BQ25756: Doubts regarding reset conditions of BQ25756

Hello Juan,

Thanks for working with this. Did you make sure to disable the watchdog timer when you were doing these tests?

1. The device shouldn't POR if the input is VAC is unplugged. Can you tell me what the input and battery voltage is?

2. The IC should do this automatically

3. That's strange. What's the charge current and battery voltage where the device always reports that it's in CC mode? The device should only report that it's in CV mode when the FB pin reaches its set voltage. Does setting EN_PFM=0 change anything here?

4. Turning EN_CHG ON/OFF shouldn't POR the registers.Do you know if input or battery voltage is dropping during these tests?

Best Regards,
Ethan Galloway

Hello Ethan:

To give you some context, we tried to test the behavior of the bq25756 after a timer expires. After observing that the BQ stops switching when a timer expires, we tried to search for methods to recover its normal functioning. For that reason we have the following doubts. 

We tried to make sure of disabling the watchdog timer, to avoid resetting the registers, but we are confused regarding the reset conditions of the device, if i understand correctly you say that a POR should not happen if the watchdog timer is disabled, however i find in the bq25756 datasheet the following information:

1. Unplugging the VAC and having the battery connected, produced a POR. The input voltage was 24 Volts, the battery voltage was 26 Volts. Unplugging the battery (having the input voltage source connected) did not cause a POR.

2. Could you elaborate if possible how the IC does this? 

3. Our input conditions were 24 Volts, our battery voltage 26 volts, We didn't change the value of the EN_PFM bit, we left it to its default value, is it necessary to modify this to see a change between CV/CC?

4. At least the watchdog timer, it returned to its default value (40 seconds), this was after a test in which the pre-charge timer had expired. Neither the input voltage or the battery voltage suffered any voltage drop. 

5. Can the expiration of any timer produce a reset in the registers?

Thank you kindly.
Juan.

Hello Luke,

Thanks for being patient with this. I'll get back to you later this week.

Best Regards,
Ethan Galloway

Hello Juan,

Thanks for being patient with this. This week has been really busy for us.

1. Can you talk to the device when only the battery is plugged in? Can you measure the voltage on the SRN pin with only the battery plugged in?

2. I think it's a diode OR function.

3. What's the battery termination voltage or charge voltage here? Have you changed the FB pin feedback resistors?

4. Try setting watchdog to disabled and then reading the registers again afterward. Sometimes using BQStudio causes writing to the watchdog timer to not stick.

5. Only the watchdog timer expiring will reset the registers.

Best Regards,
Ethan Galloway

Hello Ethan:

1.Yes we can, We didn't measure the voltage on the SRN pin, but should it be similar the battery voltage right? We could make this measure in the next test.

2.So, you mean that if we don't have input voltage and we produce an OFF-ON in the battery voltage, would that produce a RESET?

3.Yes we have changed the FB pin feedback resistors we've reduced this resistor by 10 times. The charge voltage is 29.4 Volts and there are 1.536 in the FB pin

4. Ok we will try this, I will come back with the test results, but one question i have is:  Will the sequence turning ON/OFF the EN_CHG produces a POR after the precharge timer has expired ? The problem is that this sequence produces a reset in our registers and we think it is because it causes a POR. 



5.Ok understood, but a POR will cause a reset in the registers right? If so which causes can produce a POR?

Best regards
Juan

Hello Juan,

1. The SRN pin voltage should be the same as the battery voltage, but I just want to make sure.

2. Yes, having no voltage on either the input or the output will produce a RESET.

3. A 29.4V charge voltage is the default setting of the EVM.,

4.

Juan López Jódar said:

Will the sequence turning ON/OFF the EN_CHG produces a POR after the precharge timer has expired ?

I will try to look into this and get back to you. This shouldn't produce a reset in the registers.

5. Yes, a POR will reset the registers. If VAC<4.2 or VSRN<3V, the device will probably reset the registers.

Best Regards,
Ethan Galloway

Hello Ethan,

1. Ok we will return to you with the mesurement of the SRN pin.

2. We made the following test : 

a) With the battery connected and the input voltage connected, we did a OFF-ON in the input voltage, in this situation a timer had expired and we were able to reactivate the MOSFETs switching.
 
b) With the battery connected and the input voltage connected, we did a OFF-ON in the battery, in this situation a timer had expired and we were not able to reactivate the MOSFETs switching.

Could you explain this behaviour?

3.Yes we had the same charge voltage value but we still obtained CV when we really were in CC mode of charge. Is this an IC error? 

4. A reset in the MOSFETs switching can produce a reset in the watchdog? 

5.Two questions to your answer
 
a) When a timer has expired and the converter has stopped switching and we reactivate the switching, is this a POR? 

b) When a timer has expired and the converter has stopped switching, in which way can we reactivate the switching? 

Thank you kindly.
Juan.

Hello Juan,

Thanks for being patient with this.

Juan López Jódar said:

b) With the battery connected and the input voltage connected, we did a OFF-ON in the battery, in this situation a timer had expired and we were not able to reactivate the MOSFETs switching.

Can you send me what the status registers say when you are unable to start switching again?

Juan López Jódar said:

3.Yes we had the same charge voltage value but we still obtained CV when we really were in CC mode of charge. Is this an IC error? 

I don't think this is an IC error. Make sure EN_PFM is set to 0 when the charge termination current is less than 2A.

Juan López Jódar said:

4. A reset in the MOSFETs switching can produce a reset in the watchdog? 

This shouldn't reset the watchdog.

Juan López Jódar said:

a) When a timer has expired and the converter has stopped switching and we reactivate the switching, is this a POR? 

This is not a POR.

Juan López Jódar said:

b) When a timer has expired and the converter has stopped switching, in which way can we reactivate the switching? 

All the registers reset when the watchdog timer expires. The EN_CHG register resets back to EN_CHG=1 so this shouldn't stop switching. Have you made any hardware/jumper changes to EVM?

For your testing, what is the input power supply and the VBAT load being used here?

Best Regards,
Ethan Galloway

Hello Ethan,

1b) Ok as soon as we are able to re-make these tests i will send you the status registers information.

3)Ok understood

4) Ok we will remake this test.

5b) We haven't made any modification of the EVM and in all cases when a timer had expired the converter has stopped switching (we have forced a timer expiration while maintaing the converter in pre-charge during more than 2 hours for example ) (Is this a normal behavior? Does the expiration of any timer produce an stop in the switching? 

The input power supply was 28 volts and the VBAT was 28 volts.

Thank you kindly.
Juan.

Hello Juan.

For question 5, can you let me know what the status and flag registers say before and after the reset if possible?

Are you using a CV load for VBAT or a real battery?

Best Regards,
Ethan Galloway

Hello Ethan, 

For question 5 i am unable to tell you the value of the registers in order to tell you this we should do a new test.

We are using a CV load. 

Best regards.
Juan

Hello Juan,

Thanks for letting me know.

Let me know what you can find from the registers.

For the CV load on VBAT, is the voltage on VBAT dropping and resetting the registers when VAC is removed?

Best Regards,
Ethan Galloway

Hello Ethan. 
Perfect, yes the voltage on VBAT did not drop.
I will tell you as soon as i can about the rest.
Best regards.
Juan