L6924U datasheet(22/38 Pages) STMICROELECTRONICS

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Applications information: charging process

L6924U

22/38

7.8

Maximum charging time

To avoid the charging of a dead battery for a long time, the L6924U has the possibility can

be set a maximum charging time starting from the beginning of the fast-charge phase. This

timer can be set with a capacitor, connected between the TPRG pin and GND. The CTPRG is

the external capacitor (in nF) and is given by the following formula:

Equation 10

Note:

The maximum recommended CTPRG value must be less than 50 nF.

Figure 15.

TPRG pin connection

Where,

RPRG = resistor which sets the current (RUSB or RAC)

VREF = 1.8 V,

KT = 279 x 10

VBG = 1.23 V, and

TMAXCH is the charging time given in seconds.

If the battery does not reach the end-of-charge condition before the time expires, a fault is

issued.

Also during the pre-charge phase there is a safety timer, given by:

Equation 11

If this timer expires and the battery voltage is still lower than VPRETH, a fault signal is

generated, and the charge process finishes.

Note:

When the device is charged in low power USB mode, in order to take into account the

reduced charge current, the maximum charging time is proportionally increased (five times

the maximum charging time calculated with RUSB)

⎟

⎠

⎞

⎜

⎝

⎛

REF

PRG

MAXCH

TPRG

MAXCH

MAXPRECH

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