CHG-IN

Batt

RSense

Diff-Amp

120 m:

0.583V

50 mA

Charge Current

750 mA

1.74V

ICHG =

(VDIFF - 0.497)

1.655

LP3947

www.ti.com

SNVS298B – NOVEMBER 2004 – REVISED APRIL 2013

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 I2C interface.

Table 2. 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

I2C.

HIGH

LOW

USB Mode

LOW

HIGH

100 mA charge current

LOW

LOW

Default 500 mA charge current. Can be reprogrammed via I2C.

BATTERY VOLTAGE SELECTION

The battery voltage regulation can be set to 4.1V or 4.2V by default. Please refer to Ordering Information for

more details.

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

provides 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:

(1)

Where voltage at Diff Amp output (VDIFF) is in volt, and charge current (ICHG) is in amps.

Figure 5. Charge Current Monitoring Circuit (Diff-Amp)

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 3 shows all the conditions.

Copyright © 2004–2013, Texas Instruments Incorporated

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