Brown-Out Problem with PIC

Hey,

My circuit is a very simple uC controlled solenoid.

The board is powered from three AA batteries, and managed by a PIC12F629. The PIC simply controls a MOSFET gate, that short-out the batteries across the solenoid once every five minutes or so, for about 30ms. (The solenoid requires a few Amps of current to operate).

Problem is, every time the solenoid gets activated the PIC gets a reset by the Brown-Out Detection.

The PIC has a 100pF bypass capacitor near its Vdd and Vss, and delivers current to the MOSFET's gate through a 330Ω resistor. (Qg is about 20nC)

I suspect that the fact the RDSon of the MOSFET is 50mΩ causes too much current to choose this path and the PIC's Vdd voltage drops too low.

I'm not sure what to do about this...

Thanks in advance, help appreciated.

Hi,
You should have a bulk capacitor of about 1000uF that can supply short high current "spikes" to the solenoid when required so that the voltage does not drop. I think this should help.

Hope this helps.
Tahmid.

Okay, that may help (unfortunately fitting a 1mF capacitor in my board isn't an option at the time...)

I was wondering what can you do in cases where the solenoid (or other high-current load) would be operated from the same power supply for a long time - Long enough that a capacitor is not an option. What would I do then?

Sounds like your project have a design FLAW! Sometimes a person has to redesign things to fix the bigger problem, instead of just attaching a capacitor.

You need to sit down and consider your WORST case power supply usages for everything in the system, then step back and try to determine how to keep your microcontroller working in the WORST case situation.

You might need to have seperate power supplies or split power supplies, where one is split off for the digital controls to ensure they have well regulated power. In this case, you split off the power and re-regulate it to ensure the power is very clean.

May the fix is easier than it sounds, but since we don't have your entire design, then you are only going to get generic answers.