Datasheet

Current Monitor
Cell Balance Control
bq77PL900
www.ti.com ...................................................................................................................................................... SLUS844B – JUNE 2008 – REVISED JANUARY 2009
Vout(0V) = V
OUT
(2.5V) + K
ACT
× VREF_2.5V
OR
Vout(0V) = V
OUT
(0.975V or 1.2V) + K
ACT
× VREF_m
Cell voltage is calculated by as follows:
VCn – VC(n + 1) = {Vout(0V) – V
OUT
} / K
ACT
Discharge and charge currents are translated to allow a host controller to measure accurately current, which
measurement can then be used for additional safety features or calculating the remaining capacity of the battery.
The sense resistor voltage is converted using the following equation. The typical offset voltage is V
CELL_OFF
(1.2 V typical), although it can be presented on the IOUT pin for measurement, if required.
The output voltage increases when current is positive (discharging) and decreases when current is negative
(charging).
V
CURR
= 1.2 + (I
PACK
× R
SENSE
) × (IGAIN)
where
State_Control [IGAIN] = 1 then IGAIN = 50
State_Control [IGAIN] = 0 then IGAIN = 10
The current monitor amplifier can present the offset voltage as shown in Table 8 . The IOUT pin is enabled or
disabled by FUNCTION_CONTROL [IACAL, IAEN] and has a default state of OFF. IOUT is internally connected
to ground when disabled.
Table 8. IACAL and IAEN Configuration
IACAL IAEN CONDITION
0 1 NORMAL
1 1 OFFSET
X 0 OFF
The integrated cell balance FETs allow a bypass path to be enabled for any one series element. The purpose of
this bypass path is to reduce the current into any one cell during charging to bring the series elements to the
same voltage. Series resistors placed between the input pins and the positive series element nodes limits the
bypass current value. Series input resistors between 500 Ω and 1 k Ω are recommended for effective cell
balancing.
In host-control mode, individual series element selection is made via CELL_BALANCE [CBAL1, CBAL2, CBAL3,
CBAL4, CBAL5, CBAL6, CBAL7, and CBAL8] and FUNCTION_CONTROL [CBAL9, CBAL10].
In stand-alone mode, cell balancing works as shown in Figure 19 . When a certain cell (cell A) voltage reaches
cell overvoltage, the battery charging stops and then cell balance starts working at ta. The cell-A voltage
decreases by the bypass current until the voltage reaches (V
OV
– Δ V
OVH
). Cell-B voltage does not change during
the period because cell balancing works only for the cell that reached V
OV
. At tb, battery charging starts again.
Cell A and cell B have been charged in this period until cell-A voltage reaches V
OV
again. The voltage difference
between cell A and cell B becomes smaller when the bq77PL900 repeats the foregoing cycle. The bq77PL900
stops cell balance when cell overvoltage protection has released.
The bq77PL900 is designed to prevent cell balancing on adjacent cells or on every other cell. For example, if cell
overvoltage happened to cell 8, cell 7 (cell 7 is next to cell 8) and cell 3 (cell 3 is not next to cell 8 or cell 7), then
cell balancing starts for cell 8 and cell 3 first. When the cell-8 voltage is back to normal, then cell balancing starts
for cell 7.
While the bq77PL900 monitors the overvoltage and undervoltage, cell balancing is automatically turned off. This
configuration is supported for both modes (host-control and stand-alone modes).
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