BQ40z60 EVM charging voltage feedback tolerance issues.

Tony Carter

Other Parts Discussed in Thread: BQ40Z60

After trying to set the charging voltage for a 2S2P to 8400mV to within 100mV (50mV per cell) doesn't seem likely without some type of trimming. Adding up worst case using 1% resistors and the 2% VFB tolerance this could add up to almost 4% around 300mV on 8400. Please someone show me I'm wrong, or an easy way to trim it.

The second problem the FB network on the EVM using 1% resistors can exceed the 12200 mV limint using 4200mV cells. This can be 1235mV in my case and I will need to change them to prevent this..

over 8 years ago

Tony Carter over 8 years ago

Intellectual 360 points

Is this trimmed using Pack Gain Setting?

ThomasCosby over 8 years ago in reply to Tony Carter

TI__Guru 73830 points

yes

ThomasCosby over 8 years ago

TI__Guru 73830 points

Tony
I have an application note on the bq40z60 website that Charging Voltage Compensation. You can try the steps in that document to see whether you can improve the accuracy.
Tom

Tony Carter over 8 years ago in reply to ThomasCosby

Intellectual 360 points

Thomas, I have read this and used it to correct compensation. I thought my issue was I needed to calibrate the Pack Gain but when I did the charge voltage was too high and charge current started surging from full to off about every second. I noticed that the VFB was exceeding 1220mV when it was doing this which caused me to look at the feedback circuit worst case tolerances and came up with a charger voltage of 8400mV and the EVM resistor divider at worst case would provide at VFB voltage of 1% of 1237mV which would exceed spec then throw in the 1220-2% = 1195.6mV. I'm going to increase R1 to 357K to prevent from exceeding this input spec.

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BQ40z60 EVM charging voltage feedback tolerance issues.