Standalone Linear Li-ion Battery Charger with Thermal Regulation
FSP4054
OPERATION
The FSP4054 is a single cell lithium-ion battery charger using a constant-current/constant-voltage algorithm. It can
deliver up to 800mA of charge current (using a good thermal PCB layout) with a final voltage accuracy of ±1%. The
FSP4054 includes an internal P-channel power MOSFET and thermal regulation circuitry. No blocking diode or
external current sense resistor is required; thus, the basic charger circuit requires only two external components.
Furthermore, the FSP4054 is capable of operating from a USB power source.
Normal charge cycle
A charge cycle begins when the voltage at the Vcc pin rises above the UVLO threshold level and a 1% program
resistor is connected from the PROG pin to ground or when a battery is connected to the charger output. If the BAT
pin is less than 2.9V, the charger enters trickle charge mode. In this mode, the FSP4054 supplies approximately
1/10 the programmed charge current to bring the battery voltage up to a safe level for full current charging.
When the BAT pin voltage rises above 2.9V, the charger enters constant-current mode, where the programmed
charge current is supplied to the battery. When the BAT pin approaches the final float voltage (4.2V), the FSP4054
enters constant-voltage mode and the charge current begins to decrease. When the charge current drops to 1/10 of
the programmed value, the charge cycle ends.
Programmed charge current
The charge current is programmed using a single resistor from the PROG pin to ground. The battery charge current
is 1000 times the current out of the PROG pin. The program resistor and the current are calculated using the
following equations:
The charge current out of the BAT pin can be determined at any time by monitoring the PROG pin voltage using the
following equation:
Charge termination
A charge cycle is terminated when the charge current falls to 1/10
th
the programmed value after the final float voltage
is reached. This condition is detected by using an internal, filtered comparator to monitor the PROG pin. When the
PROG pin voltage falls below 100mV for longer than t
TERM
, charging is terminated. The charge current is latched off
and the FSP4054 enters standby mode, where the input supply current drops to 200uA. (Note: C/10 termination is
disabled in trickle charging and thermal limiting modes)
When charging, transient loads on the BAT pin can cause the PROG pin to fall below 100mV for short periods of
time before the DC charge current has dropped to 1/10
th
the programmed value. The 1ms filter time on the
termination comparator ensures that transient loads of this nature do not result in premature charge cycle
termination. Once the average charge current drops below 1/10
th
the programmed value, the FSP4054 terminates
the charge cycle and ceases to provide any current through the BAT pin. In this state, all loads on the BAT pin must
be supplied by the battery.
The FSP4054 constantly monitor the BAT pin voltage in standby mode. If this voltage drops below the 4.05V
recharge threshold, another charge cycle begins and current is once again supplied to the battery. To manually
restart a charge cycle when in standby mode, the input voltage must be removed and reapplied, or the charger must
be shut down and restarted using the PROG pin. The figure 1shows the state diagram of a typical charge cycle.
Charge status indicator
The charge status output has three different states: strong pull-down(10mA), weak pull-down (20uA) and high
impedance. The strong pull-down state indicates that the FSP4054 is in a charge cycle. Once the charge cycle has
terminated, the pin state is determined by undervoltage lockout conditions. A weal pull-down indicates that Vcc
meets the UVLO conditions and the FSP4054 is ready to charge. High impedance indicates that the FSP4054 is in
undervoltage lockout mode: either VCC is less than 100mV above the BAT pin voltage or insufficient voltage is
applied to the VCC pin. A microprocessor can be used to distinguish between these three states—this method is
discussed in the Applications Information section.
Thermal limiting
An internal thermal feedback loop reduces the programmed charge current if the die temperature attempts to rise
above a preset value of approximately 120°C. This feature protects the FSP4054 from excessive temperature and
allows the user to push the limits of the power handling capability of a given circuit board without risk of damaging
the FSP4054. The charge current can be set according to typical (not worst-case) ambient temperature with the
3/15
2007-7-3
FOSLINK [ FOSLINK SEMICONDUCTOR CO.,LTD ]
