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BQ2954SNG4 Datasheet(PDF) 9 Page - Texas Instruments |
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BQ2954SNG4 Datasheet(HTML) 9 Page - Texas Instruments |
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9 / 23 page  minimum current threshold, IMIN. The IFULL and IMIN thresholds are programmed using the ITERM input pin (See Table 4.) Figures 4 and 5 show the bq2954 configured for display mode 2 and IFULL =IMAX/5 while IMIN =IMAX/10. Voltage and Current Monitoring In low-side current sensing, the bq2954 monitors the battery pack voltage as a differential voltage between BAT and pins. In high-side current sensing, the bq2954 monitors the battery pack voltage as a differential volt- age between BAT and VSS pins. This voltage is derived by scaling the battery voltage with a voltage divider. (See Figures 6 and 7.) The resistance of the voltage di- vider must be high enough to minimize battery drain but low enough to minimize noise susceptibility. RB1 + RB2 is typically between 150k Ω and 1MΩ. The volt- age-divider resistors are calculated from the following: RB1 RB2 NV V CELL REG = ∗ − 1 (1) where VCELL = Manufacturer-specified charging cell voltage N = Number of cells in series VREG = 2.05V The current sense resistor, RSNS (see Figures 6 and 7), determines the fast-charge current. The value of RSNS is given by the following: R V I SNS MAX = 025 . (2) where IMAX is the current during the constant-current phase of the charge cycle. (See Table 1.) Battery Insertion and Removal VBAT is interpreted by the bq2954 to detect the presence or absence of a battery. The bq2954 determines that a battery is present when V BAT is between the High-Voltage Cutoff (VHCO =VREG + 0.25V) and the Low-Voltage Cutoff (VLCO = 0.8V). When VBAT is outside this range, the bq2954 determines that no battery is present and transitions to the battery test state, testing for valid battery voltage. The bq2954 detects battery re- moval when VBAT falls below VLCO. The BTST pin is driven high during battery test and can activate an exter- nal battery contact pull-up. This pull-up may be used to activate an over-discharged Li-Ion battery pack. The VHCO limit implicitly serves as an over-voltage charge fault. The CHG output can be used to disconnect capacitors from the regulation circuitry in order to quickly detect a battery-re- moved condition. Battery insertion is detected within 500ms. Transition to the fast-charge phase, however, will not occur for time tHO (approximately one second), even if voltage qualifi- cation VMIN is reached. This delay prevents a voltage spike at the BAT input from causing premature entry into the fast-charge phase. It also creates a delay in detection of battery removal if the battery is removed during this hold-off period. Temperature Monitoring Temperature is measured as a differential voltage be- tween TS and BAT-. This voltage is typically generated by a NTC (negative temperature coefficient) thermistor and thermistor linearization network. The bq2954 com- pares this voltage to its internal threshold voltages to determine if charging is allowed. These thresholds are the following: I High-Temperature Cutoff Voltage: VTCO = 0.4 ∗ VCC This voltage corresponds to the maximum temperature (TCO) at which charging is allowed. I High-Temperature Fault Voltage: VHTF = 0.44 ∗ VCC This voltage corresponds to the temperature (HTF) at which charging resumes after exceeding TCO. I Low-Temperature Fault Voltage: VLTF = 0.6 ∗ VCC This voltage corresponds to the minimum temperature (LTF) at which charging is allowed. Charging is inhibited if the temperature is outside the LTF—TCO window. Once the temperature exceeds TCO, it must drop below HTF before charging resumes. RT1 and RT2 for the thermistor linearization network are determined as follows: 06 1 . () () ∗= + ∗+ ∗ V V RT1 RT2 R RT2 R CC LTF LTF (3) 044 1 1 . () () = + ∗+ ∗ RT1 RT2 R RT2 R HTF HTF (4) where RLTF = thermistor resistance at LTF RHTF = thermistor resistance at HTF V=VCC - 0.250 in low-side current sensing V= VCC in high-side current sensing TCO is determined by the values of RT1 and RT2. 1% resistors are recommended. 9 bq2954 |
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