Inverting regulator (-ve rail from +ve supply) - current requirements?

[peskywinnets]

Rather than go the single supply/ 'virtual' ground way of things, I'm making up a little circuit with a TL072 preamp & I'd like to go with dual rails. Because of space constraints, I only want to feed the overall circuit wiith 9V ...from this I will regulate down to +5v. So I need an inverting regulator....which will give me -5V from that

A quick check & it seems all the low priced inverting regulators have somewhat low current delivery ability (10-15mA)...so to my line of questioning.

How much current do you think the negative rail of a typical dual supply TL072 preampifier amplifier will pull? (using non-inverting ->non inverting stages)

10mA, 15mA, more?

 

[BobEllis]

The data sheet is your friend - http://focus.ti.com/lit/ds/symlink/tl072.pdf look for Icc on page 8, you'll see that 10 mA supply is plenty.

What may not be plenty is your rail voltage - take a look at the output swing @15V rails, it looks like you'll only be able to swing 2V on 5V rails (or less if your load is under 10K). Is that enough for your application? Not a lot of headroom. Higher rails or a rail to rail op amp may be more appropriate.

 

[boyofwax]

The TL082 in my preamp draws 3.6 mA @ 2x15v.

 

[wakibaki]

EXAR|SP6661EN-L|CHARGE PUMP INVERTER 200MA, 6661 | Farnell United Kingdom

This'll give you a -5V rail @ 200mA

And here's a -9V one:

http://www.geofex.com/circuits/+9_to_-9.htm

w

 

[peskywinnets]

thanks guys - I suspected it was in single digits of mA draw, but it's nice to have it reaffirmed.

Wakibike, I'd rather keep the cost as low as possible (I might need a few), and by proxy that means using one with the least amount of current 'supply' I can get away with....I'm actually eyeing this one...

MICROCHIP|TCM829ECT713|CHARGE PUMP, CONV, SOT23A-5 | Farnell United Kingdom

I bit smaller than I like to work with, but not impossibly so.

 

[macboy]

I have to ask, if you are using a 9V battery, and want +/- 5 V rails, why not just create a virtualy ground at 4.5 V, giving you +/- 4.5 V rails? You can use an opamp voltage follower to create a very low impedance virtual ground will very little wasted power (compared to just using a voltage divider).

 

[peskywinnets]

Yes, I know. ..hammer to crack a nut - never built a dual rail & just fancied having a dabble - just educating myself & experimenting really.

 

[macboy]

Yes, I know. ..hammer to crack a nut - never built a dual rail & just fancied having a dabble - just educating myself & experimenting really.

That's actually a perfectly good reason.

 

[jackinnj]

You can use any number of SMPS chips to derive an inverted voltage -- I used the National Semi "Simple Switchers" followed by an LM337L, (allowing for the overhead of the linear regulator). Pete Millet also used a Murata DC-DC converter in his sound-card adapter -- but make sure to put a choke on the input or you'll have switching noise all over your system.

 

[peskywinnets]

While I'm here...with LDO (pwm type) regulators...in a 1/2 rail virtual earth arrangement they seem a little noisy - are there any tips here to tame their noise a little?

Also, you say to put a choke on the input to the regulator? (I would have expected output). What size choke are we talking (value wise) for a low current draw setup.

 

[jackinnj]

While I'm here...with LDO (pwm type) regulators...in a 1/2 rail virtual earth arrangement they seem a little noisy - are there any tips here to tame their noise a little?

Also, you say to put a choke on the input to the regulator? (I would have expected output). What size choke are we talking (value wise) for a low current draw setup.

For low noise switching regulators the shape of the pulse is smoothed off -- so you have less efficiency -- but far less noise. Linear Tech has several and the datasheets tell you how to calculate the energy lost.

Yupp, you need a choke on the input of the Murata DC-DC converters. Only some microHenries. Same reason you need EMI protection on the input of any switch-mode power supply.

The choke which is attached to the switching node of the SMPS chip momentarily stores energy for boost, buck or invert, depending on how it's oriented. Sanjaya Maniktala at National Semi has a good power point presentation on SMPS magnetics.