FC5754

2014. 11. 04

15/19

Revision No : 0

discussed further in the Applications Information section.

Undervoltage Lockout (UVLO)

An internal undervoltage lockout circuit monitors the

input voltage and keeps the charger in shutdown mode

until VIN rises above the undervoltage lockout threshold.

The UVLO circuit has a built-in hysteresis of 200mV.

Furthermore, to protect against reverse current in the

power MOSFET, the UVLO circuit keeps the charger in

shutdown mode if VIN falls to within 30mV of the battery

voltage. If the UVLO comparator is tripped, the charger

will not come out of shutdown mode until VIN rises 80mV

above the battery voltage.

Manual Shutdown

At any point in the charge cycle, the FC5754 can be

the PROG pin. This reduces the battery drain current to

about to 0υA and the supply current to less than 45υA. A

new charge cycle can be initiated by reconnecting the

program resistor.

In manual shutdown, the CHGB pin is in a weak

pull-down state as long as VIN is high enough to exceed

the UVLO conditions. The CHGB pin is in a high

impedance state if the FC57754 is in undervoltage

lockout mode: either VIN is within 80mV of the BAT pin

voltage or insufficient voltage is applied to the VIN pin.

Automatic Recharge

Once the charge cycle is terminated, the FC5754

continuously monitors the voltage on the BAT pin using a

cycle restarts when the battery voltage falls below

4.05V (which corresponds to approximately 80% to 90%

battery capacity). This ensures that the battery is kept at

or near a fully charged condition and eliminates the

need for periodic charge cycle initiations. CHGB output

enters a strong pulldown state during recharge cycles.

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Application Information

Stability Considerations

The constant-voltage mode feedback loop is stable

without an output capacitor provided a battery is

connected to the charger output. With no battery

present, an output capacitor is recommended to

reduce ripple voltage. When using high value, low ESR

ceramic capacitors, it is recommended to add a 1Ω

resistor in series with the capacitor. No series resistor is

needed if tantalum capacitors are used.

In constant-current mode, the PROG pin is in the

feedback loop, not the battery. The constant-current

mode stability is affected by the impedance at the

PROG pin. With no additional capacitance on the

PROG pin, the charger is stable with program resistor

values as high as 100k. However, additional

capacitance on this node reduces the maximum

allowed program resistor. The pole frequency at the

PROG pin should be kept above 100kHz. Therefore, if

following equation can be used to calculate the

PROG

PROG

C

R

∗

∗

=

5

10

2

1

∏

Average, rather than instantaneous, charge current

may be of interest to the user. For example, if a

switching power supply operating in low current mode is

connected in parallel with the battery, the average

current being pulled out of the BAT pin is typically of

more interest than the instantaneous current pulses. In

such a case, a simple RC filter can be used on the

PROG pin to measure the average battery current as

shown in Figure 1. A 10㎑ resistor has been added

between the PROG pin and the filter capacitor to

ensure stability.

Power Dissipation

The conditions that cause the FC5754 to reduce

charge current through thermal feedback can be

approximated by considering the power dissipated in

the IC. Nearly all of this power dissipation is generated

by the internal MOSFET—this is calculated to be

approximately:

current. The approximate ambient temperature at

which the thermal feedback begins to protect the IC is:

Example: An FC5754 operating from a 5V USB supply is

programmed to supply 500mA full-scale current to a

discharged Li-Ion battery with a voltage of 3.7V.

which the FC5754 will begin to reduce the charge

current is approximately:

the charge current will be reduced from 500mA. The

Figure 1. Isolating Capacitive load on PROG Pin and Filtering

θ

θ