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Hidden current path in MPR121 when VDD is low

Question asked by shva on Jun 3, 2015
Latest reply on Jun 11, 2015 by Jose Alberto Reyes Morales

I am using MPR121 together with another MCU. There are two versions of circuits. In both versions, the SDA and SDL lines are pulled up by two 10 kOhm resistors. The only difference is that MPR121 shares power and ground with the external MCU in the first version, and the MPR121 is powered by a GPIO pin on the MCU in the second version. The purpose of the latter is MPR121 can be completely turned off if I don't need data from it because I only need to read from MPR121 infrequently. The first version works fine, having only a few uA of background current in idling state. However, in the second version, as long as I turn the GPIO pin low, MPR121 draws about 400uA of background current. After a closer look, it turns out when the VDD of MPR121 is driven to GND, the current actually comes out of VDD pin, meaning that there may be a current path from power supply to the pull-up resistors to the internal circuits of MPR121 and finally to GND. Is this an expected behavior? If so, is it possible to do a few tweaks with MPR121 registers rather than changing the hardware design to terminate the current path?

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