I am using the BQ24079T to control power for a wireless, battery powered sensor node. The MCU will operate in one of two modes: periodic sample mode where it takes sensor readings and then sleeps, or idle mode where it does nothing but sleep and wait for the next command. If it is in idle mode for too long, it should shut down completely to save battery power. To turn it on again, it would have to be attached to the charger.
I figured that an easy way to do this would be to use the SYSOFF pin on the charger to disconnect the battery from the system when commanded by the MCU. Here is my current circuit for this .
The SYSOFF pin goes to an I/O expander (PCA9557)) chip which is held low as long as the device wants to stay powered. When it wants to turn off, the pin is released and pulled to VPP, turning VCO off.
Maximum possible battery life is very important for this project, so I am trying to eliminate as much load during sleep mode. All peripherals on the board are on switchable power and are turned off while the MCU is asleep. The only thing it cannot control power for is the charger chip itself and related circuitry.
The 200k pull-up on SYSOFF will constantly draw between 15 and 20 uA, depending on battery voltage, while the MCU is powered on, and the charger itself will draw around 6.5 uA. This is not a huge amount of current, but as a constant load, it amounts to several months from the 300 mAh Li-ion battery that I am using.
I was thinking that I could use a flip-flop or latch controlled by the MCU in place of the pull-up to reduce static load, but I am not sure how that would work.
How would you handle this?