L6924U datasheet(19/38 Pages) STMICROELECTRONICS

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L6924U

Applications information: charging process

19/38

7.4

Fast charge phase

When the battery voltage reaches the pre-charge voltage threshold (VPRETH), the L6924U

enters the fast-charge phase.

In this phase the device charges the battery with a constant current, whose value can be set

by external resistors connected to IAC pin (AC adapter mode selected) or to IUSB pin (USB

mode) with an accuracy of 7 %.

In AC adapter mode (MODE pin low), the resistor RAC can be calculated as:

Equation 6

Where VBG is the internal reference equal to 1.23 V, whereas KPRG is a constant equal to

9500.

Figure 11.

IAC pin connection

In USB mode (MODE pin high), the resistor RUSB can be selected as:

Equation 7

Where VBG and KPRG have the same meaning and value above mentioned.

The charge current in USB mode depends on RUSB as well as the state of the ISEL pin.

When this pin is high, the “high-power” USB mode is selected and the charge current is

determined by the equation 7.

The charge current in USB mode should be set in accordance with the typical USB current

capability (up to 500 mA). If ISEL is low, the “low-power” USB mode is selected and the

charge current is a fifth of the high-power USB mode charge current (up to 100 mA)

During low power USB mode operation, since the charge current is reduced by one fifth, the

maximum charging time is proportionally increased (Section 7.8: Maximum charging time).

PRG

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PRG

USB

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