capacitancecircuit-designinrush-currentspark
I have a motor driver that handles 2 150watt motors powered by 2 Li-po 12V batteries (connected in series). Obviously, I need filtering caps on my driver so I used 2 1200uF caps connected in parallel.
My issue is that every time I connect the batteries there is a spark because of the caps. How do I prevent this spark from happening? I am pretty sure that there has to be a nice solution for this. Also, the motors never have to run right after connecting the batteries.
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One possible source of latch-up is USB_VBUS. The datasheet of the LPC43xx allow VBUS to be 5 V only when VCC is present - but in your case this takes a small amount of time until the regulators are fired up. Consider adding a resistor in this path.
Another issue is the ON-Switch of the FPGAs 3V3 LDO. It will be enabled by default, as the GPIO pins are "weak high" when not configured. The resulting voltage level is high enough to switch the regulator on. The SNS pin must be connected to VOUT, if present. You should change the drawing in case they are not.
Best Answer
The way you describe it, it sounds like the inrush current caused by your capacitors charging up.
For that you could use a limiter, such as a thermistor, or a soft-start circuit, such as a large pass FET with an RC timing circuit. The latter's RC would turn the FET on slowly enough for the capacitors to charge smoothly but not so slowly that the FET gets hot (500 ms?).
Are you sure that your motor load is off when you plug in, though, and have you got any power-up circuitry to ensure this?