I've developed a circuit for my model aircraft, which requires 3.3V @ 100mA. My aircraft uses a 3S LiPoly, which ranges from 9V – 12.6V. When the motor is off, the 3.3V supply is good; very low ripple. However, when it spins up, the battery voltage ripples, and the supply voltage to my MCU occasionally dips below ~2.5V, and this occasionally resets it, and at the very least it causes problems with the rest of the analog circuitry.
I've attached a diagram of my system.
What can I do to reduce this problem?
Best Answer
The 100uF cap at the input of the regulator should supply power for short dips, less than 1 ms in duration. 100 uF droops at 1V / ms at 100 mA current draw.
If your dip is longer than a few ms, you can try a bigger cap.
Also, you can gain a couple volts of headroom by using a low dropout regulator instead of the LM317, and a Schottky instead of the 1N400x diode. That might be enough, but it would be nicer to solve the problem altogether.
100 mA is a pretty high current draw. Look at your power budget. Can you arrange to have the high-current parts shut off at brownout, so the microcontroller isn't in danger of resetting?
If you stick with the LM317, you can improve the regulation by bypassing the ADJ terminal: