SC820 datasheet(12/21 Pages) SEMTECH

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SC820

Applications Information (continued)

input also provides 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. It is also useful where

there is no signal available to indicate whether USB low or

high power mode should be selected.

Constant Current Mode Fast-charge Current

Programming

The Constant Current (CC) mode is active when the

battery voltage is above the pre-charge threshold voltage

(VT

PreQ) and less than VCV. When VAD is the selected input,

the programmed CC regulation fast-charge (FQ) current

is inversely proportional to the IPRGM pin resistance to

GND according to the equation

When VUSB is the selected input, the programmed CC

mode fast-charge current is inversely proportional to the

IPUSB pin resistance to GND according to the equation

The fast-charge current can be programmed for a

minimum of 70mA and a maximum of 995mA for either

input, nominally. The VAD input is designed for lower

dropout voltage at high current, which ensures charging

without thermal limiting with a charging adapter operat-

ing in current limit of at least 700mA.

Current regulation accuracy is dominated by gain error at

high current settings and offset error at low current set-

tings. The range of expected fast-charge output current

versus programming resistance R

IPRGM

or R

IPUSB

(for VAD or

VUSB input selected, respectively) is shown in Figures 1a

and 1b. The figures show the nominal current versus

nominal R

IPRGM

or R

IPUSB

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 1a and 1b

show low and high resistance ranges, respectively.

Pre-charge Mode

This mode is automatically enabled when the battery

voltage is below the pre-charge threshold voltage (VT

PreQ).

Pre-charge current conditions the battery for fast charg-

ing. The pre-charge current value is fixed at 20%

nominally of the fast-charge current for the selected

input. The fast-charge current is programmed by the

2.5

3.5

4.5

5.5

6.5

250

300

350

400

450

500

550

600

650

700

750

800

850

900

950

1000

1050

1100

IPRGM

or R

IPUSB

(kΩ), R-tol = 1%

Figure 1a — Fast-charge Current Tolerance versus

Programming Resistance, Low Resistance Range

11121314

15161718

19202122

232425

26272829

100

125

150

175

200

225

250

275

300

325

IPRGM

or R

IPUSB

(kΩ), R-tol = 1%

Figure 1b — Fast-charge Current Tolerance versus

Programming Resistance, High Resistance Range

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