Application idea: dual input management with AC priority

L6924U

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Doc ID 14716 Rev 2

10

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

possible to AC.

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

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

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

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

AC

IN

V

V

V

−

=

(

)

USB

2

Q

_

DSon

1

Q

_

DSon

USB

IN

I

R

R

V

V

⋅

+

−

=