Cypress AN2309, CY8C29x66, CY8C24794 specifications Battery Measurement, R15

AN2309

Figure 6. PSoC Internal User Module Configuration

Battery Measurement

To provide a correct implementation of the charge and cell- balancing algorithms, the charge current, battery voltage and temperature must be measured accurately.

These three parameters are measured as the voltage drops on corresponding resistors by using the instrumental amplifier INA. The measurement is implemented as a two- stage procedure to eliminate any voltage offset from the INA and ADC inputs. The INA inputs are shorted together in the first stage. This state is used to measure INA and ADC offset voltage. Then the real signal is measured. At this point the difference between the ADC codes corresponding to the first and second stages is directly proportional to the battery measurement parameter without the influence of the INA and ADC offset voltage.

To transform the battery current (voltage drop on the current-sense resistor) and battery voltage into levels suitable for PSoC signals, precise resistive dividers are used. To limit the current flow from the battery to the powered-down battery charger, divider resistors of large nominal resistance are employed.

To provide higher current measurement accuracy, a current- sense resistor was put in the pack current path close to the negative battery voltage. In this case, the voltage drop on the resistive divider (R15, R16, R20, and R21) is independent of the battery pack voltage level. This is not true if a current-sense resistor is placed close to the positive voltage. At the beginning of the charging process, the voltage bias on the current-sense resistor is measured and during subsequent processes it is subtracted from the measured values. In this way, the difference between resistor values in the resistive divider is partly compensated.

The following equation represents the current measurement scheme:

VADC			Gina I Ibat Rsense	Equation 16
n nmax		nmax
	V		V
	ref		ref

The value n is the ADC code without the influence of the INA and ADC offset voltage and without the voltage bias on the current-sense resistor ( n nmeas noffset nbias ).

The value nmax is the maximum ADC code, which is equal to 2048 for the 12-bit incremental ADC in bipolar mode.

The value Ibat is the battery current, Gina is INA gain (4), Vref is the bandgap reference voltage (1.3V), and I is the resistive divider coefficient (0,833333):

1

I 1 R20Equation 17

R15

November 25, 2007 Document No. 001-17394 Rev. *B - 9 -

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