L6924UTR datasheet(19/37 Pages) STMICROELECTRONICS

19 / 37 page

L6924U

Applications information: charging process

Doc ID 14716 Rev 2

19/37

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 RUSB resistor 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 7.7: Maximum charging time).

Figure 12.

IUSB pin connection

Regardless of the operation mode (AC adapter or USB), during the fast-charge phase the

battery voltage increases until it reaches the programmed output voltage (4.2 V). A safety

timer is also present. If the Fast-charge phase does not finish within the programmed time

(see Chapter 7.7: Maximum charging time on page 21), a fault is given.

PRG

⋅

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

PRG

USB

⋅

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

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