Re: max668 problems

On Thu, 22 Jun 2006 11:11:30 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@xxxxxxxxxxxxxxx> wrote:

On Thu, 22 Jun 2006 18:57:22 +0100, Malcolm Reeves
<mreeves@xxxxxxxxxxxxxxx> wrote:

Hi,

Anyone know anything about this part. I have a design with 2 of
these. The first is a standard 5V boost, 0.3A. That seems to work
fine. The second is 10W flyback giving +/-70V on the output. This
uses a transformer. I'm blowing up max668s like they are going out of
fashion :-(. One board now has damaged tracks so I've moved on to
board 2. On this board I made sure the 70V psu was working before
fitting the CPU etc. It was working fine. I loaded the supply and
took measurements. But first time I tried the supply after fitting
the CPU it has blown up.

The odd thing about the way it fails is that the feedback pin takes
too much current. The current "blown" part takes about 2mA into the
feedback pin. I've taken out the external mosfet and put a 10K to the
feedback node to a bench supply. The max668 output is running at 80%
as expected. When I increase the bench supply the feedback pin
voltage increases until it hits about 1.25V (Vref) then the output
drops to 0%. All as expected, except that I'm pushing ~2mA in and the
FB resistors are 18K and 1M and the feedback pin should only need
20nA.

It is the high current that causes the problems. In effect the
feedback fails, the transformer is driven flat out and the mosfet
blows up.

I don't understand what is causing this failure and it is odd that the
max668 works, except for the feedback pin current. The only thing
different from when I had it working is the addition of the CPU. The
on/off pin is now driven hard from a CPU port rather than a 100k pull
down and 10K link to 5V. Could a port blow a max668?

TIA

Malcolm


Is the CPU "chattering" the ON/OFF pin?

Have you tried 100K between CPU and ON/OFF pin?

...Jim Thompson

The CPU shouldn't be "chattering" the ON/OFF as I'm running with an
emulator and switching it by hand at the moment. Plus, the shutdown
pin is also sync so shouldn't mind chattering.

To answer roger

What compensation capacitor are you using on the FB pin?. If this is not
right, the circuit could oscillate beautifully..

Currently 1nF+18k is the feedback but it was working before I added
the CPU.

I've added a 33k in series when I replaced the blown part. My theory
is that perhaps the GND at the MAX668 is lifting due to high currents
and that means CPU 0V is below MAX668 0V so shutdown is taken negative
and that blows the chip.

I've just replaced the blown part and check for shorts/continuity with
a DVM so make sure it is all soldered right. I've left the mosfet out
and use a bench supply via a 10K to feedback node to check the part is
working before any big currents fly about. As expected enable it and
output is 80% odd pulse train. Raise FB to Vref and it is a 0% pulse
train. BUT, FB pin is taking 1-2mA and I haven't run this part in
anger yet. No mosfet, no large currents. I work on a static mat area
and the DVM I used to check shorts/continuity measures 0.4V, 0.6uA on
another meter so why is this part broken?

Very odd.



--

Malcolm

Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
(mreeves@xxxxxxxxxxxxxxx, mreeves@xxxxxxxxxxxxxxxxx or mreeves@xxxxxxx).
Design Service for Analogue/Digital H/W & S/W Railway Signalling and Power
electronics. More details plus freeware, Win95/98 DUN and Pspice tips, see:

http://www.fullcircuit.com or http://www.fullcircuit.co.uk

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