MAX232 overheating

njepsen · Joined: 13 Aug 2007 Posts: 469

I use 3 Max 202CSE chips on my boards to interface to various external RS232 devices, and for some time have been plagued with intermittent overheating of one MAX device that connects to my debug port. There are various comments of similar problems on other forums, but no real conclusions.
My circuit is correct and I have made about 100 of these boards, all of which have been successful (for 5 years) up until now.
I have changed my source of the MX202CWE devices - theres a clue.

Anyway - I have found that if I have my laptop with a USB/Ser convertor plugged into my board BEFORE i power up the board, the Max device will latch and burn your fingers. On the other hand, if I plug the USB/ser header in AFTER I power on the PCB, the Max device runs perfectly. I have repeated this test about 10 times, with same result.

I use 0.1uF caps on the charge pumps. Changing these to 1uF polarized made no difference. The 0v of my PCB is connected through the ribbon cable to pin 5 of the USB/ser convertor, so its not a common earth problem. Also - if I run the laptop on battery only, with nothing else plugged into it - that doesnt help.
A poster "Prodigy" on this forum also found the same scenario.

albertsm · Joined: 09 Apr 2004 Posts: 5921 Location: Holland

and how is the housing of the connector connected?
I mean the metal frame.

you can try another brand too.

you are 100 sure you connected the max the right way?

njepsen · Joined: 13 Aug 2007 Posts: 469

hi mark,
100% sure, but i will post the cct. i have not changed brandds but will do that. they are smd and i have some on order from RS.
The debug cable is a ribbon cable , with a 10pin box header so no metal shroud. all spare wires are to 0v.
ive done some chevking and when the atmega board is off, and the laptop running and plugged in, there is -6v dc from the usb/rs232 converter appeating on the rs242 receiver input of the max, and i suspect this is the culprit, but no proog. a 1k to ground on both rx and tx of the max made no fifference.
thanks for taking an interest Mark. Appreciated.
neil

albertsm · Joined: 09 Apr 2004 Posts: 5921 Location: Holland

you might want to try a different usb/ser converter too.
and also: try a normal serial port to see what happens.

when you have multiple devices connected to each other and they all have a different power, there is a sequence of how to turn them on. in most cases it does not matter.
in your case it appears the TX input can not handle the signal when the chip has no power. if this was never a problem before i would change back to the old chip. or a different brand.
a protection in the input might also work.