LM25576 problem

[Diver300]

I am trying to use an LM25576 and I am having a problem. It is designed for 3.8V at 2A, using a 10 :mu:H inductore

At light loads, it oscillates at about 5 - 10 Hz. The output voltage rises very fast, limited by the output capacitors, up to about the correct voltage. Then it seems to shut down and the output voltage decays down to about 3V, which takes 100 - 200 ms, and the whole lot starts again.

I can't see what would stop the IC from working for that long.

I have had this problem with the evaluation kit, as well as with the prototype that I have designed.

Any ideas?

 

[kchriste]

Do you you mean the LM2576? Are you satisfying the minimum load requirement of the regulator circuit? Most switching regulators will be unstable if they do not have a minimum load resistor on their output while output has no load.

 

[Hero999]

I can't help you I'm afraid.

Have you spoce to the manufacturer about the problem?

As it happened with the evaluation kit, it sound like it isn't your fault.

 

[Diver300]

Thank you for the replies.

It is the LM25576. I can't see anything in the data sheet about minimum loads.

I queried the problem with National but I haven't had a reply.

 

[aussiepoof]

I am trying to use an LM25576 and I am having a problem. It is designed for 3.8V at 2A, using a 10 :mu:H inductor

What's the input voltage? I just did a quick Webench design and it suggests 12uH which is close. What's your output capacitance? Are you using low ESR caps?

At light loads, it oscillates at about 5 - 10 Hz. The output voltage rises very fast, limited by the output capacitors, up to about the correct voltage. Then it seems to shut down and the output voltage decays down to about 3V, which takes 100 - 200 ms, and the whole lot starts again.

Does it work fine at higher loads? There's no need for a load resistor as far as I can see, but you should make sure the feedback resistors are the right values!

Happy to help
P.

 

[eng1]

I am trying to use an LM25576 and I am having a problem. It is designed for 3.8V at 2A, using a 10 :mu:H inductore

At light loads, it oscillates at about 5 - 10 Hz. The output voltage rises very fast, limited by the output capacitors, up to about the correct voltage. Then it seems to shut down and the output voltage decays down to about 3V, which takes 100 - 200 ms, and the whole lot starts again.

I can't see what would stop the IC from working for that long.

I have had this problem with the evaluation kit, as well as with the prototype that I have designed.

Any ideas?

I don't know this IC, but most app notes I've read recommend running your converter in conitinuous mode. At light loads the energy is transferred to the load in a small fraction of the commutation period and the converter works in discontinuous mode. The output voltage depends on a number of variables: Vin, duty cycle + inductor value, commutation period, load. This might be the problem?

 

[Diver300]

Thank you for the answers.

The input voltage is 12 V. The output is 3 x 100 :mu:F tanatalum capacitors.

I can't run in continuous mode because the output power is too low. I need the peak current capability and lowish supply current most of the time.

However the problem is not that I a running in burst mode. Burst mode is what I expected. The problem is that the converter shuts off for 100ms or more. The output dips to 2.8 V or so each time.

In burst mode there is a single pulse from the LM25576, after which the current in the inductor decays to zero. This takes a few :mu:s. The output voltage is kicked up by this by a few mV at most. It only takes a few ms for the output voltage to decay these few mV, and the process starts again.

That is normal, and what I want to happen. It is oscillation at about 1 kHz

What I am getting is continuous mode while the circuit charges its output capacitors up to 3.8 V. That takes less than 1ms. Then a big pause of 100 ms while the output decays by about a volt, and this process repeats.

That is not normal. The voltage is dipping way too low. It is oscillation at about 5 - 10 Hz.

I don't understand why the LM25576 doesn't kick in during this time. The feedback voltage is low for ages, and the response time has to be in :mu:s for it to work at all.

Any ideas?

I may just go and find the board space for the LM2676 and its 22 :mu:H inductor.