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BQ2040SN Datasheet(PDF) 10 Page - Texas Instruments |
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BQ2040SN Datasheet(HTML) 10 Page - Texas Instruments |
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10 / 36 page  Error Summary Capacity Inaccurate The FCC is susceptible to error on initialization or if no updates occur. On initialization, the FCC value includes the error between the design capacity and the actual ca- pacity. This error is present until a qualified discharge occurs and FCC is updated (see the DCR description). The other cause of FCC error is battery wear-out. As the battery ages, the measured capacity must be adjusted to account for changes in actual battery capacity. Periodic qualified discharges from full to empty will minimize er- rors in FCC. Current-Sensing Error Table 3 illustrates the current-sensing error as a func- tion of VSR. A digital filter eliminates charge and dis- charge counts to the RM register when -VSRD <VSRO < +VSRD. Display The bq2040 can directly display capacity information us- ing low-power LEDs. The bq2040 displays the battery charge state in either absolute or relative mode. In rela- tive mode, the battery charge is represented as a per- centage of the FCC. Each LED segment represents 25% of the FCC. In absolute mode, each segment represents a fixed amount of charge, 25% of the DesignCapacity. As the battery wears out over time, it is possible for the FCC to be below the design capacity. In this case, all of the LEDs may not turn on in absolute mode, representing the reduction in the actual battery capacity. When DISP is tied to VCC, the LED1-4 outputs are inac- tive. When DISP is left floating, the display becomes ac- tive whenever the bq2040 detects a charge rate of 100mA or more. When pulled low, the segment outputs become active immediately for a period of approximately 4 seconds. The DISP pin must be returned to float or VCC to reactivate the display. LED1 blinks at a 4Hz rate indicating a low battery con- dition whenever the display is active, EDVF is not set, and Remaining_Capacity_Alarm is set. VSB below EDVF (EDVF = 1) disables the display output. Microregulator The bq2040 can operate directly from three nickel chem- istry cells. To facilitate the power supply requirements of the bq2040, an REF output is provided to regulate an external low-threshold n-FET. A micropower source for the bq2040 can be built inexpensively using a 2N7002 or BSS138 FET and an external resistor. (See Figure 1.) The value of R11 depends on the battery pack’s nominal voltage. Communicating With the bq2040 The bq2040 includes a simple two-pin (SMBC and SMBD) bi-directional serial data interface. A host proc- essor uses the interface to access various bq2040 regis- ters; see Table 4. This method allows battery character- istics to be monitored easily. The open-drain SMBD and SMBC pins on the bq2040 are pulled up by the host sys- tem, or may be connected to VSS, if the serial interface is not used. The interface uses a command-based protocol, where the host processor sends the battery address and an eight- bit command byte to the bq2040. The command directs the bq2040 to either store the next data received to a register specified by the command byte or output the data specified by the command byte. bq2040 Data Protocols The host system, acting in the role of a Bus master, uses the read word and write word protocols to communicate integer data with the bq2040. (See Figure 3). Host-to-bq2040 Message Protocol The Bus Host communicates with the bq2040 using one of three protocols: n Read word n Write word 10 bq2040 Symbol Parameter Typical Maximum Units Notes VOS Offset referred to VSR ± 75 ± 150 µV DISP = VCC. INL Integrated non-linearity error ± 1 ± 4 % Add 0.1% per °C above or below 25°C and 1% per volt above or below 4.25V. INR Integrated non- repeatability error ± 0.5 ± 1 % Measurement repeatability given similar operating conditions. Table 3. bq2040 Current-Sensing Errors Not Recommended For New Designs |
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