CHARGE CURRENT SELECTION IN CONSTANT

CURRENT MODE

In the AC adaptor mode, the LP3947 is designed to provide

a charge current ranging from 100 mA to 750 mA, in steps of

50 mA, to support batteries with different capacity ratings.

The default value is 500 mA. No external resistor is required

to set the charge current in the LP3947. In the USB mode,

the LP3947 will initially charge with 100 mA (ISEL = high). By

setting the ISEL pin low, charge current can be programmed

to 500 mA. In addition, with ISEL = low, the charge current

can be programmed to different values via the I

TABLE 1. Charge Current Selection in AC Adaptor/USB Mode

MODE Pin

ISEL Pin

Functions

AC Adaptor Mode

HIGH

HIGH

ISEL polarity is irrelevant. Default 500 mA charge current. Can be

reprogrammed via I

HIGH

LOW

USB Mode

LOW

HIGH

100 mA charge current

LOW

LOW

Default 500 mA charge current. Can be reprogrammed via I

BATTERY VOLTAGE SELECTION

The battery voltage regulation can be set to 4.1V or 4.2V by

default. Please refer to the Ordering Information table for

more detail.

END OF CHARGE (EOC) CURRENT SELECTION

The EOC thresholds can be programmed to 0.1C, 0.15C or

0.2C in the LP3947. The default value is 0.1C, which pro-

vides the highest energy storage, but at the expense of

longer charging time. On the other hand, 0.2C takes the

least amount of charging time, but yields the least energy

storage.

CHARGE CURRENT SENSE DIFFERENTIAL AMPLIFIER

The charge current is monitored across the internal 120 m

Ω

current sense resistor. The differential amplifier provides the

analog representation of the charge current. Charge current

can be calculated using the following equation:

Where voltage at Diff Amp output (V

charge current (I

Monitoring the Diff Amp output during constant voltage cycle

can provide an accurate indication of the battery charge

status and time remaining to EOC. This feature is particularly

useful during constant voltage mode. The current sense

circuit is operational in the LDO mode as well. It can be used

to monitor the system current consumption during testing.

LED CHARGE STATUS INDICATORS

The LP3947 is equipped with two open drain outputs to drive

a green LED and a red LED. These two LEDs work together

in combinations to indicate charge status or fault conditions.

Table 2 shows all the conditions.

20111009

FIGURE 4. Charge Current Monitoring Circuit (Diff-Amp)

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