LDO Regulators and low voltage PICs

[Stellarcore]

Hello folks,

I'm running a PIC16LF4585 at low voltage (3.3V) so that I can interface directly to an SD card without any level conversion circuitry.

I'm using an LM2936 LDO regulator to supply the PIC. I have a 0.1uF cap on the input and output of the regulator as well as a larger 47uF electrolytic cap on the output of the regulator to keep things stable.

The problem I'm having is that when I kill the power and bring it back up before the 47uF cap has discharged through leakage currents or whatever, the PIC wont start. I either have to remove the PIC from the socket and drop it back in, or short the cap to discharge it completely before the PIC will start back up.

In order to get the PIC to operate at low voltage, it was neccessary to disable brown out detection. My theory is that the PIC is not shutting down correctly when the power goes down leaving the PIC stuck in some kind of quasi non functioning state.

Can anybody think of a simple solution to this problem? One solution that I thought of could be to put a resistor between the cap and ground so that on power down, the cap will discharge quicker, but I'd prefer a better solution.
Perhaps a circuit that toggles the RESET pin on the PIC when the voltage drops below an operating level?

Thanks for your help.

 

[justDIY]

use the brown out detect / reset feature built into the PIC. since you're operating at 3.3v, set VBOR = 2.8 or 2.1v, check page 345 of the data (pdf page 347)

power-up timer might help too, the pic might be trying to boot before the supply has stabilized, and is crashing. same datasheet page as brownout

 

[Papabravo]

Try a reverse diode across the regulator. That'll discharge the 47 uF cap without much trouble when the input voltage drops.

 

[Gayan Soyza]

One solution that I thought of could be to put a resistor between the cap and ground so that on power down, the cap will discharge quicker, but I'd prefer a better solution.
Thanks for your help.

Did that method worked for you?

I don’t know whether you giving supply from batteries or from a power pack to the regulator.

If it is from a power pack,
Tell how much capacity (big) is your smoothing capacitor after the bridge rectifier.

How do you power OFF the circuit? By the AC wall out switch or from…….?


You place the ON/OFF switch after the regulator & 47uF cap.

Also make sure to clear all GP registers in the first part of your program.

 

[Stellarcore]

Well, I haven't had a chance just yet to try out the solution. I can't stand being in my basement for more than 20-30 mins before it gets unbearably hot in the summer (no AC).

I'll give a couple of your ideas a try tonight when I get home from work and let you know how I made out.

As for my power supply, I'm currently using the 5V from an old computer power supply and just flipping the PSU switch on/off. However, I have also run into the same problem with a more expensive lab PSU, running from a simple 9V battery and running from a DC-DC up converter.

 

[Stellarcore]

I feel like a bit of a boob.. I turned brown out detection back on, set it to 2.7volts and also enabled power up timer. Now it resets like a charm.

I didnt need to try the reverse diode trick. For some reason, when I first started playing with the low voltage PIC I couldn't get it to work with BOD, maybe I just had it set too high.

Although I still have to work on my power circuitry for my final design so that its protected from things like over voltage and reverse battery (I accidentally smoked some wirewrap wire doing the later).

Anywho, thanks for the ideas. I'll definately be around the forums, it seems like a pretty cool place.

BTW, JustDIY, you're website is awesome, a true inspiration for a site that I'd like to start on in the coming months.