6

FN9174.4

July 9, 2007

to LOW when the trickle charge starts and rises to HIGH at

the EOC. After the EOC is reached, the charge current has

again, as shown in Figure 4. The current surge after EOC

can be caused by a load connected to the battery.

A thermal foldback function reduces the charge current

anytime when the die temperature reaches typically +115°C.

This function guarantees safe operation when the printed

circuit board (PCB) is not capable of dissipating the heat

generated by the linear charger. The ISL6294 accepts an input

voltage up to 28V but disables charging when the input

voltage exceeds the OVP threshold, typically 6.8V, to protect

against unqualified or faulty AC adapters.

PPR Indication

The PPR pin is an open-drain output to indicate the

presence of the AC adapter. Whenever the input voltage is

higher than the POR threshold, the PPR pin turns on the

internal open-drain MOSFET to indicate a logic LOW signal,

independent on the EN pin input. When the internal open-

drain FET is turned off, the PPR pin should leak less than

1µA current. When turned on, the PPR pin should be able to

sink at least 10mA current under all operating conditions.

The PPR pin can be used to drive an LED (see Figure 1) or

to interface with a microprocessor.

Power-Good Range

The power-good range is defined by the following three

conditions:

overvoltage protection threshold given “Electrical

hysteresis, as given in “Electrical Specifications” on page 2.

The charger will not charge the battery if the input voltage is

not in the power-good range.

Input and Output Comparator

The charger will not be enabled unless the input voltage is

higher than the battery voltage by an offset voltage VOS.

The purpose of this comparator is to ensure that the charger

is turned off when the input power is removed from the

charger. Without this comparator, it is possible that the

charger will fail to power down when the input is removed

and the current can leak through the PFET pass element to

continue biasing the POR and the Pre-Regulator blocks

shown in the Block Diagram on page 5.

CHG Indication

The CHG is an open-drain output capable to at least 10mA

current when the charger starts to charge and turns off when

the EOC current is reached. The CHG signal is interfaced

either with a micro-processor GPIO or an LED for indication.

EN Input

EN is an active-low logic input to enable the charger. Drive

the EN pin to LOW or leave it floating to enable the charger.

This pin has a 200k

Ω internal pulldown resistor so when left

floating, the input is equivalent to logic LOW. Drive this pin to

HIGH to disable the charger. The threshold for HIGH is given

in “Electrical Specifications”on page 2.

IREF Pin

The IREF pin has the two functions as described in the Pin

Description section. When setting the fast charge current,

the charge current is guaranteed to have 10% accuracy with

the charge current set at 500mA. When monitoring the

charge current, the accuracy of the IREF pin voltage vs the

actual charge current has the same accuracy as the gain

from the IREF pin current to the actual charge current. The

accuracy is 10% at 500mA and is expected to drop to 30% of

the actual current (not the set constant charge current) when

the current drops to 50mA.

Operation Without the Battery

The ISL6294 relies on a battery for stability and is not

guaranteed to be stable if the battery is not connected. With

a battery, the charger will be stable with an output ceramic

decoupling capacitor in the range of 1µF to 200µF. The

maximum load current is limited by the dropout voltage or

the thermal foldback.

Dropout Voltage

The constant current may not be maintained due to the

resistance of the pass FET is 1.2

Ω the maximum operating

temperature, thus if tested with 0.5A current and 3.8V

battery voltage, constant current could not be maintained

when the input voltage is below 4.4V.

Thermal Foldback

The thermal foldback function starts to reduce the charge

current when the internal temperature reaches a typical

value of +115°C.

ISL6294