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Topic: Problem with MAX756 Boost converter (Read 663 times) previous topic - next topic

zetaInvestigator

Jul 06, 2017, 03:29 pm Last Edit: Jul 06, 2017, 05:18 pm by zetaInvestigator
I'm using MAX756 as a DC-DC boost converter to provide 3.3V from of a single 1.5 AA battery. I've implemented the circuit on page 1 of datasheet with 1N5819 instead of 1N5817 and 144uF caps between Vin and Gnd instead of 150uF. Also Vin comes from bench power supply. 

I've tested three 22uH inductors with different current capabilities (all above the required 1.2A due to datasheet (page 6).   
 
~SHDN and 3/~5 is connected to out. LBO is left open.   
The problem is the Vout is 3.3V in no-load. But as soon as I insert the n * 220 Ohm resistors in parallel, Vout drops and in Iout = 60mA, the Vout drops below 2V. 

Here is the picture of the setup: lower rail in Vin and upper rail is Vout


I've shown the other two inductors that I tested. 
Anyone knows where the problem might be?
Thanks in advance.
P.S. I've also posted this question here

jremington

#1
Jul 06, 2017, 05:02 pm Last Edit: Jul 06, 2017, 05:04 pm by jremington
Quote
But as soon as I insert the 220Ohm resistors, Vout drops and in Iout = 60mA, the Vout drops below 2V. 
The above is very confusing and possibly wrong (220 Ohm?). Your StackExchange post explains this much better.

There are several possibilities to explain the problem, but I would get rid of the breadboard for starters. They are limited in current handling capabilities, and generally terrible for critical circuitry like this. I use wires and solder all connections for initial testing of circuits like this.

zetaInvestigator

#2
Jul 06, 2017, 05:20 pm Last Edit: Jul 06, 2017, 05:26 pm by zetaInvestigator
@jremington Fixed it. So you are suggesting perfboard and soldier? Can I use Dip-8 socket for IC then, because this IC is rather expensive here? Or will I face this inductive-bounce phenomena even there?

jremington


MarkT

Basically at high switching rates (this chip does upto 0.5MHz), stray inductance is very important
(basically the higher the current,the higher the frequency and the lower the voltages, the greater the problem.)

Reducing stray inductance is all about tight layout with short wide leads, loops as small as possible.
PCB with a ground-plane is good for this as all the current paths can run over/beside their ground
returns.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

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