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BQ26501 Datasheet(PDF) 6 Page - Texas Instruments |
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BQ26501 Datasheet(HTML) 6 Page - Texas Instruments |
6 / 33 page bq26501 SLUS586B − OCTOBER 2003 − REVISED MARCH 2004 6 www.ti.com 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 RS in Figure 2). Available capacity is reported with a resolution of 0.003/RS(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, RS, 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-k Ω pull-up to VCC 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. Not Recommended For New Designs |
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