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DC DC converter problem (loading problem)

P

praveen

Jan 1, 1970
0
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

http://www.semtech.com/pdf/sc2442.pdf

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
 
F

Frank Bemelman

Jan 1, 1970
0
praveen said:
The DC DC converter which i am using is in the URL below

http://www.semtech.com/pdf/sc2442.pdf

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).

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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