Regulators failing

**Diver300** · 29th March 2009 08:28 PM

I have an LM25576 regulating approximately 12 V down to 4 V. The LM26676 is a buck switching regulator and in my power supply it is rated to 2A. It has a soft start capacitor so that the output voltage comes up fairly slowly.

From that I am feeding a ZXCL300 to give me 3 V at a few mA.

The problem is that as I turn on the 12 V, the ZXCL300 sometimes goes up in smoke.

I've looked at the 4 V line with a scope and there don't seem to be any spikes, but I am not entirely sure. When I switch on the 12 V with a transistor there isn't a spike and I can just see the ramp controlled by the LM25576. There is a 100 uF capacitor on the output of the buck regulator, and a couple of ceramics as well.

Any ideas?

**Sceadwian** · 29th March 2009 08:40 PM

We can't help you with anything unless you post the entire schematic of your circuit.

**colin55** · 29th March 2009 09:19 PM

Why don't you put a couple of diodes to drop the voltage. The chip you are using is obviously a very delicate device.

**Mikebits** · 29th March 2009 09:27 PM

What is being powered off of the ZXL part? Maybe something is latching up.

**Sceadwian** · 29th March 2009 09:34 PM

Or the SMPS isn't set up correctly and it's getting straight 12 volts for some fraction of a cycle. There's any number of a hundred things that could be going wrong and we could guess until the cows came home but we can't offer any advice without a schematic. There should be a lock on all threads like this without schematics because they drag on for 3-5 pages and usually then the poster comes out with some random bit of information that if inclded in the original post would have led to an immediate answer. But they get bogged down in what ifs and people aruging about symantics from a circuit that isn't even fully defined.

**Sceadwian** · 29th March 2009 09:35 PM

See. The what ifs are already starting =)

**tcmtech** · 29th March 2009 11:14 PM

You mean like, What if something, something, something, something, 555 timer IC, something, something, something, something, Microcontroller, something, something, something, something, SMPS, something, something, something, something, super ultra multistage battery charger,something, something, something, something, line harmoncs.

Is that what you are saying?

**Diver300** · 30th March 2009 10:23 AM

Here is the schematic. I should have posted it when I started the thread.

I am running a PIC from the 3V line, and using the 3V as the ADC source, so I want a well-regulated voltage.

I don't think that the PIC is taking excessive current. I've never blown one of them. The ZXCL300 should have thermal shutdown to protect it against too much current.

**sPuDd** · 30th March 2009 10:52 AM

Why such a complex buck psu when you could just use an 7805 for 5v rail, and an LM317LZ for the other 4V and 3V rails. Assuming the current draw is less than 100mA per reg.

sPuDd..

**colin55** · 30th March 2009 10:52 AM

These devices get damaged much faster with excess voltage, rather than excess current. Try putting a large inductor on the input line to the regulator and put an electro both ends of the inductor.

**Diver300** · 30th March 2009 11:23 AM

Originally Posted by sPuDd

Why such a complex buck psu when you could just use an 7805 for 5v rail, and an LM317LZ for the other 4V and 3V rails. Assuming the current draw is less than 100mA per reg.

sPuDd..

2 Amp peaks on the 4 V rails.

1/2 amp continuous on the 5 V rail.

It can run from 24 V. With a linear regulator I would dissipate 10 W continuous. At the moment, I don't have a heat sink.

I like Colin55's idea of an inductor, even if I never find the real cause.

**electronica85** · 30th March 2009 11:35 AM

Hi, when you see the smoke, is the IC ZXCL300 loaded or not? Maybe you are over loading it.

**Boncuk** · 30th March 2009 11:50 AM

The first thing I'd try is using a 100n capacitor at the ZXCL300 input and output pins.

If it oscillates at a very high frequency, which you haven't checked yet it fries to death.

Boncuk

**mvs sarma** · 30th March 2009 11:54 AM

Originally Posted by Diver300

Here is the schematic. I should have posted it when I started the thread.

I am running a PIC from the 3V line, and using the 3V as the ADC source, so I want a well-regulated voltage.

I don't think that the PIC is taking excessive current. I've never blown one of them. The ZXCL300 should have thermal shutdown to protect it against too much current.

Put one 0,1uF cap across input and Gnd of the ZXCL300. Instead, you may also use a conventional TL431 and few components around to get 3.3V or even 3.0V

**Boncuk** · 30th March 2009 12:02 PM

Looking at the Zetex data sheet and the current boost circuit with the ZXCL300 I see a 1µF capacitor each at the input and output pins.

(I guess they're not there for fun. Zetex obviously knows about the oscillation tendency of that chip.)

Using two additional 100nF capacitors won't hurt or be astronomically expensive but keep the regulator alife.

Boncuk

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