SC820 datasheet(14/22 Pages) SEMTECH

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SC820

Applications Information (continued)

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 threshold current 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

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% tolerance resistor. The dots on each plot indicate

the currents obtained with the Electronic Industries

Association (EIA) E96 standard value 1% tolerance resis-

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

ranges, respectively.

Enable Input

The ENB pin is a tri-level logical input that allows selection

of the following behaviors:

Charging enabled with float-charging after ter-

mination (ENB = low range)

•

Charging enabled with float-charging disabled

and battery monitoring at termination (ENB =

mid range)

Charging disabled (ENB = high range).

This input is designed to interface to a processor GPIO

port powered from a peripheral supply voltage as low as

1.8V or as high as a fully charged battery. While a con-

nected GPIO port is configured as an output, the proces-

sor writes a 0 to select ENB low-range, and 1 to select

high-range. The GPIO port is configured as an input to

select mid-range.

ENB can also be permanently grounded to select low-

range or left unconnected to select mid-range if it will not

be necessary to change the level selection.

The equivalent circuit looking into the ENB pin is a vari-

able resistance, minimum 15kΩ, to an approximately 1V

source. The input will float to mid range whenever the

external driver sinks or sources less than 5μA, a common

worst-case characteristic of a high impedance or a weak

pull-up or pull-down GPIO configured as an input. The

driving GPIO must be able to sink at least 25μA or source

at least 50μA to ensure a low or high state, respectively.

(See the Electrical Characteristics table.)

With the ENB input voltage floating to mid-range, the

charger is enabled but it will turn off its output following

charge termination and will enter the monitor state. This

•

Figure 3a — Termination Current Tolerance versus

Programming Resistance, Low Resistance Range

22.5

33.5

44.5

55.5

66.5

100

105

110

115

IPRGM

(kΩ), R-tol = 1%

Figure 3b — Termination Current Tolerance versus

Programming Resistance, High Resistance Range

11121314

15161718

19202122

232425

26272829

IPRGM

(kΩ), R-tol = 1%

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