ICS1722
With the batteries removed, the current source must be capable of
raising the voltage at the VIN pin above the voltage at the OPREF
pin to ensure proper polling. With the batteries in-stalled, the
current source overshoot characteristics when turned on and off
must not cause the voltage at the VIN pin to exceed the voltage at
the OPREF pin. If the voltage at OPREF exceeds the voltage at
VIN when a charge pulse is applied or removed, the polling feature
will be activated.
Applications Information
To ensure proper operation of the ICS1722, external components
must be properly selected. The external current source used must
meet several important criteria to ensure optimal performance of
the charging system. The charging current should be constant when
using voltage slope termination.
VIN and OPREF Divider Resistors
Figure 9 shows a typical application using the ICS1722. R1
through R3 must be carefully selected to ensure that battery
detection and voltage termination methods operate properly.
R1 and R2 are selected to scale the battery voltage down to the
voltage of one cell. The following table shows some typical values.
Additional information is available in the Voltage Input section.
PC Board Design Considerations
It is very important that care be taken to minimize noise coupling
and ground bounce. In addition, wires and connectors can add
significant resistance and inductance to the charge and discharge
circuits.
When designing the printed circuit board, make sure ground and
power traces are wide and bypass capacitors are used right at the
controller. Use separate grounds for the signal, charge and
discharge circuits. Separate ground planes on the component side
of the PC board are recommended. Be sure to connect these
grounds together at the negative lead of the battery only. For the
discharge circuit, keep the physical separation between power and
return (ground) to a minimum to minimize field radiation effects.
This precaution is also applicable to the constant current source,
particularly if it is a switch mode type. Keep the ICS1722 and the
constant current source control circuits outside the power and
return loop described above. These precautions will prevent high
circulating currents and coupled noise from disturbing normal
operation.
Cells
R1
Short
2.0k
2.0k
3.0k
12k
10k
12k
9.1k
R2
Open
2.0k
1.0k
1.0k
3.0k
2.0k
2.0k
1.3k
1
2
3
4
5
6
7
8
The current source should prevent ripple voltage from appearing on
the battery. The effects of ripple on the battery voltage may
interfere with proper operation.
Using the Voltage Slope Termination Method
In general, the voltage slope termination method works best
for equipment where the battery is fast charged with the equipment
R3 is used to set the open circuit (no battery) reference voltage on
the OPREF pin. The function of this pin is discussed in the Open
Circuit Reference section.
off or the battery is removed from the equipment for fast charge.
VOPREF
R3
1.82 V
1.93 V
2.02 V
2.14 V
43k
47k
51k
56k
15
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