SC820 datasheet(11/22 Pages) SEMTECH

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SC820

Charger Operation

The SC820 is a dual-input stand-alone Li-ion battery

charger. The VAD input pin is optimized for a charging

adapter. The VUSB pin is optimized for charging from the

USB Vbus supply. The device is independently pro-

grammed 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 a valid 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 programmed fast-charge current for

the selected input.

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 the VAD input is selected and by

the IPUSB resistance to ground when the VUSB input is

selected.

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

input selected. This is known as charge termination.

Optional Float-charging or Monitoring

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

tion the SC820 either operates indefinitely as a voltage

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

automatically and the process repeats. A forced re-charge

cycle can also be periodically commanded by the proces-

sor 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 Selection

The SC820 has two charging supply input pins. VAD is

optimized for adapter charging. VUSB is optimized for

charging from the USB Vbus power supply. The inputs

differ in selection rising and de-selection falling thresh-

olds, their behavior when overloading their respective

charging sources, and in which current programming pin

determines the fast-charge and pre-charge current. Both

use the same Over-Voltage Protection (OVP) threshold.

Glitch filtering is performed on the VAD and VUSB inputs,

so an applied input voltage that is ringing across its selec-

tion threshold will not be selected until the ringing has

ceased. When both inputs exceed their respective UVLO

thresholds, VAD is selected even when VAD voltage is

applied while already charging from the VUSB input. VAD

is also selected in the case that the VAD voltage exceeds

its OVP threshold, so that an excessive VAD voltage will

disable charging despite the presence of a valid VUSB

input voltage.

When a valid input (defined as greater than its selection

threshold and less than the OVP threshold) is first

selected, a charge cycle is initiated and the STATB output

is asserted. When a new input selection is made (when

VAD is applied or removed while VUSB is present), the

charge cycle is immediately halted and re-initiated with

the newly selected input. There is a momentary (approxi-

mately 1ms) interruption in output current and a release

and re -assertion of the STATB pin during input

res-election.

If the VAD input charging current loads the adapter

beyond its current limit, the VAD input voltage will be

pulled down to just above the battery voltage. This is

referred to as Current-Limited-Adapter (CLA) operation.

The adapter input de-selection falling threshold is set

close to the battery voltage pre-charge threshold to

permit low-dissipation charging from a current limited

adapter.

Applications Information

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