SC813 datasheet(12/23 Pages) SEMTECH

12 / 23 page

SC811 / SC813

Charger Operation

The SC811/3 is a single input tri-mode stand-alone Li-ion

battery charger. (The SC811 differs from the SC813 only in

the input voltage Over Voltage Protection threshold.) It

provides selections of adapter mode and USB high and

low power mode charging. The device is independently

programmed for battery capacity dependent currents

(adapter fast-charge current and termination current)

using the IPRGM pin. Charging currents from the USB

Vbus supply, which has a maximum load specification, are

programmed using the IPUSB pin when either of the USB

modes is selected.

When an input supply is first detected, a charge cycle is

initiated and the STATB open-drain output goes low. If the

battery voltage is less than the pre-charge threshold

voltage, the pre-charge current is supplied. Pre-charge

current is 20% of the IPRGM (adapter or USB high power

modes) or IPUSB (USB low power mode) programmed

fast-charge current.

When the battery voltage exceeds the pre-charge thresh-

old, typically within seconds for a standard battery with a

starting cell voltage greater than 2V, the fast-charge

Constant Current (CC) mode begins. The charge current

soft-starts in three steps (20%, 60%, and 100% of pro-

grammed fast-charge current) to reduce adapter load

transients. CC current is programmed by the IPRGM resis-

tance to ground when adapter mode is selected and by

the IPUSB resistance to ground when either USB mode is

selected. In USB low power mode, the CC current is held

at 20% of the IPUSB programmed fast-charge current.

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. In CV

regulation, the output voltage is regulated, and as the

battery charges, the charge current gradually decreases.

The STATB output goes high when I

BAT

drops below the

termination threshold current, which is 10% of the IPRGM

pin programmed fast-charge current regardless of the

mode selected. This is known as charge termination.

Optional Float-charging or Monitoring

Depending on the state of the ENB input, upon termina-

tion the SC811/3 either operates indefinitely as a voltage

regulator (known as float-charging) or it turns off its

output. If the output is turned off upon termination, 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). A re-charge cycle then begins

automatically and the process repeats. A forced re-

charge cycle can also be periodically commanded by the

processor to keep the battery topped-off without float-

charging. See the Monitor State section for details.

Re-charge cycles are not indicated by the STATB pin.

Charging Input Pin Mode Dependencies

The UVLO rising and falling thresholds are adjusted with

the charging mode selected. In adapter mode, if the

charging current loads the adapter beyond its current

limit, the input voltage will be pulled down to just above

the battery voltage. The adapter mode UVLO falling

threshold is set close to the battery voltage pre-charge

threshold to permit low-dissipation charging from a

current limited adapter.

The USB modes provide a higher UVLO falling threshold

applicable to the USB specification. The USB modes also

provide Under-Voltage Load Regulation (UVLR), in which

the charging current is reduced if needed to prevent

overloading of the USB Vbus supply. UVLR can serve as a

low-cost alternative to directly programming the USB

low power charge current. This can be beneficial for

charging small batteries, for which the USB high power

fast-charge current must be programmed to less than

500mA. The fixed 20% USB low power mode fast-charge

current would be less than 100mA and, therefore, is

unsuitable for minimum charge-time applications. UVLR

can also be used where there is no signal available to

indicate whether USB low or high power mode should be

selected.

All modes use the same input Over-Voltage Protection

(OVP) threshold as defined in the Electrical Characteristics

section for the device being used.

Constant Current Mode Fast-charge Current

Programming

Constant Current (CC) regulation is active when the

battery voltage is above VT

PreQ and less than VCV. When

adapter mode is selected, the programmed CC regula-

tion fast-charge (FQ) current is inversely proportional to

Applications Information

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