output ripple at the lm2676

[sfiber]

hi my friends I want to ask a question about buck converters.I designed a circuit that give 7.3 volts as an output.(it should be 7.2 v) but there is a ripple at the output like this when I use 10 mh inductor.


actually I should use 22 uH for this circuit but I have'nt one until next week.To see whats going on with 5 mh I parallel connect to inductors.Then I gave this output signal like under.what do you think about this.Is this ripple,important for electronic circuits.



I will check the circuit with 22 uh inductor soon.

[madires]

Are you using a 10mH (milli Henry) inductor for the buck converter? Anyway, even with the correct inductor you'll get ripple. To reduce the ripple I'd suggest to add a LC filter at the output, something like 10 or 22 µH and a low ESR cap with a few hundred µF. I can't tell you if the ripple will cause any trouble without any knowledge about the circuit. If you like to get some feedback please tell us about your circuit.

[IanJ]

Board layout is important with switchers, especially the higher frequency ones.

http://www.ti.com/lit/an/snva021c/snva021c.pdf

Ian.

[moffy]

Does it have a dummy load? All SMPS need a minimum load to work correctly. It looks like the supply is bursting for several cycles then turnning off until the voltage falls below threshold hence the noise pattern.

[johansen]

the output esr and or esl of the output capacitor is excessive, you should not be seeing 500mv with those rise times, regardless if its operating in burst mode or not.

[T3sl4co1l]

10 milli henry?!  How do you convince that thing to not oscillate?!

Unless you have external compensation available, and are willing to follow through and optimize it, you are expressly limited to the inductor and capacitor values recommended by the manufacturer.

Tim

[sfiber]

yes I know my friends but as I said I havent one now,so I just put it to there.Then in this morning I check my circuit with two digital high torque servo and avg voltage dont fell down to 5 volts.it stayed at 7 volts and I think its efficient for me.I reply the circuit paralelly and connected them together with high watt diodes.But there is one little question to you.I recognized that;I couldnt use my servos if I did not connect my osc probes black hook to my reference point.

[madires]

Does it have a dummy load? All SMPS need a minimum load to work correctly. It looks like the supply is bursting for several cycles then turnning off until the voltage falls below threshold hence the noise pattern.

Where do you got that wisdom from? Some SMPS topologies need a minimal load but not a LM2676 buck converter. You could consider the voltage divider for the feedback a load but it's a few k.

[madires]

yes I know my friends but as I said I havent one now,so I just put it to there.Then in this morning I check my circuit with two digital high torque servo and avg voltage dont fell down to 5 volts.it stayed at 7 volts and I think its efficient for me.I reply the circuit paralelly and connected them together with high watt diodes.But there is one little question to you.I recognized that;I couldnt use my servos if I did not connect my osc probes black hook to my reference point.

Sound's like a ground issue. A circuit diagram would help. BTW, what kind of inductor are you using for the buck converter? Is it a iron power toroid (distributed air gap) or a ferrite toroid with a gap?