Circuit Description / Overview
When charging batteries with a current-limited input source,
such as USB, a switching charger’s high efficiency over a
wide range of output voltages minimizes charging time.
FAN540X combines a highly integrated synchronous buck
regulator for charging with a synchronous boost regulator,
which can supply 5V to USB On-The-Go (OTG) peripherals.
The regulator employs synchronous rectification for both the
charger and boost regulators to maintain high efficiency over
a wide range of battery voltages and charge states.
The FAN540X has three operating modes:
1. Charge Mode:
Charges a single-cell Li-ion or Li-polymer battery.
2. Boost Mode:
Provides 5V power to USB-OTG with an integrated
synchronous rectification boost regulator using the
battery as input.
3. High-Impedance Mode:
Both the boost and charging circuits are OFF in this
mode. Current flow from VBUS to the battery or from the
battery to VBUS is blocked in this mode. This mode
consumes very little current from VBUS or the battery.
Note: Default settings are denoted by bold typeface.
In Charge Mode, FAN540X employs four regulation loops:
1. Input Current: Limits the amount of current drawn from
VBUS. This current is sensed internally and can be
programmed through the I2C interface.
2. Charging Current: Limits the maximum charging current.
This current is sensed using an external RSENSE resistor.
3. Charge Voltage: The regulator is restricted from
exceeding this voltage. As the internal battery voltage
rises, the battery’s internal impedance and RSENSE work
in conjunction with the charge voltage regulation to
decrease the amount of current flowing to the battery.
Battery charging is completed when the voltage across
RSENSE drops below the ITERM threshold.
4. Temperature: If the IC’s junction temperature reaches
120°C, charge current is continuously reduced until the
IC’s temperature stabilizes at 120°C.
In addition, the FAN5403-05 employ an additional loop to
limit the amount of drop on VBUS to a programmable voltage
(VSP) to accommodate “special chargers” that limit current to
a lower current than might be available from a “normal” USB
Battery Charging Curve
If the battery voltage is below VSHORT, a linear current source
pre-charges the battery until VBAT reaches VSHORT. The PWM
charging circuit is then started and the battery is charged
with a constant current if sufficient input power is available.
The current slew rate is limited to prevent overshoot.
The FAN540X is designed to work with a current-limited
input source at VBUS. During the current regulation phase of
charging, IINLIM or the programmed charging current limits the
amount of current available to charge the battery and power
the system. The effect of IINLIM on ICHARGE can be seen in
PRE- CURRENT REGULATION
Figure 35. Charge Curve, ICHARGE Not Limited by IINLIM
PRE- CURRENT REGULATION
Figure 36. Charge Curve, IINLIM Limits ICHARGE
Assuming that VOREG is programmed to the cell’s fully
charged “float” voltage, the current that the battery accepts
with the PWM regulator limiting its output (sensed at VBAT)
to VOREG declines, and the charger enters the voltage
regulation phase of charging. When the current declines to
the programmed ITERM value, the charge cycle is complete.
Charge current termination can be disabled by resetting the
TE bit (REG1).
The charger output or “float” voltage can be programmed by
the OREG bits from 3.5V to 4.44V in 20mV increments, as
shown in Table 3.
© 2009 Fairchild Semiconductor Corporation
FAN5400 Family • Rev. 1.0.7