ISL9217 Intersil Corporation, ISL9217 Datasheet - Page 25

• Manufacturer Part Number: ISL9217
• Description: (ISL9216 / ISL9217) 8 to 12 Cell Li-Ion Battery Overcurrent Protection and Analog Front End Chip Set
• Manufacturer: Intersil Corporation

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DEFINITION OF CELL BALANCING
Cell balancing is defined as the application of differential
currents to individual cells (or combinations of cells) in a
series string. Normally, of course, cells in a series string
receive identical currents. A battery pack requires additional
components and circuitry to achieve cell balancing. For the
ISL9216 and ISL9217 devices, the only external
components required are balancing resistors.
CELL BALANCE OPERATION
Cell balancing is accomplished through a microcontroller
algorithm. This algorithm compares the cell voltages (a
representation of the pack capacity) and turns on balancing
for the cells that have the higher voltages. There are many
parameters that should be considered when writing this
algorithm. An example cell balancing algorithm is available
in the ISL9216EVAL1Z evaluation kit.
The microcontroller turns on the specific cell balancing by
setting a bit in the Cell Balance Register. Each bit in the
register corresponds to one cell’s balancing control. When
the bit is set, an internal cell balancing FET turns on. This
shorts an external resistor across the specified cell. The
maximum current that can be drawn from (or bypassed
around) the cell is 200mA. This current is set by selecting
the value of the external resistor. Figure 7 shows an example
with a 200mA (maximum) balancing current.
With lower balancing current, more balancing FETs can be
turned on at once, without exceeding the device power
dissipation limits or generating excessive balancing current
that will heat the external resistor.
External VMON/CFET Protection Mechanisms
When there is a single charge/discharge path, a blocking
diode is required in the ISL9216 VMON to P- path. See D1 in
Figure 8. This diode is to protect against a negative voltage
on the VMON pin that can occur when the FETs are off and
the charger connects to the pack. This diode is not needed
when there is a separate charge and discharge path,
because the voltages on P- (discharge) are likely always
positive.
For the cascaded combination of ISL9216 and ISL9217, a
zener diode (D2 in Figure 8) needs to be in the ISL9216
VMON path to the P- pin to protect the ISL9216 from an
overvoltage condition when the FETs open due to a short
circuit or overcurrent condition.
With the single set of charge/discharge FETs, the ISL9216
CFET pin needs to be protected in the event of an over-
current or short circuit shut-down. When this happens, the
FET opens suddenly. The flyback voltage from the motor
windings will likely exceed the maximum input voltage on the
CFET pin. So, when operating in this configuration, an
additional external series diode must be placed between the
CFET pin of the ISL9216 and the gate of the Charge FET.
25
ISL9216, ISL9217
See Diode D3 in Figure 8. This will reduce the CFET gate
voltage, but not significantly.
Finally, in all configurations, to protect the Charge FET itself
in the event of a large negative voltage on the Pack- pin,
zener diode D4 is added. The large negative voltage can
occur when the P- pin goes significantly negative, while the
CFET pin is being internally clamped at VSS. The zener
voltage of D4 should be less than the V
specification of the FET.
FIGURE 8. USE OF A DIODES FOR PROTECTING THE
FIGURE 7. CELL BALANCING CONTROL EXAMPLE WITH
200mA
CFET AND VMON PINS.
21Ω
1W
1W
21Ω
ISL9217
ISL9216
100mA BALANCING CURRENT
VMON
CFET
DFET
VCELL1
VSS
CB1
VC7/VCC
CB7
D3
MUST ASSUME ZERO r
FOR MAX CURRENT CALCULATION
D1
ISL9216, ISL9217
10M
7 6 5 4 3 2 1
GS
D2
1M
(max)
D4
November 2, 2007
CELL
BALANCE
REG
DS(ON)
PACK+
PACK-
FN6488.1