SLUUCI8 User guide

SLUUCI8 November 2023 BQ76905

5.5 Measurement Calibration

The BQ76905 device includes optional capability for the customer to calibrate offset and gain for the cell voltage measurement, the current measurement, and the internal temperature measurement. It also supports gain calibration for the top-of-stack (TOS) voltage measurement and offset calibration for the TS pin measurement. Table Table 5-11 shows the calibration configuration values provided.

Table 5-11 Calibration Configuration Settings

Name	Description
*Calibration:Voltage:Cell 1 Gain*	Cell 1 voltage gain calibration value in unsigned 16-bit format
*Calibration:Voltage:Cell 2 Gain Delta*	Signed 8-bit value which is added to Cell 1 Gain to obtain the Cell 2 Gain calibration value.
*Calibration:Voltage:Cell 3 Gain Delta*	Signed 8-bit value which is added to Cell 1 Gain to obtain the Cell 3 Gain calibration value.
*Calibration:Voltage:Cell 4 Gain Delta*	Signed 8-bit value which is added to Cell 1 Gain to obtain the Cell 4 Gain calibration value.
*Calibration:Voltage:Cell 5 Gain Delta*	Signed 8-bit value which is added to Cell 1 Gain to obtain the Cell 5 Gain calibration value.
*Calibration:Voltage:TOS Gain*	Top-of-stack (TOS) voltage gain calibration value in unsigned 16-bit format
*Calibration:Current:Curr Offset*	Current offset calibration value in units of signed 24-bit ADC counts (this is a 16-bit value, so only has range of +/- full-scale raw current range / 256)
*Calibration:Current:Curr Gain*	Current gain calibration value in unsigned 16-bit format
*Calibration:Current:CC1 Offset*	CC1 Current offset calibration value in units of signed 24-bit ADC counts (this is a 16-bit value, so only has range of +/- full-scale raw current range / 256)
*Calibration:Current:CC1 Gain*	CC1 Current gain calibration value in unsigned 16-bit format
*Calibration:Temperature:TS Offset*	TS measurement offset calibration value in units of signed 16-bit ADC counts.
*Calibration:Temperature:Int Temp Offset*	Internal temperature offset calibration value in units of signed 16-bit ADC counts
*Calibration:Temperature:Int Temp Gain*	Internal temperature gain calibration value in unsigned 16-bit format

If the host does not overwrite values for the calibration gain or offset configurations, the BQ76905 device uses the factory trim values stored in OTP for each respective setting. When a calibration configuration value is written, the device uses that in place of the respective factory trim value.

Each cell voltage has an associated Cell # Gain value, which is used to scale the raw ADC result to convert it into units of mV. The Cell 1 Gain is stored directly in data memory, while the remaining Cell # Gain values for cells 2 to 5 are encoded as a delta value versus the Cell 1 Gain value, as shown below.

Equation 1.

C e l l 2 G a i n = (C e l l 1 G a i n) + (C e l l 2 G a i n D e l t a)

Equation 2.

C e l l 3 G a i n = (C e l l 1 G a i n) + (C e l l 3 G a i n D e l t a)

Equation 3.

C e l l 4 G a i n = (C e l l 1 G a i n) + (C e l l 4 G a i n D e l t a)

Equation 4.

C e l l 5 G a i n = (C e l l 1 G a i n) + (C e l l 5 G a i n D e l t a)

The BQ76905 uses these calibration values to calculate the reported measurements from the raw ADC data as described below.

Equation 5.

C e l l 1 V o l t a g e () = \frac{[(16 - b i t A D C c o u n t s) \times (C ell 1 Gain)]}{65536}

Equation 6.

C e l l 2 V o l t a g e () = \frac{[(16 - b i t A D C c o u n t s) \times (C ell 2 Gain)]}{65536}

Equation 7.

C e l l 3 V o l t a g e () = \frac{[(16 - b i t A D C c o u n t s) \times (C ell 3 Gain)]}{65536}

Equation 8.

C e l l 4 V o l t a g e () = \frac{[(16 - b i t A D C c o u n t s) \times (C ell 4 Gain)]}{65536}

Equation 9.

C e l l 5 V o l t a g e () = \frac{[(16 - b i t A D C c o u n t s) \times (C ell 5 Gain)]}{65536}

Equation 10.

