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VFQFPN16

USB compatible battery charger system with integrated power switch for Li-Ion/Li-Polymer

STMicroelectronics 

L6924U

6

Operation description

Operation description

The L6924U is a fully integrated battery charger that allows a very compact battery

management system for space limited applications. It integrates in a small package all the

power elements: power MOSFET, reverse blocking diode and the sense resistor.

It normally works as a linear charger when powered from an external voltage regulated

adapter or USB port.

However, thanks to its very low minimum input voltage (down to 2.5 V) the L6924U can also

work as a quasi-pulse charger when powered from a current limited adapter. To work in this

condition, it is enough to set the device’s charging current higher than the adapter’s one

(Chapter 6.2: Quasi-pulse mode on page 14). The advantage of the linear charging

approach is that the device has a direct control of the charging current and so the designer

needn’t to rely on power source. However, the advantage of the quasi-pulse approach is that

the power dissipated inside the portable equipment is dramatically reduced.

The L6924U charges the battery in three phases:

● Pre-charge constant current: in this phase (active when the battery is deeply

discharged) the battery is charged with a low current (internally set to 10 % of the fast-

charge current).

● Fast-charge constant current: in this phase the device charges the battery with the

maximum current (IAC for AC adapter mode, IUSB for USB mode).

● Constant voltage: when the battery voltage reaches the selected output voltage, the

device starts to reduce the current, until the charge termination is done.

The full flexibility is provided by:

● Programmable fast-charging current (IAC or IUSB) (Chapter 7.3 on page 18).

● Programmable end of charge current threshold (IENDTH) (Chapter 7.4 on page 20).

● Programmable end of charge timer (TMAXCH) (Chapter 7.7 on page 21).

If a PTC or NTC resistor is used, the device can monitor the battery temperature in order to

protect the battery from operating under unsafe thermal conditions.

Beside the good thermal behavior guaranteed by low thermal resistance of the package,

additional safety is provided by the built-in temperature control loop. The IC monitors

continuously its junction temperature. When the temperature reaches approximately 120 °C,

the thermal control loop starts working, and reduces the charging current, in order to keep

the IC junction temperature at 120 °C.

Two open collector outputs are available for diagnostic purpose (status pins ST1 and ST2).

They can be also used to drive external LEDs or to interface with a microcontroller. The

voltage across the resistor connected between IEND and GND gives information about the

actual charging current (working as a gas gauge), and it can be easily fed into a

microcontroller ADC.

Battery disconnection control is provided thanks to the differentiated sensing and forcing

output pins. A small current is sunk and forced through VOUT. If VOSNS doesn’t detect the

battery, the IC goes into a standby mode.

Figure 5 on page 12 shows the real charging profile of a Li-Ion battery, with a fast charge

current of 450 mA (R1 or R2 = 26 kΩ).

Doc ID 14716 Rev 2

11/37

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