3. TECHNICAL BRIEF
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3.6 Chargind IC (ISL6299, U508)
The ISL6299 is designed for a single-cell Li-ion or Li-polymer battery charging circuit that accepts both a
USB port and a desktop cradle as its power source.
Input Auto Selection
When both input sources are present, the charger selects only one power source to charge the battery.
When the CRDL input is higher than the POR threshold, CRDL is selected as the power source. Otherwise
the USB input is selected. If the CRDL input voltage is below the battery voltage but the USB input voltage
is higher than the battery voltage, then the USB input is used to charge the battery. The control circuit
always breaks both internal power devices before switching from one power
source to the other to avoid a cross conduction of both power MOSFETs.
USB Charge Current
When the USB port is selected as the power source, the charge current enabled by the logic input at the
USBON pin. When the USBON is driven to logic LOW, the charger is disabled. When the USBON is driven
to logic HIGH, the charge current is fixed at a typical value of 380mA. Thus for the USB input, the USBON
pin has a similar function as the EN pin. The following table describes the USB charge control by both the
USBON pin and EN pin The USBON pin is equivalent to a logic LOW when left floating. Typically the P-
channel MOSFET for the USB input has an rDS(ON) of 700mΩ at room temperature. With a 380mA charge
current, the typical head room is 260mV. Thus, if the input voltage drops to a level that the voltage
difference between the USB pin and the BAT pin is less than 260mV, the rDS(ON) becomes a limiting factor
of the charge current; and the charger drops out the constant current regulation.
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