S t a c k V o l t a g e () = \frac{(16 - b i t A D C c o u n t s) \times (TOS Gain)}{16384}

Equation 11.

T S M e a s u r e m e n t () = (16 - b i t A D C c o u n t s) - (TS Offset)

Equation 12.

I n t T e m p e r a t u r e () = \frac{[(16 - b i t A D C c o u n t s) - (Int Temp Offset)] \times (Int Temp Gain)}{65536}

Equation 13.

C C 1 C u r r e n t () = \frac{[(16 - b i t A D C c o u n t s) - (CC1 Offset / 256)] \times (CC1 Gain)}{32}

Equation 14.

C u r r e n t () = \frac{[(24 - b i t A D C c o u n t s) - (Curr Offset)] \times (Curr Gain)}{8192}

The cell voltage calibration is implemented by writing the Calibration:Voltage:Cell 1 Gain value to 65535 and all Calibration:Voltage:Cell # Gain Delta values to 0, then applying two precise DC voltages to a cell input (such as 2.5 V and 4.5 V) and averaging multiple raw ADC measurements in 16-bit counts obtained from the Cell # Voltage() command for each voltage. The gain is then calculated as:

Equation 15.

C e l l # G a i n = \frac{65536 \times (V o l t a g {e 1}_{i n m V} - {V o l t a g e 2}_{i n m V})}{{(16 - b i t A D C C o u n t s)}_{V o l t a g e 1} - {(16 - b i t A D C C o u n t s)}_{V o l t a g e 2}}

The calibration for the TOS measurement gain is implemented by similarly writing the Calibration:Voltage:TOS Gain value to 16384, then applying two precise DC voltages to the VC5 pin and averaging multiple raw ADC measurements in 16-bit counts obtained from the 0x26 Stack Voltage() command. Note: one of the voltages can be skipped to reduce calibration time. The gain is then calculated as:

Equation 16.

T O S G a i n = \frac{16384 \times (V o l t a g {e 1}_{i n m V} - {V o l t a g e 2}_{i n m V})}{{(16 - b i t A D C C o u n t s)}_{V o l t a g e 1} - {(16 - b i t A D C C o u n t s)}_{V o l t a g e 2}}

The calibration for the TS pin measurement offset is implemented by similarly first writing the Calibration:Temperature:TS Offset value to zero. A precise DC voltage can be applied to the TS pin, and the 16-bit ADC counts obtained from the 0x6A TS Measurement() command. The offset is then calculated as:

Equation 17.

T S O f f s e t = {(16 - b i t A D C C o u n t s)}_{T S_V o l t a g e} - \frac{32768 \times 3 \times (T S_V o l t a g e_{i n V})}{V R E F 1 \times 5}

Although it is typically not practical in a production environment, the Internal Temperature measurement can be calibrated by first writing the Calibration:Temperature:Int Temp Gain value to 65535 and Calibration:Temperature:Int Temp Offset value to zero, then exposing the system to two precise temperatures and averaging multiple raw ADC measurements in 16-bit counts obtained from the Int Temperature() command for each temperature. The gain is then calculated as:

Equation 18.

I n t T e m p G a i n = \frac{65536 \times (T e m p {_1}_{i n ° C} - {T e m p_2}_{i n ° C})}{{(16 - b i t A D C C o u n t s)}_{T e m p_1} - {(16 - b i t A D C C o u n t s)}_{T e m p_2}}

The offset for the Internal Temperature measurement is calculated using this new value of Calibration:Temperature:Int Temp Gain as shown below:

Equation 19.

I n t T e m p O f f s e t = {(16 - b i t A D C C o u n t s)}_{T e m p_1} - \frac{65536 \times ({T e m p_1}_{i n ° C})}{I n t T e m p G a i n}

The 0x3A Current() current measurement can be calibrated by applying two precise DC currents and averaging multiple raw ADC measurements in 24-bit counts obtained from the 0x36 Raw Current() command for each current. The 0x3C CC1 Current() current measurement, which is used to calculate the charge integration data available from the 0x0004 PASSQ() command, can be calibrated by applying two precise DC currents and averaging multiple raw ADC measurements in 16-bit counts obtained from the 0x3C CC1 Current() command for each current, while the value of Calibration:Current:CC1 Gain is set to 32.

These gains are then calculated as:

Equation 20.

