BQ76920 for Ultracapacitors

Digant Dalal

Prodigy 110 points

Other Parts Discussed in Thread: BQ76920

Hello there,

I am just curious whether one can monitor 3 Ultracaps in series with bq76920.

Apart from errors from ADC at lower cell voltage, is there any significant risk to the device?

Thanks,

Digant

over 9 years ago

0 WM5295 over 9 years ago

TI__Guru 54715 points

In addition to the individual cell input voltage, be sure to look at the total supply voltage. If your caps are low voltage you may below the operating voltage of the device.

0 Digant Dalal over 9 years ago in reply to WM5295

Prodigy 110 points

I have 3 of 2.5 V charged supercaps in series, so total 7.5V across the pack. The problem is that I am not able to clear UV fault from system register. Is there any way to lower UV limit from device?

0 Vish Nadarajah over 9 years ago in reply to Digant Dalal

TI__Expert 6085 points

The programmable range of the UV threshold is 1.58V to 3.1V. the UV threshold is stored in the UV_TRIP register.
refer to d/s pp25. the ADC reads out conform to the equation , Vcell =Gain x ADC(cell) +offset; spec in d/s UVrange = 0x1000 ---> 0x1FF0. typical ADC Gain ~ 378uV/LSB and ADC offset 43mV

0 Digant Dalal over 9 years ago in reply to Vish Nadarajah

Prodigy 110 points

Hi Vish,
Thanks!
I need help for coulomb counter. I am having values in uV as per datasheet. Now how can I translate it into current. I know it sounds like a stupid question but is it really I = V/R translation of this voltage I get.
I am currently using bq76920 EVM and developing an interface in simulink to read data over i2c.
regards,
Digant

0 Vish Nadarajah over 9 years ago in reply to Digant Dalal

TI__Expert 6085 points

Hi Digant:
Current = CC value *CC LSB / sense resistor ; LSB ~ 8.44uV... note....CC is not to calculate instantaneous current. CC gives average over 250mS integration period. CC recommended range is +/-200mV . Positive value are CHARGE CURRENT; Neg values are DIS CHARGE current. Data is 2's complement, so NEG current has the highest bit set.

0 Digant Dalal over 9 years ago in reply to Vish Nadarajah

Prodigy 110 points

Hi Vish,
Thanks again for the reply. Yes, I understand . However, the value I get after CCvalue(already 2's complement)*8.44uv/0.001ohm is average over 250ms period. so, what is the unit of this quantity which I get it after calculation? uA?
Thanks again
regards,
Digant

0 Digant Dalal over 9 years ago in reply to Digant Dalal

Prodigy 110 points

Hello Vish,

This is how my circuit for 3 series super-capacitor looks like.

Can you verify please. The one on right is DC/DC voltage regulator.

Thanks and best regards,

Digant

0 Vish Nadarajah over 9 years ago in reply to Digant Dalal

TI__Expert 6085 points

Current units is mA. i.e [ CC value * CC LSB value ]---> micro Volts. sense resistor----> m Ohm
I = uV/mohm = [ 1E-6]/[1E-3] =mA.......

0 Digant Dalal over 9 years ago in reply to Vish Nadarajah

Prodigy 110 points

Hi Vish,
Do i need to subtract ADC offset as stored in 0X51?
The value I am getting seems to have some sort of OFFSET. It is difficult to interpret the value given by CC.
Thanks in advance.
BR,
Digant

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BQ76920 for Ultracapacitors