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System Present Operation

BATTERY PARAMETER MEASUREMENTS

Charge and Discharge Counting

Voltage

Current

Auto Calibration

Temperature

COMMUNICATIONS

SMBus On and Off State

bq20z80

bq20z80A

SLUS782 – JULY 2007

FEATURE SET (continued)

The performance of the internal oscillator depends on the tolerance of the 100-k

Ω resistor between RSOC (pin

33) and VSSA (pin 34). Choose a resistor with a tolerance of

±0.1%, and 50-ppm or better temperature drift.

Place this resistor as close as possible to the bq20z80. If a 12-pF crystal is used, place it as close as possible to

the XCK1 (pin 34) and XCK2 (pin 33) pins. If not properly implemented, the PCB layout in this area can degrade

oscillator performance.

The bq20z80 pulls the PU pin high periodically (1 s). Connect this pin to the PRES pin of the bq20z80 via a

resistor of approximately 5 k

Ω. The bq20z80 measures the PRES input during the PU-active period to determine

its state. If PRES input is pulled to ground by external system, the bq20z80 detects this as system present.

The bq20z80 uses an integrating delta-sigma analog-to-digital converter (ADC) for current measurement, and a

second delta-sigma ADC for individual cell and battery voltage, and temperature measurement.

The integrating delta-sigma ADC measures the charge/discharge flow of the battery by measuring the voltage

drop across a small-value sense resistor between the SR1 and SR2 pins. The integrating ADC measures bipolar

time, using an internal counter. The fundamental rate of the counter is 0.65 nVh.

The bq20z80 updates the individual series cell voltages through the bq29312A at one second intervals. The

bq20z80 configures the bq29312A to connect the selected cell, cell offset, or bq29312A VREF to the CELL pin

of the bq29312A, which is required to be connected to VIN of the bq20z80. The internal ADC of the bq20z80

measures the voltage, scales and calibrates it appropriately. This data is also used to calculate the impedance

of the cell for the Impedance Track™ gas-gauging.

The bq20z80 uses the SR1 and SR2 inputs to measure and calculate the battery charge and discharge current

using a 5 m

Ω to 20 mΩ (typical) sense resistor.

The bq20z80 provides an auto-calibration feature to cancel the voltage offset error across SR1 and SR2 for

maximum charge measurement accuracy. The bq20z80 performs auto-calibration when the SMBus lines stay

low continuously for a minimum of 5 s.

The bq20z80 TS1 and TS2 inputs, in conjunction with two identical NTC thermistors (default are Semitec

103AT), measure the battery environmental temperature. The bq20z80 can also be configured to use its internal

temperature sensor.

The bq20z80 uses SMBus v1.1 with Master Mode and package error checking (PEC) options per the SBS

specification.

The bq20z80 detects an SMBus off state when SMBC and SMBD are logic-low greater than an adjustable

period of time. Clearing this state requires either SMBC or SMBD to transition high. Within 1 ms, the

communication bus is available.

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