L6924D datasheet(19/38 Pages) STMICROELECTRONICS

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L6924D

Applications information: charging process

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7.3

Pre-charge voltage

If the VPRE pin is floating, a default value of VPRETH is set, equal to 2.8V (VPRETHDefault).

Otherwise, the device offers the possibility to program this value, with a resistor connected

between the VPRE pin and GND Figure 12. In this case, the RVPRE is given by the equation 8.3:

Figure 12. VPRE pin connection

Where RVPRE is the resistor between VPRE and GND, and RPRG is the resistor used to set the

charge current (see Section 7.4: Fast charge current), and VPRETH is the selected threshold.

A safety timer is also present. If the battery voltage doesn't rise over VPRETH, before this time is

expired, a fault is given (see Section 7.8: Maximum charging time). If at the beginning of the

charge process, the battery voltage is higher than the VPRETH, the Pre-Charge phase is

skipped.

7.4

Fast charge current

When the battery voltage reaches the Pre-charge voltage threshold (VPRETH), the L6924D

starts the Fast-charge Phase. In this phase, the device charges the battery with a constant

current, ICHG, programmable by an external resistor that sets the charge current with an

accuracy of 7% Figure 13. The formula used to select the RPRG as follows:

Figure 13. IPRG pin connection

Where KPRG is a constant, equal to 9500.

During this phase, the battery voltage increases until it reaches the programmed output

voltage. A safety timer is also present. If this time expires, a fault is given (Section 7.8:

Maximum charging time).

⎟

⎠

⎞

⎜

⎝

⎛

PRETHDefau

PRETH

PRG

VPRE

Eq: 8-3

VPRE

L6924D

PRE

VPRE

L6924D

PRE

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

CHG

PRG

Eq: 8-4

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