carsten Thiele
Carsten,
Thanks for the ideas. I'm thinking of the OPA727 as a replacement. It should be able to handle +/- 5V. It can also handle the 1MHz bandwidth requirement easily.
I plan on using the USB Vcc as the direct +5V supply. I was considering then using the TPS54060 SMPS in an inverting configuration to provide the -5V Vee. How does this sound to you?
I ended up using a power supply.