SC820 datasheet(13/21 Pages) SEMTECH

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SC820

resistance between IPRGM and GND for the VAD input,

and by the resistance between IPUSB and GND for the

VUSB input.

Pre-charge current regulation accuracy is dominated by

offset error. The range of expected pre-charge output

current versus programming resistance is shown in Figures

2a and 2b. The figures show the nominal pre-charge

current versus nominal resistance as the center plot and

two theoretical limit plots indicating maximum and

minimum current versus nominal programming resis-

tance. These plots are derived from models of the

expected worst-case contribution of error sources

depending on programmed current. The current range

includes the uncertainty due to 1% tolerance resistors.

The dots on each plot indicate the currents obtained with

standard value 1% tolerance resistors. Figures 2a and 2b

show low and high resistance ranges, respectively.

Termination

When the battery voltage reaches V

, the SC820 transi-

tions from constant current regulation to constant voltage

regulation. While V

BAT

is regulated to V

, the current into

the battery decreases as the battery becomes fully

charged. When the output current drops below the termi-

nation current threshold, charging terminates. Upon

termination, the STATB pin open drain output turns off

and the charger either enters monitor state or float-

charges the battery, depending on the logical state of the

ENB input pin.

Applications Information (continued)

The termination current threshold is fixed at 10% of the

VAD input fast-charge current, as programmed by the

resistance between IPRGM and GND. The IPRGM pin

resistance determines the termination current threshold

regardless of whether the selected charging input is VAD

or VUSB.

Charger output current is the sum of the battery charge

current and the system load current. Battery charge

current changes gradually, and establishes a slowly

diminishing lower bound on the output current while

charging in CV mode. The load current into a typical

digital system is highly transient in nature. Charge cycle

termination is detected when the sum of the battery

charging current and the greatest load current occurring

within the immediate 300μs to 550μs past interval is less

than the programmed termination current. This timing

behavior permits charge cycle termination to occur

during a brief low-load-current interval, and does not

require that the longer interval average load current be

small.

Termination current threshold accuracy is dominated by

offset error. The range of expected termination current

versus programming resistance R

IPRGM

(for either VAD or

VUSB input selected) is shown in Figures 3a and 3b. The

figures show the nominal termination current versus

nominal R

IPRGM

resistance as the center plot and two theo-

retical limit plots indicating maximum and minimum

current versus nominal programming resistance. These

Figure 2a — Pre-charge Current Tolerance versus

Programming Resistance, Low Resistance Range

2.5

3.5

4.5

5.5

6.5

100

110

120

130

140

150

160

170

180

190

200

210

220

230

240

250

260

270

IPRGM

or R

IPUSB

(kΩ), R-tol = 1%

Figure 2b — Pre-charge Current Tolerance versus

Programming Resistance, High Resistance Range

11121314

15161718

19202122

232425

26272829

IPRGM

or R

IPUSB

(kΩ), R-tol = 1%

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