BQ29311_15 Datasheet, PDF (11/24 Pages) Texas Instruments – FOUR-CELL LITHIUM-ION

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Not Recommended for New Designs
bq29311
SLUS487D − DECEMBER 2001 − REVISED NOVEMBER 2003
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
V_OUT1 = 0.975 + (1 + K) × V_OS (V)
where K = VCELL scaling factor
V_OS = 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
V_OUT2 = 0.975 + (1 + K) × V_OS – K × 0.975 (V)
3. Calculate (V_OUT1 – V_OUT2)/0.975
The result is the actual scaling factor, K_ACT and is
represented by
K_ACT = (V_OUT1–V _OUT2)/0.975 = (0.975 + (1 + K) ×
V_OS) − (0.975 + (1 + K) × V_OS – K × 0.975)/0.975
= K × 0.975/0.975 = K
4. Calculate the actual offset value where
V_OS(ACT) = (V_OUT1 – 0.975)/(1 + K_ACT)
5. Calibrated cell voltage is calculated by
VCn − VC(n+1) = [0.975 + (1 + K_ACT ) × V_OS(ACT) –
V_CELLOUT]/K_ACT
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 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.
20.0
16.7
15.0
10.0
5.0
3.5
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).
0 3.8 5.25 10
15
20
25
30
VPACK − Pack Positive Voltage − V
Figure 3. PCHG Drive Output vs. Pack Voltage
(Design Simulation)
11