BQ29312_15 Datasheet, PDF(26/39 Page) Texas Instruments

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BQ29312_15 Datasheet, PDF (26/39 Pages) Texas Instruments – FOUR CELL LITHIUM-ION

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bq29312

SLUS546E â MARCH 2003 â REVISED MARCH 2005

APPLICATION INFORMATION (continued)

www.ti.com

V(PACK)

REG

ZVCHG

GHD

OFF

3.5 V+VGS(ZVCHGFET)

3.3 V

ZVCHG FET = OFF

V = VPACK*(1/2)

3.5 V (typ.)

CHG FET = ON

âLâ (1 V)

OFF

DSG

CHG FET = ON

âLâ (1 V)

Battery

Voltage

Charge

Current

Fast Charge Current

0 V and Precharge Current

0 V Charge

Mode

Fast Charge Mode

Precharge

Mode

Figure 10. Signal Timing of Pins During 0 V Charging and Precharging (0 V Charge FET)

Common FET

This mode does not require a dedicated precharge FET (ZVCHG-FET). The charge FET (CHG-FET) is ON at

initialization of the bq29312 when PMS = V(PACK) allowing for 0 V or precharge current to flow. The application

circuit is shown in Figure 11. The charger is expected to provide the precharge function in this mode, where the

charger provides a precharge current level suitable to charge cells below a set level, typically below 3.0 V per

cell. When the lowest cell voltage rises above this level then a fast charging current is applied.

When the charger is connected the voltage at PMS rises. Once it is above 0.7 V, the CHG output is driven to

GND which turns ON the CHG-FET. The charging current flows through the CHG-FET and a back diode of

DSG-FET. The pack voltage is represented by the following equation.

V(PACK) = V(BAT) + VF + VDS(CHG-FET)

Where VF = 0.7 V is the forward voltage of a DSG-FET back diode and is typically 0.7 V.

While V(PACK) is maintained above 0.7 V the precharging current is maintained. While V(PACK) and V(BAT) are

under the bq29312 supply voltage then the bq29312 regulator is inactive and the host controller is not functional.

Thus, any protection features of this chipset do not function during this period. This state continues until V(PACK)

goes higher than the bq29312 minimum supply voltage.

When V(BAT) rises and V(PACK) reaches bq29312 minimum supply voltage, the REG output is active providing a

3.3 V (typ) supply to the host. When this level is reached the CHG pin changes its state from GND to the level

controlled with CHG bit in bq29312 registers. In this state, the CHG output level is driven by a clamp circuit so

that its voltage level changes from 0 V to 1 V. Also, the host controller is active and can turn ON the DSG-FET.

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