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3. TECHNICAL BRIEF
USB Charge Current
When the USB port is selected as the power source, the charge current is 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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