SC824 datasheet(12/29 Pages) SEMTECH

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SC824

Charger Operation

The SC824 is a single input, tri-mode, stand-alone Li-Ion

battery charger. It has a tri-state MODE input pin that

allows the device to be put in USB high power, USB low

power, or adapter mode. Fast-charge current is pro-

grammed with a resistor from the IPRGM pin to ground.

USB high power mode is equal to adapter mode current.

USB low power current is 20% of fast-charge current.

When a valid input supply is detected, the STAT0 output

goes low. A charge cycle is initiated and the STAT1 output

goes low. When the battery voltage is less than the pre-

charge threshold voltage, the pre-charge current is output

to the battery. Pre-charge current is fixed at 20% of the

programmed adapter mode or USB high power mode

fast-charge current, and is equal to the USB low power

mode fast-charge current.

Fast-charge Constant Current (CC) regulation begins when

the battery voltage exceeds the pre-charge threshold.

The charge current soft-starts in three steps (20%, 60%,

and 100% of programmed fast charge current) to reduce

adapter load transients. In USB low power mode, the CC

current is held at the 20% step.

The charger begins Constant Voltage (CV) regulation

when the battery voltage rises to the fully-charged single-

cell Li-Ion regulation voltage (V

), nominally 4.2V. When

regulating the output voltage, the charge current gradu-

ally decreases as the battery charges. The STAT1 output

goes high, after a 20 second delay, when the output

current I

BAT

drops below the termination current threshold.

This is known as charge termination. The termination

current threshold is 10% of the IPRGM-programmed fast-

charge current regardless of the mode selected.

Each step of the charge cycle is separately timed using the

optional programmable charge timer. Time-out of the

fixed 45 minute pre-charge stage timer or the fixed or

programmable CC stage timer is indicated as a fault, which

is encoded in the STAT0 and STAT1 outputs. Time-out

during the CV timer stage has the same result as normal

charge termination. When the timer is used, the output

remains on for 45 minutes following termination to ensure

that the battery is fully topped-off, then turns off. If the

timer is not used, the charger turns off immediately upon

reaching the termination current.

Optional Top-off Charging, and Monitoring

Depending on the state of the RTIME pin, upon termina-

tion the SC824 either tops-off the battery by operating as

a voltage regulator (known as float charging) for 45

minutes or it immediately turns off its output. Once the

output is turned off, the device enters the monitor state.

In this state, the output remains off until the BAT pin

voltage decreases by the re - charge threshold

(VT

ReQ = 100mV typically). A re-charge cycle then begins

automatically and the process repeats. If the timer is

enabled, then the multistage timer also protects the re-

charge cycle. Re-charge cycles are not indicated by the

STAT1 pin.

A forced re-charge cycle can also be periodically com-

manded by the processor to maintain the battery in a

fully charged state without discharging to the re-charge

threshold, and without top-off charging. See the Monitor

State section for details.

Charging Input Mode Dependencies

In adapter mode, a programmed charging current greater

than the adapter’s current limit will pull down the VIN pin

voltage to the battery voltage plus charger dropout

voltage. This is referred to as Current-Limited-Adapter

(CLA) operation. The UVLO falling threshold is set close

to the battery voltage pre-charge threshold to permit

low-dissipation charging from a current limited adapter.

Both USB modes provide Under-Voltage Load Regulation

(UVLR) in which the charging current is reduced if needed

to prevent overloading of the USB Vbus supply. UVLR

ensures the integrity of the USB Vbus supply for all

devices sharing a host or hub supply. UVLR can also serve

as a low-cost alternative to commanding USB low power

charge current where there is no signal available to indi-

cate whether USB low or high power mode should be

selected.

CC Fast-charge Current Programming

CC regulation is active when the battery voltage is above

PreQ and less than VCV. When either adapter mode or

USB high power mode is selected, the programmed CC

regulation fast-charge (FQ) current is inversely propor-

tional to the IPRGM pin resistance to GND according to

the following equation:

Applications Information

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