SC820 datasheet(14/21 Pages) SEMTECH

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SC820

Applications Information (continued)

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 standard value 1% tolerance

resistors. 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 processor

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 or source at least 75μA

to ensure a low or high state, respectively, although the

drive current is typically far less. (See the Electrical

Characteristics table.)

If the ENB input voltage is permitted to float to mid-range,

the charger is enabled but it will turn off its output follow-

ing charge termination and will enter the monitor state.

This state is explained in the next section. Mid-range can

be selected either by floating the input (sourcing or

sinking less than 5μA) or by being externally forced such

that V

ENB

falls within the midrange limits specified in the

Electrical Characteristics table.

When driven low (V

ENB

< Max V

), the charger is enabled

and will continue to float-charge the battery following

termination. If the charger is already in monitor state fol-

lowing a previous termination, it will exit the monitor state

and begin float-charging.

When ENB is driven high (V

ENB

> Min V

), the charger is

disabled and the ENB input pin enters a high impedance

state, suspending tri-level functionality. The specified

high level input current I

is required only until a high

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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