DC DC converter problem (loading problem)

what is your working plane? a breadboard or a pcb? if you work on a breadboard it may cause the problems like yours. try it on a pcb using adequate track widths?

And can you call Semtech? Perhaps they have an engineer that can help you directly.

-M

I would be suspicious that ay no load, the converter error amplifier is saturating when the output voltage overshoots, and outputs are then locked off, entirely for brief periods, so that the output voltage you measure is the average of the operating and shut down periods. When you apply a slight load, the converter operates continuously, and you get to see what the actual operating setpoint voltage is.

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John Popelish

Hi, Praveen -

When you say no load, how much is no load? If you really, really mean no load, and the converter is a buck with series switch, you may have enough leakage through the switch or other parts of the circuit to charge up the output capacitor. It is also possible that there are very narrow switching spikes charging the output cap. Or, maybe the switcher can't go to low enough duty factor at no load. If any of these is the case, simply add a light (10 mA) load to the output.

Good luck.

John

Hello,

I have designed a power supply for 3.3V/5V from 30V using the Semtech DC DC converter(SC2442H).I have taken the reference schematics given from semtech. The problem is the under no load condition i am getting required 3.3V and 5V voltage but when i start loading the voltage starts to decrease. I have designed the pwer supply for 3.3V/3A and

5V/2A. Decrease is very Drastic for example 5V become 4.5V for just 10mA load. Can someone point out how can i debug it. Any suggestion will be very much appreciated.I need help from you people.Where am i going wrong?

what is your working plane? a breadboard or a pcb? if you work on a breadboard it may cause the problems like yours. try it on a pcb using adequate track widths?

-----working plane is on PCB

When you say no load, how much is no load? If you really, really mean no load, and the converter is a buck with series switch, you may have enough leakage through the switch or other parts of the circuit to charge up the output capacitor. It is also possible that there are very narrow switching spikes charging the output cap. Or, maybe the switcher can't go to low enough duty factor at no load. If any of these is the case, simply add a light (10 mA) load to the output.

------I did apply a load of 10 ohms on the 3.3V, the result is that voltage has dropped to low voltage i.e 0.8V.

I really cannot figure the problem...

And can you call Semtech? Perhaps they have an engineer that can help you directly.

--- I contacted the semtech...the semtech application person did come here but he could not figure why it is behaviour like this...

The DC DC converter which i am using is in the URL below

How do i debug this??? i have checked everything....

The other observation is that the Upper FET of 3.3V supply switches (but the on period is narrow) but the lower FET doesnot switch (low).

Thanks and waiting for reply Praveen

"praveen" schreef in bericht news: snipped-for-privacy@posting.google.com...

How much is the feedback voltage for FB1 (pin 6) and FB2 (pin 19), at

10mA load and at 1A load? It should be close to 0.75 volt. What resistor values did you use for the feedback divider, R13 & R15, R14 & R16?

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Thanks, Frank.
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