Hi, all-
I hope someone could offer me some advice.
My design has a buck switched mode power supply (12v to 1.8v,
MAX1993).
On startup, there's an overshoot on the voltage, and the contoller
shuts down on
the over-voltage protection.
What could be the causes? maybe wrong values on the output inductor,
or capacitor?
maybe wrong ESR value on the cap? maybe layout design problem?
Thanks for any help, guys,
Harpoon
Good luck with that one. It is a highly complex, read I've never really
looked very hard at that stuff much before, controller. Working out what it
thinks its transfer function might be could be hard. It's probably some
wierd thing that is not suited to 'normal' linear analysis.
The dirty sheet does mention possible instabilities due to the capacitor ESR
zero being too high. Voltage feedback is a simple divider and there is no
feedback available to tailor the response of the internal error amplifier if
there is one in there.
That might suggest that the overall voltage loop including the capacitor is
second order up to the ESR zero where it becomes first order and crosses
over at some higher frequency. If the ESR zero is too high then is crosses
over second order and is unstable...... BTW this is speculation.
Anyway, one possibility is that with such a large VIN/VOUT differential you
can rapidly ramp up your filter inductor current to some excessive level and
when the output voltage reaches the set level the controller switches things
off BUT you have still put energy into the inductor and that gets delivered
to the output causing the overvoltage.
Cures for that would be reducing the absolute current limit level and/or
increasing the inductor value so it doesn't store too much excess energy.
If I'm right about the loop behaviour (pigs might fly) then you might split
the feedback resistor from the output node into two in series with a ten to
one ratio and add a capacitor across the larger resistor..... and then
experiment with the value of the capacitor. That has two effects.
The first is it introduces a low frequency zero at the input making the loop
first order up to the frequency where the smaller resistor cancels that
zero. This is good because is makes the loop less conditionally stable.
The second is that such a response results in a more damped behaviour and,
in particular, a less snappy start up. Rather than starting up balls to the
wall it should bring the output voltage up 'gradually'.
Further than that I wouldn't like to guess. All of the above is speculation
based on 'normal' controllers and should but might not apply to this one.
Good Luck
DNA
