SC820 datasheet(12/22 Pages) SEMTECH

12 / 22 page

SC820

Applications Information (continued)

The VUSB input provides a higher de-selection falling

threshold appropriate to the USB specification. The USB

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 (by switching the

IPUSB resistor), or 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 nominal fast-charge current for either input can be

programmed to the minimum of 70mA (R

IPxxx

= 29.4kΩ).

The maximum fast-charge current for the VAD input is

995mA nominally (R

IPRGM

= 2.05kΩ), and for the VUSB

input, the programmed fast-charge current should not

exceed 450mA (R

IPUSB

= 4.42kΩ) nominally. (If a greater

USB input fast-charge current is desired, please contact

your Semtech Field Applications Engineer for assistance.)

The VAD input is designed for lower dropout voltage at

high current, which ensures charging without thermal

limiting with a charging adapter operating 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

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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