Datasheet

VREF VCOUT
VCOUT
VCOUT
VCOUT GC + OC
VCn = × (1 + GC )
G
´
( )
( )
( )
( ) ( )
VCOUT
VCOUT
VREF
GC = VCn_GC_4 << 4 VCn_GAIN_CORR 0.001,
OC = VCn_OC_4 << 4 VCn_OFFSET_CORR 0.001,
GC = (1 + VREF_GC_4 << 4 VREF_GAIN_CORR 0.001)
VREF_OC_5 << 5 VREF_OC_4 << 4 VREF_OF
+
é ù
+ ´
ë û
é ù
+ ´
ë û
é ù
+ ´
ë û
+ +
NOMINAL
FSET_CORR 0.001
VREF
é ù
´
ë û
(VC2 VC1) 0.6 > (VC1 VSS)- ´ -
bq76925
SLUSAM9B JULY 2011REVISED DECEMBER 2011
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The actual cell voltage (VCn) is calculated from the measured voltage (VCOUT) as shown in the following
equations:
(1)
spacer
spacer
(2)
Cell Amplifier Headroom Under Extreme Cell Imbalance
For cell voltages across (VC1 VC0) that are less than ~2.64 V, extreme cell voltage imbalances between
(VC1 VC0) and (VC2 VC1) can lead to a loss of gain in the (VC2 VC1) amplifier. The cell imbalance at
which the loss of gain occurs is determined by the following equation:
(3)
Assuming VC0 = VSS, it can be seen that when (VC1 VC0) > 2.64 volts, the voltage across (VC2 VC1) can
range up to the limit of 4.4 V without any loss of gain. At the minimum value of (VC1 VC0) = 1.4 V, an
imbalance of more than 900 mV is tolerated before any loss of gain in the (VC2 VC1) amplifier. For higher
values of (VC1 VC0), increasingly large imbalances are tolerated. For example, when (VC1 VC0) = 2.0 V, an
imbalance up to 1.33 V (i.e. (VC2 VC1) = 3.33 V) results in no degradation of amplifier performance.
Normally, cell imbalances greater than 900 mV will signal a faulty condition of the battery pack and its use should
be discontinued. The loss of gain on the second cell input does not affect the ability of the system to detect this
condition. The gain fall-off is gradual so that the measured imbalance will never be less than the critical
imbalance set by Equation 3.
Therefore if the measured (VC2 VC1) is greater than (VC1 VSS) / 0.6, a severe imbalance is detected and
the pack should enter a fault state which prevents further use. In this severe cell imbalance condition
comparisons of the measured (VC2 VC1) to any over-voltage limits will be optimistic due to the reduced gain in
the amplifier, further emphasizing the need to enter a fault state.
Cell Amplifier Headroom Under BAT Voltage Drop
Voltage differences between BAT and the top cell potential come from two sources as shown in Figure 6: V3P3
regulator current that flows through the R
BAT
filter resistor, and the voltage drop in the series diode D
BAT
of the
hold-up circuit. These effects cause BAT to be less than the top cell voltage measured by the cell amplifier.
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