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AD5821A load current when Vdd=0

I am using an AD5821A to provide a custom current pulse through an IR LED for an engineering prototype. The microcontroller uses a LiPo battery for a power source. I use the 3.3V regulator on the uC board for Vdd, and a direct connection from the battery through the LED (plus a series current measuring resistor) to ISINK. DGND and AGND are shared, tied back to the - battery terminal. The setup works perfectly when the uC is on. I use the XSHUTDOWN pin to bring the LED current to zero. But when I "turn off" the system (disable the on-board 3.3V regulator), I see current flowing through the LED. In this state, I suppose XSHUTDOWN could be floating, but even if I tie XSHUTDOWN to GND in the off state, I still get current through the LED. This is more of an annoyance than a progress limiting problem as I come back the next day to a dead battery. Is there a solution other than a physical switch to disconnect the battery from the system?

  • Ah. Dug in a bit more and realized it was the output protection diode. Shutting down Vdd just made current flow through my load, through the protection diode to Vdd. Tying Vdd to the battery instead of the 3.3V regulator seems to be the best path forward for this prototype. Curious how this is used in an actual mobile device when it is powered down? 

  • Hi, 

    In your case, since a battery is used to power the load, a switch (mechanical or solid-state) should be used to disconnect the load supply. On the prototype, a mechanical switch should be fine, but in your final design, you could consider the ADG7421F.

    Tying Vdd to the battery would work, however, you should consider the logic input levels and the shutdown current should be acceptable in your design. 

    In actual mobile apps, they should also be using a switch to dc the load supply to avoid unnecessary consumption.

    Best regards,