FC5754 datasheet(14/19 Pages) FS

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FC5754

2014. 11. 04

14/19

Revision No : 0

Operation

The FC5754 is a single cell lithium-ion battery charger

using a constant-current/constant-voltage algorithm. It

can deliver up to 1A of charge current (using a good

thermal PCB layout) with a final float voltage accuracy

of ±1%. The FC5754 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 FC5754 is

capable of operating from a USB power source.

Normal Charge Cycle

A charge cycle begins when the voltage at the VIN 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 FC5754 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 FC5754 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.

Programming Charge Current

The charge current is programmed using a single resistor

from the PROG pin to ground. The battery charge

current is 960 times the current out of the PROG pin. The

program resistor and the charge current are calculated

using the following equations:

PROG

CHG

PROG

960

The charge current out of the BAT pin can be

determined at any time by monitoring the PROG pin

voltage using the following equation:

960

∗

PROG

BAT

Charge Termination

A charge cycle is terminated when the charge current

falls to 1/10th 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 TTERM (typically 1.1ms), charging is

terminated. The charge current is latched off and the

FC5754 enters standby mode, where the input supply

current drops to 106uA. (Note: C/10 termination is

disabled in trickle charging and thermal limiting modes).

The FC5754 draws no current from the battery in

standby mode. This feature reduces the charge and

discharge cycles on the battery, further prolonging the

battery life.

Any external source (VPROG) that holds the PROG pin

above 100mV will prevent the FC5754 from

terminating a charge cycle. However, if the PROG pin is

controlled by external source, current sourcing from the

BAT pin can be infinity (until the internal power MOSFET is

burned out or the BAT pin voltage is close to its final float

voltage), and the formula for charge current is not valid

anymore. Therefore, controlling the PROG pin by

external source below 1.1V should be avoided when a

battery is connected to BAT pin. However, with no

battery present, forcing VPROG to 0.2V~0.4V enables

FC5754 to act as a LDO (LDO mode). In the LDO

mode, load current at the BAT pin is recommended to

be below 150mA to avoid overheating.

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 1/10th the programmed value. The 1.1ms

filter time (TTERM) 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 1/10th the

programmed value, the FC5754 terminates the

charge cycle and ceases to provide any current

through the BAT pin. This is the standby mode, and all

loads on the BAT pin must be supplied by the battery. In

the standby mode, any signal below the manual

shutdown threshold voltage (typically 1.18V) on the

PROG pin is transparent to FC5754.

The FC5754 constantly monitors the BAT pin voltage in

standby mode. If this voltage drops below the 4.05V

recharge threshold (VRECHRG), 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.

Charge Status Indicator (CHGB)

The charge status output has three different states:

strong pull-down (~10mA), weak pull-down (~24υA) and

high impedance. The strong pull-down state indicates

that the FC5754 is in a charge cycle. Once the

charge cycle has terminated, the pin state is

determined by undervoltage lockout conditions. A

weak pull-down indicates that VIN meets the UVLO

conditions and the FC5754 is ready to charge. High

impedance indicates that the FC5754 is in

undervoltage lockout mode: either VIN is less than

80mV above the BAT pin voltage or insufficient voltage

is applied to the VIN 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 FC5754 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 FC5754. The charge

current can be set according to typical (not worst-case)

ambient temperature with the assurance that the

charger will automatically reduce the current in

worst-case conditions. SOT power considerations are

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