L6924U datasheet(32/37 Pages) STMICROELECTRONICS

32 / 37 page

Application idea: dual input management with AC priority

L6924U

32/37

Doc ID 14716 Rev 2

Application idea: dual input management with AC

priority

In some applications both AC adapter and USB power source may be available.

Figure 26

shows a possible schematic which provides the possibility to manage two power

sources (AC/USB) and gives the priority to AC adapter in case both sources are available at

the same time.

For simplicity, only the relevant pins of the L6924U for this application have been indicated.

If only the AC adapter is available, since the gates of Q1 and Q2 are connected to AC, both

MOSFETs are off. The AC adapter voltage is provided to the VIN pin through the diode D1.

The voltage at the VIN pin is:

A correct choice of this diode is important to limit Vdiode and keeping VIN as close as

possible to AC.

In this condition the MODE pin is low. This sets the L6924U in AC mode and the battery is

charged with the current programmed by RAC.

When only the USB power source is available, both Q1 and Q2 switch on and the pin VIN is

connected to USB.

The MODE pin is connected to the drains of Q1 and Q2 and is high. Therefore the USB

mode for the L6924U is selected and the battery is charged with a current in accordance

with the resistor connected to the pin IUSB (RUSB).

The voltage of the VIN pin is given by:

The voltage drop across the MOSFETs must be kept as low as possible to avoid reducing

too much the voltage of the VIN pin.

When both sources are present, this circuit gives the priority to the AC adapter. In fact, for

VAC ≥ 5 V, surely both Q1 and Q2 are off and VIN is connected to the AC adapter through

D1. The MODE pin is kept low and L6924U is set to AC mode.

The use of two P-channel MOSFETs connected as shown in Figure 26 is particularly useful

in this case because they remove any path between the two power sources.

diode

−

(

)

USB

DSon

USB

⋅

−

L6924U датащи(PDF) 32 Page - STMicroelectronics