BQ25892: Questions

d_zero

Part Number: BQ25892
Other Parts Discussed in Thread: BQ25896, TL7702B, TPS22953

Dear *,

please can you answer some of our questions:

1) BTST pin 47nF can we use 16V, because difference between Vsw and Vbtst per DS is 7V max, and on evaluation board 25V capacitor is used?

2) The pull-up on I2C and INT pin can be to 1.8V, our microprocessor is 1.8V signal logic ?

3) PG indicates that input voltage is ok, does it indicate that the Vsys is ok? here should we use pull-up to vsys?

4) to set the max input current limit can we connect PSEL direct to GND?

5) we want to set the input current limit to around 3A -> Rilim =120Ohm is ok? or should we use 118R or 115R ? when 115R then Ilim = 390/115=3.4A, what Kilim is used for calculation?

6) if the QON pin is not used what should we do with him?

Best Regards,

over 6 years ago

0 Jeff F over 6 years ago

TI__Guru**** 166626 points

Hi D,

Regarding 1, max voltage across BTST capacitor is VBUSmax+VREGN (6V) so voltage rating should be at least 1.2-1.5 X higher than that to account for capacitor derating from DC bias.

Regarding 2, yes, the I2C and INT pullups can be 1.8V.

Regarding 3, if battery will always be attached and you do not expect to use shipmode (I2C bit to turn off FET), then yes you can pull up to SYS.

Regarding 4, PSEL sets in the IINDPM I2C register to the maximum 3.25A The resistor on ILIM could clamp the input current limit to lower value if that bit is enabled.

Regarding 5, if you plan on using the ILIM resistor to control input current limit, then I recommend using the highest KILIM to ensure that you do not crash your 3A input source. Alternatively, you can set the IINDPM register to 3A and the set the ILIM pin for slightly higher as redundant protection.

Regarding 6, if QON is not used, please leave it floating.

Regards,
Jeff

0 d_zero over 6 years ago in reply to Jeff F

Genius 4775 points

Hi Jeff,

about the PG pin is it better to pull-up to REGN? with 100k ?

Because we will not use BQ25982 in ship mode but we will use with battery (Ce low) and also without battery ( CE high).

Best Regards,
D.

0 Jeff F over 6 years ago in reply to d_zero

TI__Guru**** 166626 points

Hi D,

Pulling /PG up to REGN is one option. Keep in mind REGN is only available with valid VBUS is applied. The other option is pull up to SYS, which would be the battery when there is no VBUS.

Regards,
Jeff

0 d_zero over 6 years ago in reply to Jeff F

Genius 4775 points

Hi Jeff,

1)if the /PG is not used, can be left floating?

So we can't pull it up ti REGN because or system will sometimes work with battery (no VBUS)and sometime without battery (only VBUS)

And i don't know if pull to SYS is good because you said :
" if battery will always be attached and you do not expect to use ship-mode (I2C bit to turn off FET), then yes you can pull up to SYS."
-> and we don't have the battery always attached :

2) If the REGN is not available when only battery ( NO VBUS) then this mean that when i the device working only on battery the TS (thermistor) is not in right temperature window and the stat led is blinking ( charge suspend) ?

Best Regards,
D.

0 Jeff F over 6 years ago in reply to d_zero

TI__Guru**** 166626 points

Hi D,

Regarding 1, /PG can be left floating.

Regarding 2, no. TS function is disabled in battery only and not OTG mode.

Regards,
Jeff

0 d_zero over 6 years ago in reply to Jeff F

Genius 4775 points

Hi Jeff,

we got the BQ25892 development board on our desk.

And we are trying to recharge a Li-ion battery but there is something strange with charging voltage.

As per DS -> page 41 table 15 -> the default charging voltage is set to 4.208V, which is equivalent to recommended charge voltage for Li-ion battery.

But we observe that the charging voltage is 4.1V when the battery is near full charge, also we have amp meter connected on Ichg and it showed 0A . And the STAT LED was not OFF but it blinked really fast (more then 1Hz).

Please can you explain this behavior?

Best Regards,
David.

0 Jeff F over 6 years ago in reply to d_zero

TI__Guru**** 166626 points

David,

First of all, an ammeter in series between BAT pin and battery has high impedance, which results in early CV/tapering, and/or the autorange function can cause the charger to oscillate. To confirm battery full charge, what is the voltage as measured at the IC BAT pin? What is the voltage on the TS pin? JEITA warm (TS pin low) results in the lower battery voltage. As per table 8, datasheet page 28, blinking LED indicates fault, including TS fault.

Regards,
Jeff

0 d_zero over 6 years ago in reply to Jeff F

Genius 4775 points

Hi Jeff,

thank you for the answer, we removed the amp meter and now the battery is charged to 4.2V and the stat led is off when charging complete.

