SC824 datasheet(14/29 Pages) SEMTECH

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SC824

Termination

When the battery voltage reaches V

CV, the SC824 transi-

tions from constant current regulation to constant voltage

regulation. The current into the battery decreases while

the BAT voltage is regulated to V

as the battery becomes

fully charged. When the output current drops below the

termination current threshold, charging terminates. Upon

termination, the charger either enters monitor state or

float charges the battery for the top-off timer duration,

depending on the configuration of the charge timer. After

a 20 second delay following termination, the STAT1 pin

open drain output turns off.

The termination current threshold is fixed at 10% of the

fast-charge current, as programmed by the IPRGM pin

resistance to ground, for all charging modes.

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 dimin-

ishing lower bound on the output current while charging

in CV regulation. The load current into a typical digital

system is highly transient in nature. Charge cycle termina-

tion 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

Applications Information (continued)

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 any charging

mode) is shown in Figures 3a and 3b. Each figure shows

the nominal termination current versus nominal R

IPRGM

resistance as the center plot and two theoretical limit

plots indicating maximum and minimum current vs.

nominal programming resistance. 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 a 1% toler-

ance resistor. The dots on each plot indicate the currents

obtained with EIA E96 standard value 1% tolerance resis-

tors. Figures 3a and 3b show low and high resistance

ranges, respectively.

Monitoring Output Current

The output current I

BAT

is indicated by the voltage at the

IPRGM pin according to the following equation.

1000

IPRGM

BAT

Ensure that the IPRGM pin is not loaded or corrupted by

the processor Analog to Digital Converter (ADC). An RC

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 (k

Ω), R-tol = 1%

Figure 2a — Pre-charge Current and USB Low Power

Mode Fast-charge Current Tolerance vs. Programming

Resistance, Low Resistance Range

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

IPRGM (k

Ω), R-tol = 1%

Figure 2b — Pre-charge Current and USB Low Power

Mode Fast-charge Current Tolerance vs. Programming

Resistance, High Resistance Range

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