15

ICS1700A

Voltage Slope Termination

In general, the voltage slope termination method works best for

equipment where the battery is fast charged with the equipment off

or the battery is removed from the equipment for fast charge. The

voltage slope termination method works best with a constant

current flow into the battery during fast charge. If equipment draws

a known constant current while the battery is charging, this current

should be added to the fast charge cur-rent. Equipment that

randomly or periodically requires current from the battery during

fast charge needs evaluation to ensure it does not interfere with the

proper operation of the voltage slope termination method.

Charging sources that produce decreasing current as fast charge

progresses may cause a voltage inflection that may result in

termination before full charge. For example, if the charge current is

supplied through a resistor or if the charging source is a constant

current type that has insufficient input voltage, the current will

decrease and may cause a termination before full charge. Other

current source characteristics that can cause a voltage inflection

that is characteristic of a fully charged battery are inadequate ripple

and noise attenuation capability or charge current decreasing due to

thermal drift. Charging sources that have any of the above

characteristics need evaluation to access their suitability for the

application.

The controller soft start stage, built-in noise filtering, and fast

charge timer operate optimally when the constant cur-rent source

charges the battery at the rate selected. If the actual charge current

is significantly less than the rate selected, the conditioning effect of

the soft start stage and the controller noise immunity are lessened.

Also, the fast charge timer may cause termination based on time

duration rather than by the battery reaching full charge due to

inadequate charge current.

Applications Information

To ensure proper operation of the ICS1700A, external components

must be properly selected. The external current source used must

meet several important criteria to ensure optimal performance of

the charging system.

VIN Divider Resistors

Figure 12 shows a typical application using the ICS1700A. R1 and

R2 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

Cells

R1

R2

1

Short

Open

2

2.0k

2.0k

3

2.0k

1.0k

4

3.0k

1.0k

5

12k

3.0k

6

10k

2.0k

7

12k

2.0k

8

9.1k

1.3k

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

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.