Hidden current path in MPR121 when VDD is low

[shva]

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?

[reyes]

Hi Shva,

Unfortunately I do not have an internal circuit of the MPR121, but giving the reasons you mentioned, it is totally possible that may be a current path from power supply to the pull-up resistors to the internal circuits of MPR121 and finally to GND, this would explain this behavior.

The only thing I can think of by modifying the MPR121 registers to avoid this problem is to test it by setting the MPR121 into Stop Mode (all ELEPROX_EN and ELE_EN bits are zeros) and then set the GPIO bit to zero (I don’t think that this can work, but it may worth to try).

By hardware you can make a test by connecting VDD and VREG together (as appears on Figure 1 on page 3 of the datasheet), this way the internal voltage regulator can be bypassed, and probably reduce the current draw you are seen when GPIO pin is low. If this do not work then yes, unfortunately I think that you would need to do mayor changes in the hardware design.

Have a great day,
Jose Reyes

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