13

ICS1708

With the battery 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 installed, the current source overshoot characteristics when turned on and off must not cause the

voltage at the VIN pin to approach the voltage at the OPREF pin. If the voltage at OPREF exceeds the voltage at VIN when a charge pulse

is applied, the polling feature will be activated.

PC Board Design Considerations

It is very important that care be taken to minimize noise coupling and ground bounce. Careful placement of wires and connectors helps

minimize resistance and inductance.

When designing the printed circuit board, make sure ground and power traces are wide and bypass capacitors are used right at IC power and

ground pins. Use separate heavy grounds for both signal and power circuits, connecting their grounds together close to where the negative

lead of the battery connects. For power circuits, keep the physical separation between power and return (ground) to a minimum to minimize

field effects. This precaution is most applicable to the constant current source, particularly if it is a switch mode type. Keep the ICS1708

and the constant current source control circuits outside the power and return loop described above. These precautions will prevent high

fields and coupled noise from disturbing normal operation. Avoid jumping across power and return with signal lines.

Using the Voltage Slope Termination Method

In general the voltage slope termination method works best for products where the battery is fast charged with the product off, or the battery

is removed from the product for fast charge in a charger stand.

The voltage slope termination method used by the ICS1708 requires a nearly constant amplitude current flow into the battery during fast

charge. Charging the battery in products that draw a known and fairly constant current while the battery is charging should have this current

draw added to the fast charge current. Using the ICS1708 for charging the batteries in products that randomly or periodically requires

moderate current from the battery during fast charge needs evaluation. Products that randomly or periodically require high current from the

battery during fast charge may cause a voltage inflection that results in termination before full charge. A voltage inflection can occur due to

the charge current decreasing or fluctuating as the load changes rather than by the battery reaching full charge. The voltage slope method

will terminate charge based on voltage inflections that are characteristic of a fully charged battery. The ICS1702 and ICS1712 charge

controllers have temperature termination methods for products that randomly or periodically draw significant current from the battery during

fast charge.

Charging sources that produce decreasing current as fast charge progresses may also 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 abnormalities

that may cause a voltage inflection that is characteristic of a fully charged battery are inadequate line frequency ripple attenuation capability

or charge current decreasing due to thermal drift or thermal limiting.

Charging sources that have any of the above characteristics need

evaluation to access their suitability for the application if the use of voltage slope termination is desired.

The controller

charges the battery at the rate selected. If the actual charge current is significantly less than the rate selected, the conditioning effect of the

rather than by the battery reaching full charge due to inadequate charge current.

Charging System Status by Indicator

The Indicator Description List in Table 2 contains some displays that are caused by charging system abnormalities. When the CMN

indicator flashes, there may be voltage present at the charger terminals with the current source off with no battery connected. Check the

current source and ensure that it produces no more than 350mV at VIN pin 7 when the current source is turned off with no battery connected.

If VIN pin 7 divider resistors are not properly selected, an open circuit voltage that should produce less than 350mV with the charger off

and no battery will not divide down the open circuit voltage properly and produce the CMN flash indication. Check the VIN divider and

ensure that it properly scales the battery voltage to the 1 cell level at VIN pin 7. If the CMN flash indication occurs with the battery

installed, there may be an open connection in the charger or the battery. Check wires, connections, battery terminals, and the battery itself

for an open circuit condition.

If the MMN indicator is active at the initiation of fast charge, check the external pull-down resistor or alternate divider off 5V that sets the

voltage on OPREF pin 6. If the voltage set on OPREF is greater than 2.3V but less than 4V, the start of fast charge will be inhibited until the

voltage on OPREF is lowered.