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FN6704.0
July 28, 2008
input power is removed or the EN pin is pulled to HI. Figure 2
shows the typical charge waveforms after the power is on.
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 ISL9222A accepts
an input voltage up to 28V but disables charging when the
input voltage exceeds the OVP threshold, typically 7.2V, 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 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 VOS is the offset voltage for the input and output
voltage comparator, discussed shortly, and 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 Specification 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
shown in the Block Diagram on page 5.
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 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 the Electrical Specifications table on page 2.
IREF Pin
The IREF pin has the two functions as described in “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 is the same 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 ISL9222A 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 1.2at 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.
Auxiliary OR Gate
The auxiliary OR gate provides a booting enable signal from
from 2 possible inputs, the VIN power good signal, which is
internal to the IC, or the external JIGIN signal. The supply
voltage of the OR gate comes from VBAT. The JIGON states
are summarized in Table 1. There is an internal pull-down
resistor at the JIGIN pin so that when left floating, the input is
a logic low.
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. A 1µF or larger X5R ceramic
capacitor is recommended.
Due to the inductance of the power leads of the wall adapter
or USB source, the input capacitor type must be properly
selected to prevent high voltage transient during a hot-plug
ISL9222A
