Schematic Review: BQ27421-G1

Hello Jay,

A couple of points/questions concerning the schematic:

1. The BIN pin is responsible for allowing the gauge to detect if the battery has been inserted or removed.  Is this application have a removable battery or embedded battery?  If the battery is non-removable (i.e. embedded) then you pull the the BIN pin to ground with a 10k resistor.  If the battery is removable and the gauge needs to independently detect battery insertion/removal the BIN pin can be pulled up, but it needs to connect to the battery thermistor terminal in the pack so that the gauge can detect a HIGH to LOW transition.  If the gauge does not need to detect battery insert/removal on its own, the host can send commands to the gauge to let it know if the battery is inserted or removed.  In that case, BIN can be pulled HIGH or LOW.
2. The BIN pin cannot be pulled up to 3.3 V.  It should only be be pulled up to VDD of the gauge.
3. Make sure that C71 & C72 are as close to the IC as possible.
4. How are the SDA & SCL lines being pulled-up?  Via external pull-up or internal pull-ups from the host?
5. How is the GPOUT line being pulled up?  Via external pull-up or internal pull-up from the host?
6. The gauge should be connected between the battery and the charger/system so that it can see the current flowing into (charge) and out of (discharge) the battery.  So, the connection would be battery -> gauge -> charger/system.  So, the charger and the system should connected to the gauge's SRX pin and the battery connected to the gauge's BAT pin.

Jared,

Thank you for the help.

1. The battery can be removed. The current battery may only have two leads, +/-. So should we connect the BIN to ground with 10k?

2. For now, we can just connect to ground with 10k, correct?

3. Which version of the schematic are you reviewing? I think I have sent v61 and v71.

4. We will make certain SDA & SCL are pulled up. Do you recommend 4.7k or 10k?

5. It is not being pulled up. Should it be pulled up to VDD via 10k?

6. I’m confused about this. The way I have it per “pdmr_0p71.pdf” is: USB PWR -> charger -> battery -> power switch -> BAT on gauge -> SRX on gauge -> system. This way the battery can be charged even when the system is off. I am not sure I understand how the charger would come after the gauge. So when the system is off, USB PWR -> charger -> battery would be active.

Can you help follow-up with the customer?

Thanks,
Jay

Hello Jay,

1. Connecting BIN to ground with a 10k should be fine.  However, the gauge will not be able to detect battery insertion/removal events.  So, it would be best if the host has a way of being able to send the BAT_INSERT & BAT_REMOVE Control() Subcommands.
2. see #1
3. The document does not have a revision, but the date is 01.19.2015
4. There is a table in the DS in the Layout section concerning pull-up values (p.15).  If the pull-up voltage is 1.8 V, 10 k is typical, if the pull-up voltage is 3.3 V 5.1 k is typical (4.7 would suffice as well)
5. Yes, that should work.
6. The gauge's sense resistor is from the BAT to SRX pin.  So, with the schematic as is, the gauge will not see any current put into the battery (e.g. charge current).  The gauge needs to go right after the battery and the "node" that connects to the SRX pin would be shared by the system and the charger.  Think of how you would connect everything without the gauge in the system.  Then, insert the gauge's sense resistor at a point to where the current flowing into the battery and out of the battery can be "monitored" by the gauge.  The gauge should not have an impact on if the battery can be charged when the system is on or off.  The gauge does not really "care".  It does, however, need to be able to see both charge and discharge current for proper operation.

I hope this helps.

Jared,

We have a new schematic from the customer reflecting the updates that you discussed with Jay. Can you please review and inquire that the updated schematic attached is okay?

Thanks,

Evanlora_0p82_p4.pdf

Jared,

Do you have any updates on this updated schematic review?

Thanks,
Evan

Hello Evan,

I think the schematic is almost complete.  For the BIN connection, we would not recommend connecting the thermistor pin of the battery to both the gauge and charger (this would go for any fuel gauge).  The bias networks between the gauge and charger have the possibility of conflicting with each other and that could cause issues, mainly for the charger (it uses the bias for the JEITA settings), in this application.

It looks like with their current setup, the charger should take precedence for having the battery's thermistor connection.  If that is true, the host should be responsible for detecting if the battery is present and letting the gauge know either by I2C commands or toggling a GPO.

I hope this helps.