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Coulomb Counter Operation

SN8040

SN8045

Cool-GG Programmable Battery Management IC

SLUS753B – JANUARY 2007 – REVISED FEBRUARY 2008

The coulomb counter peripheral is a single-channel, over-sampled converter with continuous sampling and

is optimized for tracking charge and discharge activity of a rechargeable battery. The coulomb counter

(CC) in normal operation is configured to convert the differential voltage across pins GSRP and GSRN.

The coulomb counter design allows the integration of successive samples with no loss or corruption of

data. This integration further reduces quantization error over time, thereby increasing system resolution.

The coulomb counter can be turned on or off under firmware control, enabling greater power efficiency.

Coulombs are counted by integrating the voltage across a sense resistor placed in series with the

negative terminal of a battery. After a power-on reset, the default state of the CC peripheral is off. The CC

peripheral is enabled by setting the CC_ON bit to one in the CCCTL register. The CC inputs, GSRP and

GSRN, are configured for normal operation by writing CC_CAL to a zero in the CCCTL register. The

conversions begins automatically after CC_ON is set to a one. The converter asserts the CCF flag bit in

the PFLAGS register once every 250 ms. An interrupt is generated when CCF is set, assuming CCIE =

PIE = GIE = 1. CCLO and CCHI are then read and CCF cleared before another 250 ms have elapsed,

avoiding corruption or data loss when the registers are updated following the next conversion.

The data in CCHI-CCLO are stored in 2's complement format. Positive Full scale corresponds to 7FFFh,

and negative Full scale corresponds to 8000h. Full scale is referred to approximately VREF divided by four

(typically 3.969). For a typical Vref of 1.225 V, the complete input range extends from +309 mV to –309

mV. However, in order to preserve the specified accuracy, this range is limited to less than 80% of its

overall span. Thus the allowable range of voltages for GSRN and GSRP is – 0.25 V to 0.25 V, and the

differential voltage between the two inputs is limited to less than 0.25 V. The least significant bit (LSB) for

a simple conversion is 9.419 µV or 0.654 nVh. The charge LSB is determined by dividing 0.654 nVh by

the value of the sense resistor. Resolution is improved by summing (integrating) consecutive samples

since no information is lost between conversions.

Calibration cycles can be performed at any time. To perform calibration if the coulomb counter is off, set

CC_CAL and CC_ON to one. If the coulomb counter is on, and the conversion result is desired, wait until

the conversion is complete and then read the data in CCLO and CCHI. After the data are read, turn the

coulomb counter off by writing CC_ON to zero. This ensures that the converter is reset, and another

calibration cycle can then be performed by setting CC_CAL and CC_ON to one. If the coulomb counter is

on and the conversion result is not wanted, simply turn off the coulomb counter by writing CC_ON to zero.

Calibrations are then performed by setting CC_CAL and CC_ON to one. Averaging of successive

conversion results improves the offset resolution and better cancels the offset. One of several offset

cancellation algorithms is chosen, depending upon the required performance and operating conditions. A

conversion can start immediately after waking from sleep. For dc inputs, a conversion error less than one

LSB can be achieved.

To save power, turn the converter off by setting CC_ON to a logic zero.

Coulomb Counter Operation

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