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BQ25896: bq25896 ADC accuracy revisited

Part Number: BQ25896
Other Parts Discussed in Thread: BQ25895, TPS745


At least two other people before me have asked for an accuracy specification of the ADC in the bq25896, but received the surprising answer that TI doesn't have the data:

The motivation given is "The Battery monitoring of the BQ25895 is offered as an extra feature in addition to regular charger. There’s no accuracy spec for the ADC readings."

To me this is very strange and problematic, as this is a feature that is part of the marketing of the IC and for that reason it must be practically useful. A measuring device without an accuracy specification is not a tool at all. It is a toy. TI is probably the leading company in battery charging ICs, so you should be able to do better.

Please let me add that I very much like the bq25896. It provides a very neat solution to several important needs in a USB- and battery-powered application that I described in this posting:

In addition to the USB-related benefits from using the bq25896, its shipping mode also provides a very practical low-power Snow white-deep sleep mode and wake-up to an MCU-based handheld device.

The most important need our products have for better accuracy specification (compared to what is currently specified) is the possibility to reliably determine when VSYS has become too low and the down-stream TPS745 LDO output voltage is no longer within our required range. The TPS745 PG signal is not usable for this. The idea was to use the bq25896 ADC to measure VSYS (and the other metrics), but we need specifications to use such a design in production. If this is not possible we need to add a separate voltage detector IC, which we would rather avoid.

Base accuracy X% and temperature dependency Y% ?

If the temperature dependency is known we could compare ADC reading to DMM reading of VSYS at production testing when VSYS is close to our minimum voltage and count backwards to get the device's ADC reading of VSYS_our_min.

We are avoiding the use of switch-mode voltage regulation when operating on battery by using an LiIon cell with sharp-knee discharge characteristics commonly found in the so-called high-voltage cells, to TPS745-generate a 3.24 +-0.05 V system rail. This discharge curve drops very quickly at about 3.5 V (which is also dependent on load current on temperature). This discharge curve is fairly flat from 3.8 V to 3.6 V, so bad precision when detecting in the range between 3.4 V and 3.5 V means we will lose much of the battery runtime. The 20 mV REG0F->SYSV resolution sounded promising, but disappointing if its accuracy is off by 100 mV or 200 mV and a problem if its accuracy is unknown.

As a service to other readers, here is my summary of the accuracies specified in the datasheet:

Input current regulation accuracy:
 100 mA:   90 mA +11.1% -5.56% (  85 -  100 mA)
 150 mA:  135 mA +11.1% -7.4%  ( 125 -  150 mA)
 500 mA:  470 mA +-6.38%       ( 440 -  500 mA)
 900 mA:  825 mA +-9.09%       ( 750 -  900 mA)
1500 mA: 1400 mA +-7.14%       (1300 - 1500 mA)

Input voltage regulation accuracy: +-3%
VINDPM = 4.4 V, 9 V
(4.400 < 4.536 < 4.672 V) --- from datasheet
(4.462 < 4.600 < 4.738 V) --- presuming 3% accuracy also applies to 4.462 V
(4.850 < 5.000 < 5.150 V) --- presuming 3% accuracy also applies to 4.850 V

ILIM pin current regulation accuracy:

Fast charge current regulation accuracy:
Vbat 3.1 or 3.8 V, Ich  128 mA +-20%
Vbat 3.1 or 3.8 V, Ich  256 mA +-10%
Vbat 3.1 or 3.8 V, Ich 1792 mA +-5%

Charge voltage resolution accuracy: +-0.5%
VBAT = 4.208 V (REG06[7:2]=010111)   (4.18696 - 4.22904 V)
VBAT = 4.352 V (REG06[7:2]=100000)   (4.33024 - 4.37376 V)
TJ = -40 C to +85 C

VSYSMIN setting in REG03[3:1] = 3.5V is specified as minimum 3.5 V, typical 3.65 V and not specification on maximum.

Other VSYSMIN settings not specified.

Could you please escalate this?

Best regards