Standalone Linear Li-ion Battery Charger with Thermal Regulation

2007-7-3

3/15

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

is reached. This condition is detected by using an internal, filtered comparator to monitor the PROG pin. When the

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

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

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