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BQ29311PW Datasheet(PDF) 11 Page - Texas Instruments |
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BQ29311PW Datasheet(HTML) 11 Page - Texas Instruments |
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11 / 24 page  bq29311 SLUS487D − DECEMBER 2001 − REVISED NOVEMBER 2003 www.ti.com 11 CALIBRATION OF CELL VOLTAGE MONITOR AMPLIFIER GAIN The cell voltage monitor amplifier has an offset, which can be calibrated to increase accuracy. The following procedure shows how to measure and calculate the offset: 1. Set CAL2=0, CAL1=1, VM1=0, VM0=0 The output voltage includes the offset and is represented by VOUT1 = 0.975 + (1 + K) × VOS (V) where K = VCELL scaling factor VOS = offset voltage at input of the internal op-amp 2. Set CAL2=1, CAL1=0, VM1=0, VM0=0 The output voltage includes the scale factor error and offset and is represented by VOUT2 = 0.975 + (1 + K) × VOS – K × 0.975 (V) 3. Calculate (VOUT1 – VOUT2)/0.975 The result is the actual scaling factor, KACT and is represented by KACT = (VOUT1–V OUT2)/0.975 = (0.975 + (1 + K) × VOS) − (0.975 + (1 + K) × VOS – K × 0.975)/0.975 = K × 0.975/0.975 = K 4. Calculate the actual offset value where VOS(ACT) = (VOUT1 – 0.975)/(1 + KACT) 5. Calibrated cell voltage is calculated by VCn − VC(n+1) = [0.975 + (1 + KACT ) × VOS(ACT) – VCELLOUT]/KACT CELL BALANCE CONTROL The cell balance control allows a small discharge to be controlled for any one series element. The purpose of this discharge is to bring the series elements to the same voltage. Series resistors placed between the input pins and the positive series element nodes control the discharge current value. Individual series element selection is made using CELL_SEL (b4...b7). Cell balance discharge is also disabled if bits CELL_SEL (b4...b7) are zero. When all CELL_SEL (b4 …b7) bits are set to 1, then all series elements are discharged. DSG AND CHG FET DRIVER CONTROL The bq29311 drives the FET off if an OC or SC safety threshold is breached. The host can force any FET on or off only if the bq29311 integrated protection control allows. The DSG and CHG FET drive gate-to-drain voltage is clamped to 15 V (max) and 11 V (typ). The default-state of the FET drive is off. A host can control the FET drive by programming CONTROL (b1...b2) where b1 is used to control the external discharge FET and b2 is used to control the external charge FET. These controls are valid only when shutdown is not active. PCHG FET DRIVER CONTROL The PCHG FET is used when the battery is under voltage, allowing limited conditioning current modes such as pre-charge and 0 V charging. At startup of the bq29311 (charger connected to battery pack), PCHG is clamped to 3.5 V when VPACK is 3.8 V to 5.25 V. Thus the actual VPACK voltage is decided with this PCHG clamp voltage and the VGS(gate-source voltage) of external precharge FET. When VPACK is over 5.25 V, PCHG voltage would be controlled to two-thirds of VPACK. The default state of PCHG is on. There are two methods to pull up PCHG to turn off precharge FET. One is through the control register via serial communication. Setting b3 in the control register to 1 turns off the precharge FET. Another method is to use CNTL. Floating CNTL or pulling CNTL up to VREG turns off the precharge FET and also the CHG and DSG FETs. Overcurrent, overload, and short-circuit detection is not applied during precharge operations. An external resistor located in series with the external precharge FET limits the current flow for precharge operations. VPACK − Pack Positive Voltage − V 0 5.25 10 15 20 25 5.0 30 10.0 15.0 20.0 3.5 16.7 3.8 Figure 3. PCHG Drive Output vs. Pack Voltage (Design Simulation) Not Recommended for New Designs |
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