LTC4412
10
4412fb
For more information www.linear.com/LTC4412
Typical applicaTions
Figure 2. Automatic Switchover of Load Between a Battery and a
Wall Adapter with Auxiliary P-Channel MOSFET for Lowest Loss
Figure 3. Automatic Switchover of Load Between
a Battery and a Wall Adapter in Comparator Mode
V
IN
GND
CTL
SENSE
GATE
STAT
1
2
3
6
5
4
LTC4412
PRIMARY
P-CHANNEL
MOSFET
C
OUT
TO LOAD
STATUS OUTPUT
DROPS WHEN A
WALL ADAPTER
IS PRESENT
470k
4412 F02
BATTERY
CELL(S)
WALL
ADAPTER
INPUT
*
*
P-CHANNEL
MOSFET
*DRAIN-SOURCE DIODE OF MOSFET
V
IN
GND
CTL
SENSE
GATE
STAT
1
2
3
6
5
4
LTC4412
BATTERY
CHARGER
P-CHANNEL
MOSFET
C
OUT
TO LOAD
STATUS OUTPUT
IS LOW WHEN A
WALL ADAPTER
IS PRESENT
470k
*DRAIN-SOURCE DIODE OF MOSFET
4412 F03
V
CC
BATTERY
CELL(S)
*
WALL
ADAPTER
INPUT
Automatic PowerPath Control
The applications shown in Figures 1, 2 and 3 are automatic
ideal diode controllers that require no assistance from a
microcontroller. Each of these will automatically connect
the higher supply voltage, after accounting for certain
diode forward voltage drops, to the load with application
of the higher supply voltage.
Figure 1 illustrates an application circuit for automatic
switchover of a load between a battery and a wall adapter
or other power input. With application of the battery, the
load will initially be pulled up by the drain-source diode
of the P-channel MOSFET. As the LTC4412 comes into
action, it will control the MOSFET’s gate to turn it on and
reduce the MOSFET’s voltage drop from a diode drop to
20mV. The system is now in the low loss forward regula
-
tion mode. Should the wall adapter input be applied, the
Schottky diode will pull up the SENSE pin, connected to
the load, above the battery voltage and the LTC4412 will
turn the MOSFET off. The STAT pin will then sink current
indicating an auxiliary input is connected. The battery is
now supplying no load current and all the load current
flows through the Schottky diode. A
silicon diode could
be used instead of the Schottky, but will result in higher
power dissipation and heating due to the higher forward
voltage drop.
Figure 2 illustrates an application circuit for automatic
switchover of load between a battery and a wall adapter
that features lowest power loss. Operation is similar
to Figure 1 except that an auxiliary P-channel MOSFET
replaces the diode. The STAT pin is used to turn on the
MOSFET once the SENSE pin voltage exceeds the battery
voltage by 20mV. When the wall adapter input is applied,
the drain-source diode of the auxiliary MOSFET will turn
on first to pull up the SENSE pin and turn off the primary
MOSFET followed by turning on of the auxiliary MOSFE
T
.
Once the auxiliary MOSFET has turned on the voltage drop
across it can be very low depending on the MOSFE
T’s
characteristics.
Figure 3 illustrates an application circuit for the automatic
switchover of a load between a battery and a wall adapter
in the comparator mode. It also shows how a battery char
-
ger can be connected. This circuit differs from Figure 1
in the way the SENSE pin is connected. The SENSE pin is
connected directly to
the auxiliary power input and not the
load. This change
forces the LTC4412’s control circuitry
to operate in an open-loop comparator mode. While the
battery supplies the system, the GATE pin voltage will be
forced to its lowest clamped potential, instead of being
regulated to maintain a 20mV drop across the MOSFET.
This has the advantages of minimizing power loss in the
MOSFET by minimizing its R
ON
and not having the influence
of a linear control loop’s dynamics. A possible disadvantage
is if the auxiliary input ramps up slow enough the load
voltage will initially droop before rising. This is due to the
