MAX3232 power supply [solved]

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Ok so I've used the MAX3232 for a lot of my projects and it works great but I've always used it at 3.3V. However I made a new board where I want it to run on either 3.3 or 5. The datasheet has the following table for determining the cap values.

Vcc..........C1..........C2,C3,C4
3.3V..........0.1uF..........0.1uF
5V...........0.047uF........0.33uF
3V-5.5V.......0.1uF.........0.47uF

I used the recommended values for 3V-5.5 and its not working right.

If Vcc drops below 3.2V it stop working.
if Vcc goes above 4.7V it starts spitting out gibberish, but only if the baud rate is 2400 or higher. It works fine at 5V at 1200 baud and lower. I am testing by just using a loop back wire.

Was thinking of trying some of the other cap values but I don't have any 0.047uF or 0.33uF caps.

Anyone have any suggestions?

Last Edited: Wed. Apr 27, 2011 - 04:32 PM
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Quote:
If Vcc drops below 3.2V it stop working.
if Vcc goes above 4.7V it starts spitting out gibberish, but only if the baud rate is 2400 or higher. It works fine at 5V at 1200 baud and lower. I am testing by just using a loop back wire.

But this values are out off specifications isn't it?
How it works on 3,3 and 5V?

Regards,

Bruno Muswieck

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You might want to turn things round. Most people who've tried it have found that the MAX232 (not MAX3232) works just fine on 3.3V so you could use that device and it would simply work on either 5V or 3.3V

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@brunomusw
No it's not out of spec. It should run anywhere between 3V and 5.5V with those cap values and my tested range is more like 3.2 - 4.7.

@clawson
Haven't heard of that, is is still with the 1uF caps? The circuit I'm trying to get working is on a couple PCBs right now. So maybe in the future I'll try the max232 but I'd like to get this one working right now. When I get to lab I'll measure the charge pump voltages and see what they're doing.

Any idea why the baud rate would matter?

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Lower baud rates works probably because the charge pump ckt from the max works ok, but on higher baud rates, it doesn't have enough time to produce the necessary voltage levels... Did you try an oscilloscope on the C1 and the others? And on the drives lines? How is the drives lines outputs?

Regards,

Bruno Muswieck

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Alright so it took a couple days but I measured the values on the charge pump and I can't really see anything wrong. Maybe someone can point out?

Vcc.......3.3V...........4.0V............5.0V
C1+.......3.39V..........4.02V...........5.72V
C1-.......0.13V..........0.07V...........0.05V
C2+.......0.24V..........0.12V...........0.08V
C2-.......-5.12V........-5.34V...........-5.51V
V+........5.57V..........5.86V...........6.22V
V-.......-5.35V.........-5.46V...........-5.61V
T1out.....5.49V..........5.79V...........6.17V
T2out....-5.29V.........-5.40V...........-5.54V

Once again it works great at 3.3V and 4V but not 5. Any ideas?

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Ok so I figured it out. I had some activity LEDs on the data lines that had current limiting resistors I had calculated for 3.3V operation. I upped the resistor value and now it works perfect.