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BQ2050SN Datasheet(PDF) 9 Page - Texas Instruments |
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BQ2050SN Datasheet(HTML) 9 Page - Texas Instruments |
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9 / 21 page  Error Summary Capacity Inaccurate The LMD is susceptible to error on initialization or if no updates occur. On initialization, the LMD value in- cludes the error between the programmed full capacity and the actual capacity. This error is present until a valid discharge occurs and LMD is updated (see the DCR description on page 7). The other cause of LMD er- ror is battery wear-out. As the battery ages, the meas- ured capacity must be adjusted to account for changes in actual battery capacity. A Capacity Inaccurate counter (CPI) is maintained and incremented each time a valid charge occurs (qualified by NAC; see the CPI register description) and is reset whenever LMD is updated from the DCR. The counter does not wrap around but stops counting at 255. The ca- pacity inaccurate flag (CI) is set if LMD has not been up- dated following 64 valid charges. Current-Sensing Error Table 5 illustrates the current-sensing error as a func- tion of VSRO. A digital filter eliminates charge and dis- charge counts to the NAC register when VSRO is between VSRQ and VSRD. Communicating With the bq2050 The bq2050 includes a simple single-pin (DQ plus re- turn) serial data interface. A host processor uses the in- terface to access various bq2050 registers. Battery char- acteristics may be easily monitored by adding a single contact to the battery pack. The open-drain DQ pin on the bq2050 should be pulled up by the host system, or may be left floating if the serial interface is not used. The interface uses a command-based protocol, where the host processor sends a command byte to the bq2050. The command directs the bq2050 to either store the next eight bits of data received to a register specified by the command byte or output the eight bits of data specified by the command byte. The communication protocol is asynchronous return-to- one. Command and data bytes consist of a stream of eight bits that have a maximum transmission rate of 333 bits/sec. The least-significant bit of a command or data byte is transmitted first. The protocol is simple enough that it can be implemented by most host proces- sors using either polled or interrupt processing. Data input from the bq2050 may be sampled using the pulse- width capture timers available on some microcontrol- lers. If a communication error occurs, e.g. tCYCB > 6ms, the bq2050 should be sent a BREAK to reinitiate the serial interface. A BREAK is detected when the DQ pin is driven to a logic-low state for a time, tB or greater. The DQ pin should then be returned to its normal ready- high logic state for a time, tBR. The bq2050 is now ready to receive a command from the host processor. The return-to-one data bit frame consists of three dis- tinct sections. The first section is used to start the transmission by either the host or the bq2050 taking the DQ pin to a logic-low state for a period, tSTRH,B. The next section is the actual data transmission, where the data should be valid by a period, tDSU, after the negative edge used to start communication. The data should be held for a period, tDV, to allow the host or bq2050 to sample the data bit. The final section is used to stop the transmission by re- turning the DQ pin to a logic-high state by at least a pe- riod, tSSU, after the negative edge used to start commu- nication. The final logic-high state should be held until a period, tSV, to allow time to ensure that the bit trans- mission was stopped properly. The timings for data and break communication are given in the serial communi- cation timing specification and illustration sections. Communication with the bq2050 is always performed with the least-significant bit being transmitted first. Figure 3 shows an example of a communication sequence to read the bq2050 NAC register. 9 bq2050 Symbol Parameter Typical Maximum Units Notes INL Integrated non-linearity error ± 2 ± 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 ± 1 ± 2 % Measurement repeatability given similar operating conditions. Table 6. bq2050 Current-Sensing Errors |
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