BQ27220: Using BQ27220 in a 2-cell NiMH arrangement

Hi Kevin


1. I'm assuming that TI's recommendation to use the BQ27220 (intended to Li-Ion cells) will work as a coulomb counter in our 2-NiMH configuration, correct?

OA: For gauging NiMH, i would recommend bq34z100-g1. However, the bq27220 can be used for coulomb counting the charge into and out of the NIMH.


2. Since we are using the BQ27220 in the "system" side, can the chip operate as intended if we don't use the OTP Flash and just use RAM for battery profiling?

OA: Your host will have to have non-volatile memory to store your golden image should power ever gets cut off from the device otherwise the gauge will reset to defaults.

3. The register set of the BQ27220 looks much like the BQ27210. All coulomb data is read-only and data is not restorable should 3.3V go to 0V and the battery voltage down to 1.8V. This will mandate the use of a boost converter in order to retain coulomb data (TPS61xxx).
OA: Yes. If the chip looses power you will loose all data from RAM.

4. In the register table (page 16 of SLUUBD4), states that the TEMP register is R/W and the ARTTE register is RO. In the BQ27210, TEMP is RO and ARTTE is R/W. I'd assume that SLUUBD4 is incorrect.
OA: You can write the temperature of the battery to the gauge . ATTE is read only. SLUUVD4 is correct.

5. Like the BQ27210, the BQ27220 is going to be in production for at least 5 more years, correct?
OA: Correct

6. Is there any protection for the BAT, SRP, SRN pins should the batteries be put in backwards (or TI recommended external protection)? The TPS61xxx DC:DC boost is protected since the battery voltage and 3.3V are diode-or'd.
OA: Only BAT needs protection..

- It appears that only the BAT pin will need protection if SRP and SRN are used for lo-side current sensing (see datasheet, page 12).
OA: correct. If polarities of SNP and SRN are switched a charge current will have negative polarity and discharge would have positive instead of the other way around

- Should BAT be connected to the 3.3V input instead of + of the battery? I know that the battery voltage (VOLT register) will always read 3.3V.
OA: if you are coulomb counting only, then BAT can be connected to 3.3V only.

1. We started with the BQ34Z100-G1 (BQ34110), but we were told to change course late last week. This was because we couldn't read out the coulomb registers out of BQ34Z100, but we can on the BQ27220. Make sense?
2. How large is the golden image?
6a. If sensing from the lo-side, will SRP and SRN need to be swapped then?
6b. What be the best way to protect BAT from a battery reversal?

ok. Since you just need a coulomb counter, the bq27220 will be ok, Although the bq34110 also has the accumulated charge function as well and would just work as well. The bq34z100 is specially suited for NiMH because of the special taper conditions associated with cells of that type.

2. As long you make your connections such that when charging, the current gets to SRP pin first before going through the sense resistor to the SRN pin, you will be ok, the gauge will measure charge current as positive

b. The easiest method would be to use a schottky in forward bias on the Bat pin, although it would have a voltage drop across it. However, since voltage measurement doesn't seem critical for this application given that you want to connect Bat to a fixed voltage and not the battery voltage then that should be ok.

thanks
Onyx

Thanks Onyx,
1. I'll ask the sales team again regarding the use of the BQ34Z100-G1
2. How large is the "golden image"?
6b. For the BAT pin, how about a small series resistor from the battery pack and a back-biased diode with a decoupling cap at the BAT pin? This way, we can keep the battery voltage reading intact.
7. Can VCC be overridden with an external 1.8V supply?

2. Golden image is a text file which can be extracted using bqstudio and the customer can see what the size is. I don't readily have the number.
6b. Your proposal might also work but would have to be bread boarded and verified by the customer.
7. No. That is the internal LDO output

thanks
Onyx