buck converter help

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tmbg

Well-known member
Joined
Nov 7, 2004
Messages
438
Location
Atlanta, GA
Hi folks,

I designed a switching supply, a 12V -> 5V buck converter, first one I've ever done.

It's based around the MAX5033, http://pdfserv.maxim-ic.com/en/ds/MAX5033.pdf , and I pretty much used the circuit out of their notes verbatim (the one for the adjustable supply, on page 9), 47uF input cap, 33uF output cap, 220uH inductor, 30V 1A schottky diode, 41k2/13k3 thin film feedback bias network to set the output voltage.

Problem is, I'm not getting 5V out, I'm getting a 5V-8V sawtooth wave.

What exactly is going wrong here?
 
The tricky stuff with converters tends to be the inductors

Did you use the Coilcraft one DO5022P-224?

With this type circuit almost every part makes a difference. Did you note all the parts are speced in the PDF? When ever I see company and part number speced on a app sheet I will try to use the parts speced.

Sometimes trying to fix older dc to dc converter power supplys you need to use parts just like the original parts electros are a pain because of inductace and ESR and DF and........ I believe I read the newer IC based converters are designed to allow different caps etc.
 
Are you using the D version of the IC? the A/B/C are fixed voltages at 3.3/5/12 and the D version is adjustable. Just grab the B version and forget the setting resistors!
 
the other things is making sure you are using a ground plane and prototyping on a PCB, not a breadboard. many many demons come out of the woodwork with switchers if the layout is wrong.

I'm working on a buck offline to 24vdc.. non isolated and high current (1-2 amps) talk about a headache.
 
I used a Bourns inductor.

It's on a pcb, not on a breadboard, and yes there's a ground plane, and the other side has a regulated power plane.
 
Yes, I have the D ic on hand, and I have B samples coming.

I designed the PCB in such a way that I could use the FB network with the D to get 5V, or just sub in a 0ohm and an empty spot for the B version.
 
ON/OFF isn't connected at all, so I guess that'd be tracking internally.

I did try connecting it to ground, and it does shut off, and I connected it to the 12V input source, and that had no change.
 
you know, I think I know what I did wrong.

I think I took the feedback voltage from LX instead of from post-inductor voltage. DOH.
 
yeah that's exactly what it was. Had to cut a trace and run a 'wire of shame', and now it's a dead flat 5.00v :D

Thanks for the help fellas
 
[quote author="tmbg"]yeah that's exactly what it was. Had to cut a trace and run a 'wire of shame', and now it's a dead flat 5.00v :D

Thanks for the help fellas[/quote]

Heh. Wire of Shame indeed!

I had been about to weigh in with a comment to the effect that if you were getting a big sawtooth it was probably more than just a component variance issue :wink:
 

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