BQ34Z100 Datasheet, PDF (26/50 Pages) Texas Instruments – Wide Range Fuel Gauge with Impedance Track™ Technology

bq34z100
SLUSAU1 – MAY 2012
VOLTAGE DIVISION AND CALIBRATION
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The bq34z100 is shipped with factory configuration for the default case of 1 series Li-Ion cell. This can be
changed by setting the VOLTSEL bit in the Pack Configuration register and by setting the number of series cells
in the data flash configuration section.
Multi-cell applications, with voltages up to 65535 mV may be gauged by using the appropriate input scaling
resistors such that the maximum battery voltage, under all conditions, appears at the BAT input as approximately
900 mV. The actual gain function is determined by a calibration process and the resulting voltage calibration
factor is stored in the data flash location Voltage Divider.
For single-cell applications, an external divider network is not required. Inside the IC, behind the BAT pin is a
nominal 5:1 voltage divider with 88 KΩ in the top leg and 22 KΩ in the bottom leg. This internal divider network is
enabled by clearing the VOLTSEL bit in the Pack Configuration register. This ratio is optimum for directly
measuring a single cell Li-Ion cell where charge voltage is limited to 4.5 V.
For higher voltage applications, an external resistor divider network should be implemented as per the reference
designs in this document. The quality of the divider resistors is very important to avoid gauging errors over time
and temperature. It is recommended to use 0.1% resistors with 25ppm temperature coefficient. Alternately, a
matched network could be used that tracks its dividing ratio with temperature and age due to the similar
geometry of each element. Calculation of the series resistor can be made per the equation below. Note that
exceeding Vin max mV will result in a measurement with degraded linearity.
The bottom leg of the divider resistor should be in the range of 15 KΩ to 25 KΩ. Assuming we will use 16.5 KΩ:
Rseries = 16500 Ω (Vin max mV – 900 mV) / 900 mV
For all applications, the Voltage Divider value in data flash will be used by the firmware to calibrate the total
divider ratio. The nominal value for this parameter is the maximum expected value for the stack voltage. The
calibration routine adjusts the value to force the reported voltage to equal the actual applied voltage.
1S EXAMPLE
For stack voltages under 4.5 volts max, it is not necessary to provide an external voltage divider network. The
internal 5:1 divider should be selected by clearing the VOLTSEL bit in the Pack Configuration register. The
default value for Voltage Divider is 5000 (representing the internal 5000:1000 mV divider) when no external
divider resistor is used, and the default number of series cells = 1. In the 1S case, there is usually no
requirement to calibrate the voltage measurement, since the internal divider is calibrated during factory test to
within 2 mV.
7S EXAMPLE
In the multi-cell case, the hardware configuration is different. An external voltage divider network is calculated
using the Rseries formula above. The bottom leg of the divider should be in the range of 15 KΩ to 25 KΩ. For
more details on configuration, see DESIGN STEPS.
AUTOCALIBRATION
The bq34z100 provides an autocalibration feature that will measure the voltage offset error across SRP and SRN
from time-to-time as operating conditions change. It subtracts the resulting offset error from normal sense
resistor voltage, V_SR, for maximum measurement accuracy.
The gas gauge performs a single offset calibration when (1) the interface lines stay low for a minimum of Bus
Low Time and (2) Vsr > Deadband.
The gas gauge also performs a single offset when (1) the condition of AverageCurrent() ≤ Autocal Min Current
and (2) {voltage change since last offset calibration ≥ Delta Voltage} or {temperature change since last offset
calibration is greater than Delta Temperature for ≥ Autocal Time}.
Capacity and current measurements should continue at the last measured rate during the offset calibration when
these measurements cannot be performed. If the battery voltage drops more than Cal Abort during the offset
calibration, the load current has likely increased considerably; hence, the offset calibration will be aborted.
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