C C 1 G a i n = \frac{32 \times (C u r r e n t {_1}_{i n m A} - {C u r r e n t_2}_{i n m A})}{{(16 - b i t A D C C o u n t s)}_{C u r r e n t_1} - {(16 - b i t A D C C o u n t s)}_{C u r r e n t_2}}

Equation 21.

C u r r G a i n = \frac{8192 \times (C u r r e n t {_1}_{i n m A} - {C u r r e n t_2}_{i n m A})}{{(24 - b i t A D C C o u n t s)}_{C u r r e n t_1} - {(24 - b i t A D C C o u n t s)}_{C u r r e n t_2}}

then quantizing to an integer.

While a current measurement offset Calibration:Current:Curr Offset can be calculated using one of the two currents measured earlier, a more practical approach is to set the current to zero, and use the average of multiple raw ADC measurements in 24-bit counts obtained from the 0x36 Raw Current() command as Calibration:Current:Curr Offset, which uses the 16 LSBs from this calculation. The value for Calibration:Current:CC1 Offset can be obtained by setting the Calibration:Current:CC1 Gain to 32 and summing 256 samples of 0x3C CC1 Current().

A table of example gains and resulting LSB values for various sense resistor values and maximum current levels is shown below (with gains quantized to ensure support for the Max Current listed).

Table 5-12

Rsense (μΩ)	Max Current (A)	Max Voltage (V)	Min LSB Allowed (mA)	Quantized CC1 Gain	Quantized Current Gain	Quantized CC1 LSB (mA)	Quantized Current LSB (mA)
5000	3	0.015000	0.091553	524	524	0.091618	0.091618
5000	15	0.075000	0.457764	104	104	0.461614	0.461614
5000	40	0.200000	1.220703	39	39	1.230970	1.230970
2000	10	0.020000	0.305176	393	393	0.305393	0.305393
2000	50	0.100000	1.525879	78	78	1.538712	1.538712
2000	100	0.200000	3.051758	39	39	3.077424	3.077424
1000	32.767	0.032767	0.999969	240	240	1.000163	1.000163
1000	50	0.050000	1.525879	157	157	1.528911	1.528911
1000	100	0.100000	3.051758	78	78	3.077424	3.077424
1000	200	0.200000	6.103516	39	39	6.154848	6.154848
500	32.767	0.016384	0.999969	480	480	1.000163	1.000163
500	100	0.050000	3.051758	157	157	3.057822	3.057822
500	250	0.125000	7.629395	62	62	7.743196	7.743196
500	400	0.200000	12.207031	39	39	12.309696	12.309696
250	32.767	0.008192	0.999969	960	960	1.000163	1.000163
250	100	0.025000	3.051758	314	314	3.057822	3.057822
250	250	0.062500	7.629395	125	125	7.681250	7.681250
250	800	0.200000	24.414063	39	39	24.619391	24.619391
100	32.767	0.003277	0.999969	2400	2400	1.000163	1.000163
100	100	0.010000	3.051758	786	786	3.053932	3.053932
100	200	0.020000	6.103516	393	393	6.107864	6.107864
100	327.67	0.032767	9.999695	240	240	10.001628	10.001628
100	500	0.050000	15.258789	157	157	15.289112	15.289112
100	1000	0.100000	30.517578	78	78	30.774239	30.774239
100	2000	0.200000	61.035156	39	39	61.548478	61.548478
50	32.767	0.001638	0.999969	4800	4800	1.000163	1.000163
50	100	0.005000	3.051758	1573	1573	3.051991	3.051991
50	250	0.012500	7.629395	629	629	7.632403	7.632403
50	1000	0.050000	30.517578	157	157	30.578225	30.578225
50	2000	0.100000	61.035156	78	78	61.548478	61.548478
50	4000	0.200000	122.070313	39	39	123.096955	123.096955
10	32.767	0.000328	0.999969	24003	24003	1.000038	1.000038
10	100	0.001000	3.051758	7865	7865	3.051991	3.051991
10	250	0.002500	7.629395	3146	3146	7.629977	7.629977
10	1000	0.010000	30.517578	786	786	30.539321	30.539321
10	5000	0.050000	152.587891	157	157	152.891123	152.891123
10	10000	0.100000	305.175781	78	78	307.742388	307.742388
10	20000	0.200000	610.351563	39	39	615.484776	615.484776