7

FN9174.4

July 9, 2007

Applications Information

Input Capacitor Selection

The input capacitor is required to suppress the power supply

transient response during transitions. Mainly this capacitor is

selected to avoid oscillation during the start up when the

input supply is passing the POR threshold and the VIN - BAT

comparator offset voltage. When the battery voltage is above

the POR threshold, the VIN - VBAT offset voltage dominates

the hysteresis value. Typically, a 1µF X5R ceramic capacitor

should be sufficient to suppress the power supply noise.

Output Capacitor Selection

The criteria for selecting the output capacitor is to maintain

the stability of the charger as well as to bypass any transient

load current. The minimum capacitance is a 1µF X5R

ceramic capacitor. The actual capacitance connected to the

output is dependent on the actual application requirement.

Charge Current Limit

The actual charge current in the CC mode is limited by

three limits for the charge current in the CC mode. The

charge current is limited by the on resistance of the pass

element (power P-Channel MOSFET) if the input and the

output voltage are too close to each other. The solid curve

shows a typical case when the battery voltage is 4.0V and

the charge current is set to 700mA. The non-linearity on the

higher die temperature. If the battery voltage increases to

higher than 4.0V, the entire curve moves towards right side.

As the input voltage increases, the charge current may be

reduced due to the thermal foldback function. The limit

caused by the thermal limit is dependent on the thermal

impedance. As the thermal impedance increases, the

thermal-limited curve moves towards left, as shown in

Figure 1.

Layout Guidance

The ISL6294 uses a thermally-enhanced DFN package that

has an exposed thermal pad at the bottom side of the

package. The layout should connect as much as possible to

copper on the exposed pad. Typically the component layer is

more effective in dissipating heat. The thermal impedance

can be further reduced by using other layers of copper

connecting to the exposed pad through a thermal via array.

Each thermal via is recommended to have 0.3mm diameter

and 1mm distance from other thermal vias.

Input Power Sources

The input power source is typically a well-regulated wall

cube with 1-meter length wire or a USB port. The input

voltage ranges from 4.25V to 6.5V under full-load and

unloaded conditions. The ISL6294 can withstand up to 28V

on the input without damaging the IC. If the input voltage is

higher than typically 6.8V, the charger stops charging.

6.5

4.0

700

INPUT VOLTAGE (V)

4.5

5.0

5.5

6.0

θ

INCREASES

INCREASES

INCREASES

LIMITED

THERMAL

LIMITED

ISL6294

FIGURE 1. CHARGE CURRENT LIMITS IN THE CC MODE