All,
I have a circuit with an Exar XRP7659 5v 1.5A regulator and a Torex XC6210B 3.3V LDO Regulator. I am using this circuit to charge USB devices at full current and have noticed that when the device is charging I get negative voltage from any two ground points on the breadboard.
It appears that this negative voltage is coming from the GND pin on the USB port, as I read the highest negative voltage there, but it's seen throughout the circuit. The result is that the LDO goes slightly under voltage to 2.8 – 3.1 volts. The LDO returns to 3.3v when the USB device is unplugged. The issue is not apparent when using a 5V 1.5A power source but this results in ~4.4V out of the 5V regulator, which is unacceptable. I am using (and have tried multiple) a 12V 1.25A DC input source.
What could cause this negative voltage and how do I resolve it?
Here's a schematic for my circuit
Best Answer
As discussed in the comments, you have about a 0.5 Ω resistance in your ground wiring. You are losing about 0.6 volts when under load. Ohm's Law (V=I*R ==> I=V/R) says I=~0.6/~0.5 = about 1.2 amps flowing through that ground. It all adds up.
Fix the resistance in the ground wiring and you'll fix your voltage drop.