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-12v regulator problems (LM2576-12).

input is 24v DC. I'm using LM7812 to get the +12v and

formatting link

Figure 13 for softer start)

I have implemented is

get nice stable -12v in output. However, when putting 2k ohm

output and GND the LM2576 gets really hot and

also noticed that the -12v draws about 70mA from power supply when

load is 6mA (12v / 2kohm = 6mA).

You have 36V across the IC that way, use the conventional bipolar switcher: View in a fixed-width font such as Courier.

. . . .

. | | . | +-------------VIN-+ . unreg === | | . | | | . >----------+-----GND Buck PWM | . | | | . | | | . | +---FB--------SW--+ . | | | . | | | . | | | .

+---/\\/\\--+--/\\/\\----|-----\\ . | | | . | SD | | . +---|>|--------------+ | . | | | . SD | C | | . +------|>|-----|-------+------||----+ | . | | | | | . | | | | | . | | +-------+ | | . | | |o |o | . | | ) || ) | . | | ) || ) | . | | LB ) || ) LA | . | | ) || ) | . | | | | | . | +---------------+ | | . | | | | . | C | C | | . +------||------+--------||----------+-----/ . | | | . | | | . Vn GND Vp . (=-Vp) . . . . .
Reply to
Fred Bloggs
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Reply to
Spehro Pefhany

Don't give up. I have success with this same circuit but with the lower grunt LM2575. Once it goes you will have plenty of minus rail current available and you can get on with your project. I'd disconnect the delayed start circuit until I got it going - to cut down the number of variables.

The diode must be a real shottky, not an ordinary rectifier type.

That just leaves the inductor. I use a powdered iron toroid type which won't easily saturate. It will either be powdered iron or a gapped ferrite. If the inductor saturates, you get wierd behaviour and often smoke the IC.

Actually, the LM2575 will be less demanding of your inductor and still give plenty of drive - you are using a very big beast.

Your oscilloscope can tell you what is going on - the datasheet discusses principles and expected waveforms.

Roger Lascelles

Reply to
Roger Lascelles

You can "lift" the ground by feeding 12V+0..7V diode drop to the base of a pass transistor configured as a common-collector amplifier. Use the emitter as the new ground and reference everything to this +12V ground. That way, you will have +24-12=+12V and 0-12V=-12V.

Let me know if you need a schematic.

Best, Scotty

Reply to
anonymous

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