6
FN6821.2
December 11, 2008
to LOW when the trickle charge starts and rises to HIGH at
the EOC. After the EOC is reached, the charge current has
to rise to typically 76% I
REF
for the CHG signal to turn on
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 ISL6294A accepts
an input voltage up to 28V but disables charging when the
input voltage exceeds the OVP threshold, typically 7.3V, 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:
1. VIN > VPOR
2. VIN - VBAT > VOS
3. VIN < VOVP
where the VOS is the offset voltage for the input and output
voltage comparator, discussed shortly, and the VOVP is the
overvoltage protection threshold given in the “Electrical
Specifications” table on page 2. All V
POR
, V
OS
, and V
OVP
have hysteresis, as given in the “Electrical Specifications”
table 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, as
shown in Figure 3.
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 microprocessor 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 internal pull-down 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 the “Electrical Specifications” table on page 2.
IREF Pin
The IREF pin has the two functions as described in the “Pin
Descriptions” on page 3. 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 ISL6294A 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
r
DS(ON)
limit at a low input voltage. The worst case
ON-resistance of the pass FET is 1.2 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.
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 V
IN-
V
BAT
comparator offset voltage. When the battery voltage is above
the POR threshold, the V
IN
-V
BAT
offset voltage dominates
ISL6294A
