Datasheet

Application idea: dual input management with AC priority L6924U
32/37 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
MOSFETs are off. The AC adapter voltage is provided to the V
IN
pin through the diode D1.
The voltage at the V
IN
pin is:
A correct choice of this diode is important to limit V
diode
and keeping V
IN
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 R
AC
.
When only the USB power source is available, both Q1 and Q2 switch on and the pin V
IN
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 I
USB
(R
USB
).
The voltage of the V
IN
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 V
IN
pin.
When both sources are present, this circuit gives the priority to the AC adapter. In fact, for
V
AC
≥ 5 V, surely both Q1 and Q2 are off and V
IN
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.
diodeACIN
VVV
−
=
(
)
USB2Q_DSon1Q_DSonUSBIN
IRRVV
⋅
+
−
=