BQ26501_16 Datasheet, PDF (6/33 Pages) Texas Instruments – HANDHELD APPLICATIONS

bq26501
Not Recommended For New Designs
ą
SLUS586B − OCTOBER 2003 − REVISED MARCH 2004
APPLICATION INFORMATION
FUNCTIONAL DESCRIPTION
The bq26501 determines battery capacity by monitoring the amount of charge input to or removed from a Li-Ion
or Li-Pol battery. The bq26501 measures discharge and charge currents, monitors the battery for low voltage
thresholds, and compensates for temperature and self-discharge rate. Current is measured across a small
value series resistor between the negative terminal of the battery and the pack ground (see R_S in Figure 2).
Available capacity is reported with a resolution of 0.003/R_S(mAh). Time-to-empty (TTE) reporting in minutes
at host-provided at-rate currents allow the requirements for host based calculations to be greatly reduced or
eliminated; reading a single register pair provides useful and meaningful information to the application’s end
user.
Figure 2 shows a typical application circuit. Differential sense of the voltage across the current sense resistor,
R_S, improves device performance, leading to an improvement in reported time-to-empty accuracy. In the typical
application, the GPIO pin can be used as a general-purpose programmable I/O port. An internal pull-down on
the HDQ line ensures that the device detects a logic “0” on the HDQ line and automatically enters the low power
sleep mode when the system power is switched off or the pack is removed. A 100-kΩ pull-up to V_CC can be
added to the HDQ line to disable this feature. The bq26501 can operate directly from a single Li-Ion or Li-Pol
cell.
Measurements
As shown in the Figure 3, the bq26501 uses a fully differential, dynamically balanced voltage-to-frequency
converter (VFC) for charge and discharge counting and an analog-to-digital converter (ADC) for battery voltage
and temperature measurement. Both VFC and ADC are automatically compensated for offset. No user
calibration or compensation is required.
Charge and Discharge Counting
The bq26501 uses a voltage-to-frequency converter (VFC) to perform a continuous integration of the voltage
waveform across a small value sense resistor in the negative lead of the battery, as shown in Figure 2. The
integration of the voltage across the sense resistor is the charge added or removed from the battery. Since the
VFC directly integrates the waveform, the shape of the current waveform through the sense resistor has no
effect on the measurement accuracy. The low-pass filter that feeds the sense resistor voltage to the bq26501
SRP and SRN inputs serves to filter out system noise and does not affect the measurement accuracy, since
the low-pass filter does not change the integrated value of the waveform.
Offset Calibration
The offset voltage of the VFC measurement must be very low to be able to measure small signal levels
accurately. The bq26501 provides an auto-compensation feature to cancel the internal voltage offset error
across SRP and SRN for maximum charge measurement accuracy.
NOTE:NO CALIBRATION IS REQUIRED. See the Layout Considerations section for details on
minimizing PCB induced offset across the SRP and SRN pins.
6
www.ti.com