One other thing that we see when we are discharging the battery, it is discharging to 2.8V, is it possible to set the MIN discharge voltage to 3.3V or 3.4V?

For some reason when on battery the BQ25892 we see that the Vsys is < 3.5V (SYS_MIN) how is that possible ??

In table on page 13 of DS is when Vvbus >Vbat -> min sys voltage is Vsys_min (3.5V) and see here a voltage down to 2.8V ?

We measure Vbus= 0V and we have OTG pulled low and CE pulled low

Best Regards,
David.

0 Jeff F over 6 years ago in reply to d_zero

TI__Guru**** 166626 points

Hi David,

Unfortunately, the charger does not provide a battery discharge threshold. The charger expects the battery pack protector to provide over-
discharge protection.

When in MINSYS mode, if the buck converter has enough input power to drive both the system load and charge the battery, then V(SYS) = MINSYS. If not, and the battery must supplement the load, then V(SYS)=V(BAT)-ISUPP*RDSonQ4.

The buck converter can only provide MINSYS voltage when VBUS is available.

Regards,
Jeff

0 d_zero over 6 years ago in reply to Jeff F

Genius 4775 points

Hi Jeff,

if i understood correctly,

when there is no VBUS and the device is operating only from battery then the min system voltage can be as low as 2.8V ?

the table on the page 13 is only valid when VBUS is present??

3) why in this table there is Vsys_range when vbus<vbat, so this equation is not valid if vbus=0V ? and how much is the Vsys_min?

4) is there similar charger to BQ25892 that has this the feature when on battery only? we want when the battery voltage drops to 3.4V or below to turn off the system.

5) what do you suggest to use with BQ25892 in order to enable the feature?

Best regards,

David.

0 Jeff F over 6 years ago in reply to d_zero

TI__Guru**** 166626 points

Hi David,

Regarding 1, with no VBUS, the BATFET between BAT and SYS will turn on down to V(BAT)=VBATDPL=2.15V-2.5V.

Regarding 2 and 3, with no VBUS, the I2C bus only functions down to VBATUVLO=2.3V and the ADC only functions down to the MINSYS setting. MINSYS lowest setting is 3.0V.

Regarding 4, none of our chargers with SYS (NVDC power path) have a UVLO/cutoff voltage that high. The chargers were designed assuming that the battery pack has a protector that opens at the desired discharge voltage.

The BQ25892 has a bit that allows the host to turn off the BATFET. Can you write host software that uses the ADC to monitor the battery voltage and turn off the BATFET when V(BAT)=3.4V?

BTW, if you do not need ISYS or ICHRG= 5A, only 3A, you can use the BQ25896, which is the 3A version of BQ25892.

Regards,
Jeff

0 d_zero over 6 years ago in reply to Jeff F

Genius 4775 points

Hi Jeff,

1)
we also found that we can write to register of BQ25892 to turn off the bat-fet.

Does Ti has a battery pack protector solution that opens at the desired discharge voltage?

2) We would prefer a hardware solution,

we looked into a reset IC solution, but there is a problem when the reset IC cut-off the voltage then the battery voltage recovers because of no load and then it will turn back on , and then off , an on and this is not good.

so we want to cut the voltage eg at 3.4V but only enable when the voltage is a bit higher.

is there a IC with program-bale hysteresis ?

Best Regards,
David.

0 Jeff F over 6 years ago in reply to d_zero

TI__Guru**** 166626 points

Hi David,

I did a search on www.ti.com and did not find a protector with higher than 2.8V undervoltage threshold. I do not directly support protectors so you might want to submit another e2e post.

Regarding reset IC solution, you can add external hysteresis to most reset ICs with additional resistor from the output back to the external sense resistors. See www.ti.com/.../tidu020a.pdf for the concept. For example, you could use TL7702B, with external resistors connected to sense, to drive external back to back PFETs in series between BAT pin and battery.

Regards,
Jeff

0 d_zero over 6 years ago in reply to Jeff F

Genius 4775 points

Hi Jeff,

we think that the best solution for us is to use the force cut-off BATFET trough I2C on BQ25892 as primary protection.

And as secondary protection we might use a load switch TPS22953 on SYS voltage to turn off our system voltage if it falls below 3.35V.

One question that we have here is it possible and how to add a hysteresis to the load switch that it will turn ON when the sys voltage is 3.55V ?

Best Regards,
David.

0 Jeff F over 6 years ago in reply to d_zero

TI__Guru**** 166626 points

Hi David,

Sounds reasonable. I do not support the BQ22953 but based on a quick view of the datasheet, the EN pin has about 100mV of hysteresis which will scale with resistor divider used to sense. I suggest posting a question on the forum for the BQ22953 support team to answer.

Regards,
Jeff

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BQ25892: Questions