BQ40Z50-R2: Questions about WAKE button in BQ40Z50EVM

Hello Kevin,

The wake button is to make the gauge exit from shutdown/sleep mode. The purpose of the wake button is to short BAT to PACK pin. When the gauge experience a big enough voltage on the PACK pin, the gauge will exit from shutdown mode. In most designs, there is a charger IC in the design and the charger IC can apply a voltage on the PACK pin that will wake the gauge up from shutdown mode.

Regards,

Adrian

>>When the gauge experience a big enough voltage on the PACK pin, the gauge will exit from shutdown mode.

I am seeking more clarification on this point. Why doesn't the gauge wake up from the voltage it is receiving on the VC1/VC2/VC3/VC4 pins?

IOW, why does it need to see a voltage that is under its control (via FETs) to wake up? Why doesn't voltage on it's input pins do the trick?

>>In most designs, there is a charger IC in the design and the charger IC can apply a voltage on the PACK pin that will wake the gauge up from shutdown mode.

Our design does have a charger IC, so that might be a wakeup source in some scenarios, but here I am researching the behavior when the battery cells are the only source of power.

Maybe a different way to ask my question is "Why does the device initially power-up in SHUTDOWN MODE instead of (for example) in SLEEP MODE?"

Hello Kevin,

Kevin Banks said:

I am seeking more clarification on this point. Why doesn't the gauge wake up from the voltage it is receiving on the VC1/VC2/VC3/VC4 pins?

This is not how the gauge is designed to operate, it is design to wake up when voltage is applied to the PACK pin.

Kevin Banks said:

"Why does the device initially power-up in SHUTDOWN MODE instead of (for example) in SLEEP MODE?"

This is a safety feature that is built in to the design of the gauge, the FETs are turned off in shutdown mode which prohibits current and protects the entire circuit as behavior from an initial boot can be unpredictable.

Regards,

Adrian

So in a design where the BQ40Z50-R2 is in control of the charging (operating standalone, no microcontroller) and it, combined with a BQ25720 * IS * the "charger", a WAKE button or something equivalent would be required to ensure a BQ40Z50 in SHUTDOWN does not remain in SHUTDOWN forever, correct?

Hello Kevin,

The charger will act as the wake button. This wake from shutdown usually only happens after being shipped from production.

Regards,

Adrian

Adrian:

In our circuit, the charging is controlled by the gauge (in other words, it is an internal charger rather than an external charger), so it is unclear how the gauge could be woke up by a power source that is under the gauge's control.

That said, in our testing, the gauge does wake up, but only if we plug in the USB-C PD power source coming into the system TWICE (IOW, plug, unplug, re-plug - NOW the gauge is awake and communicating. We see precharging followed by charging once we do this.

Why isn't this a true deadlock (why does it work the second time)?

Hello Kevin,

When you first plug in the the USB-C PD can you measure the voltage on the PACK pin? Also, once the gauge is taken out of shutdown mode it is likely not to go back into shutdown unless the user sends it back to shutdown or the battery voltage drops too low. 

Regards,

Adrian

>>When you first plug in the the USB-C PD can you measure the voltage on the PACK pin?

I will try to get around to that, and will post a follow-up.

>>Also, once the gauge is taken out of shutdown mode it is likely not to go back into shutdown unless the user sends it back to shutdown or the battery voltage drops too low.

Agree. In our use case, the packs are run down until the gauge cuts the user off (so still plenty of power at that point to avoid shutdown) BUT then it can be an extended period of time before they get taken to a charging station. In the interim, the cells can run down further, enough to eventually put the gauge in shutdown.

It is this corner case we are trying to handle gracefully.

Hello Kevin,

I see, I fully understand the problem now with the application. Yes, let me know the results of testing the pack pin voltage, as it's possible that the USB-C PD is not providing power to the device on the first plug in.

Regards,

Adrian