Electronic – Switching between backup battery and main power supply

power supply

I have this GPS which requires a back up battery to achieve warm start. I noticed in the schematics that V3.3 and VBAT is internally connected for this GPS board. Therefore, I connect a 3.3V battery to the JST in order to power the board. A microcontroller is used to read from the GPS. However the 3.3V pin on microcontroller is not connected to 3.3V pin on the GPS. They only share a common ground. The reason for this is that I want to use a switch to turn off the power to the microcontroller but leave GPS running in low power mode(off the battery). However, When I turned off the power supply to the microcontroller, the LED (attached to 3.3V of the microcontroller) was flickering weakly. As soon as I unplugged the battery on the GPS, the LED went off.

I am suspecting that the battery is supplying power to the LED but how is that possible? The LED only shares the ground of the battery.

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There are couple 0.1uF caps between the 3.3V and GND on the microcontroller side. Also, it uses UART(TTL) to communicate with the GPS.

Best Answer

It is possible, that GPS did not went to lower power mode when you power off MCU. It seems that there's a pin '24' for this. If MCU is still driving TX pin to Vcc then your MCU have phantom power going through this connection. You should disconnect communication lines when you turn off eighter device to keep off from such effects.

schematic

simulate this circuit – Schematic created using CircuitLab

It is always good to avoid phantom power issues. There are some methods for it.

  1. There could be specific interface chips. Unfortunately I do not remember single one.

  2. There are several logic level shifters (like TXS0102) that implement disconnection (Hi-Z) when power on ether side goes low.

  3. Tristate buffers.

  4. Open-L (open-collector/open-drain) signalling with pullup at receiver side.

  5. As a low man's solution a resistor limiting current through signal wire to reasonable value.