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SC908MLTRT

Power Management IC for Single-Cell Li-Ion Devices

Semtech Corporation 

SC908

Applications Information

Charger Operation

The SC908 Li-Ion battery charger can be configured inde-

pendently with respect to fast-charge, termination current,

and timing. The charging and battery voltage status are

indicated by the four status outputs.

A charge cycle is initiated when the power adapter is con-

nected to the device and the SC908 VAD pin voltage is

between the Under-Voltage LockOut (UVLO) rising thresh-

old and the input Over Voltage Protection (OVP) threshold.

If the battery voltage is less than the pre-charge threshold,

the output current is regulated to the programmed pre-

charge current. When the pre-charge threshold voltage is

exceeded, the fast-charge Constant Current (CC) mode

begins, with the charge current rising to the programmed

fast-charge current in three soft-start current steps. The

charger enters the Constant Voltage (CV) mode when the

battery voltage rises to its final value (VCV), typically 4.2V.

In the CV mode the BAT voltage is regulated to V , and as

CV

the battery continues to charge it accepts decreasing

current. The CHRGB output turns off when I drops

BAT

below the programmed termination current. If the charge

timer is active, the battery is held in the CV charge mode

until the timer cycle ends. The charger then enters the

monitor mode, where the output remains off until the

voltage at BAT drops by VTReQ, and a new charge cycle is

initiated. If the charge timer is disabled, the monitor mode

is immediately entered upon charge termination.

Pre-Charge Mode

The pre-charge mode is automatically entered when the

battery voltage is below the pre-charge threshold voltage,

which preconditions the battery for fast charging. The

pre-charge current value is set by the resistor on the ITERM

pin, and is programmable from 14mA to 65mA. The pre-

charge current is determined by

IPr eQ

VITERM _ Typ u 130

RITERM

where VITERM_Typ designates the typical value of VITERM. (See

the Termination Current section for precharge current

accuracy.) When the timer is enabled, there is a maximum

allowed pre-charge duration. If the pre-charge time

exceeds 25% of the total charge cycle, the charger will

turn off due to a pre-charge fault. This fault is cleared

when VAD is cycled off and on, or when the EN_NTC pin

is forced low to disable the charger.

Fast-Charge Constant Current Mode

The fast-charge CC mode is active when the battery

voltage

is

above

V

TPreQ

and

less

than

V.

CV

The current can

be set to a maximum of 0.5A and is selected by the

program resistor on the IPRGM pin. The voltage on this

pin represents the charger output current. This allows

the charging current to be measured by sensing the

IPRGM pin voltage using a general purpose Analog-to-

Digital Converter (ADC) and the host microporocessor.

The fast-charge current is determined by

IFQ

VIPRGM _ Typ u 697

RIPRGM

Excellent fast-charge current accuracy is obtained by the

use of a patented polarity-switched current sense

amplifier (US Patent 6,836,095). This nullifies current

measurement offset errors, leaving only a small gain

error. The range of expected fast-charge output current

versus programming resistance RIPRGM is shown in Figures

1a and 1b.

520

500

480

460

440

420

400

380

360

340

320

300

280

260

240

220

200

180

2

2.5

3

3.5

4

4.5

5

5.5

RIPRGM (kΩ)

Figure 1a — Fast-charge Current Variation vs. IPRGM

Resistance, Low Resistance Range

15

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