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BQ76930: Current measuring is inaccurate

Alexander Roussos
Prodigy 70 points
Part Number: BQ76930

Hi there,

I've noticed some inaccurate current measuring values with my implementation of the BQ76930. I've tested different currents from 1A to 20A ( or even 25-30A) with a Fluke Current clamp and a constant current load. The Shunt-resistor is 2mOhms. ( SCD should be 120A, OCD around 80-100A) Usually the device will need 15-20 A with 40A only for 1 sec max.

Here are some values i've measured, does anyone know what could be the reason for that problem?

Kind regards, 

Alex

UNI-T UT139B Multimeter Power supply BQ76930 with 10mOhm Shunt
1,919 1,98 2,021
2,055 2,11 2,165
2,494 2,55 2,631
3,011 3,07 3,185
3,955 4,01 4,199
3,491 3,55 3,714
4,004 4,06 4,269
4,49 4,56 4,189
5,02 5,08 5,392
5,49 5,56 5,935
5,98 6,05 6,516
6,51 6,57 7,091
6,98 7,05 7,634

Agilent Multimeter Power supply BQ76930 with 10mOhm Shunt
0,24 0,29 0,252
0,531 0,58 0,555
1,037 1,09 1,085
1,239 1,29 1,297
1,532 1,59 1,606
1,733 1,79 1,819
2,024 2,08 2,125
2,566 2,62 2,698
Fluke Current Clamp BQ76930 with 2mOhm Shunt
1,08 1,11
2,09 2,165
3,05 3,161
4,01 4,178
5,02 5,25
6,06 6,247
7,08 7,431
8,1 8,529
9,07 9,579
10,29 10,909
11,19 11,841
12,04 12,85
13,11 14,069
14,13 15,365
15,08 16,081
16,09 16,963
17,1 17,754
18,13 19,015
19,08 19,864
20,15 21,151
21,24 21,298
24,98 26,936
30,07 31,566
  • 0
    TI__Guru 54715 points
    Hi Alexander,
    Other than an occasional odd value it looks like you have a calibration error.
    Be sure you have set the CC_CFG register, be sure it is set to 0x19. It is volatile, it must be set each time the part is reset.
    Calibrate with 0 current and a representative current.
    Check your board routing, but any unexpected resistance should calibrate out.
    Check the temperature coefficient of your sense resistor.
  • 0 in reply to WM5295
    Prodigy 70 points
    Hi,
    thanks for your quick answer!
    Well the CC_CFG register is set to 0x19 and my sense resistor should handle the heat.
    I couldn't find any detailed information, how to calibrate the CC, the topic 7.3.1.1.2.1 of the datasheet doesn't help me very much.
    Thanks in advance, Alex.
  • 0 in reply to Alexander Roussos
    TI__Guru 54715 points
    Hi Alexander,
    Calibration is left to the user. The CC_CFG setting should get the zero point close. The output is expected to be linear and the common expectation would be to do an offset and gain calibrations. Subtract out any residual value from your 0 current to be sure the output goes through zero. Then apply some known load and scale the output to that known current.