simple negative rail gen.

[Adam. Seychell]

I'm looking for a cheap simple method to get unregulated -5V 30mA from
+5V supply, but output ripple must be < 10mV. I've read about a using a
standard inductor based DC/DC stepdown and connecting the supply ground
to the regulator's output. There devices like the LM2664 charge-pump
voltage inverter, but switched capacitors are noisy (large ripple
current in output capacitor). Is it possible to replace the capacitor
with an inductor in one of these switched capacitor converters ?

Adam

 

[Tim Wescott]

John Popelish wrote:

"Adam. Seychell" wrote:

I'm looking for a cheap simple method to get unregulated -5V 30mA from
+5V supply, but output ripple must be < 10mV. I've read about a using a
standard inductor based DC/DC stepdown and connecting the supply ground
to the regulator's output. There devices like the LM2664 charge-pump
voltage inverter, but switched capacitors are noisy (large ripple
current in output capacitor). Is it possible to replace the capacitor
with an inductor in one of these switched capacitor converters ?

Adam


Switched capacitor (charge pump) supplies are never very efficient,
just very simple. I think you should look at an inverting supply
based on a flyback inductor and something like the LM2594.
http://cache.national.com/ds/LM/LM2594.pdf
It can supply up to 200 mA of -5 volt with a 5 volt input.
See figure 25.
You may need a second stage of LC filter to get the ripple down to 10
mV.

-- or just use an LC (or ever RC) filter following a switched cap filter.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Adam. Seychell]

John Popelish wrote:

Switched capacitor (charge pump) supplies are never very efficient,
just very simple. I think you should look at an inverting supply
based on a flyback inductor and something like the LM2594.
http://cache.national.com/ds/LM/LM2594.pdf
It can supply up to 200 mA of -5 volt with a 5 volt input.
See figure 25.
You may need a second stage of LC filter to get the ripple down to 10
mV.

Thanks, I checked out Fig 25 in the LM2594 sheet. It looks like it'll
fit the bill. But then I found the newer LM2734, which is much faster,
smaller and doesn't cost any more. It seems manufactures of power supply
IC are superseding their previous products at such a rapid rate that it
has resulted in a obscene number of devices existing today.

Adam