Unstable Switch mode supply

[J. Justesen]

Hi.

I have used the mic2183 chip as a switch mode power supply.
I runs in stable mode until 12.15V, but suddently goes unstable at
12.2V and above. I have configured it for 3.3V with the voltage
divider resistors as described in the datasheet.
I have tryed to use a 1R resistor as current sense resistor, but this
makes no difference.
Further the threshold where it goes unstable(12.2V) is NOT current
dependent...

Can anyone please help me?

Thanks.

J. Justesen

 

[Kevin Kilzer]

On 17 Aug 2004 00:15:26 -0700, jakob.j@get2net.dk (J. Justesen) wrote:

I have used the mic2183 chip as a switch mode power supply.
I runs in stable mode until 12.15V, but suddently goes unstable at
12.2V and above. I have configured it for 3.3V with the voltage
divider resistors as described in the datasheet.
I have tryed to use a 1R resistor as current sense resistor, but this
makes no difference.
Further the threshold where it goes unstable(12.2V) is NOT current
dependent...

I assume you mean the 12V is the input and 3.3V is the output?

Anyway, instability is always caused by a loop-gain that is too high
(yeah, I know phase is involved, but that's harder to explain). So,
you need to reduce the loop-gain.

You would do this by increasing the output capacitance, increasing the
inductor, or perhaps by increasing the operating frequency. You can
also try to change the compensation capacitor/resistor to larger
values. The inductor is the best bet.

Kevin

 

[J. Justesen]

Hi.

I found the problem. (with "some" help from an older analog
engineer... ;-) )
It was a bad 0V connection from the VSS supple of the mic2183 chip to
the VSS plane...
So at high voltages the mic2183 chip could not draw enough current, to
pull the gate pin of the MOSFET high fast.