Datasheet

LTC4412
11
4412fb
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Typical applicaTions
SENSE pin voltage rising above the battery voltage and
turning off the MOSFET before the Schottky diode turns
on. The factors that determine the magnitude of the voltage
droop are the auxiliary input rise time, the type of diode
used, the value of C
OUT
and the load current.
Ideal Diode Control with a Microcontroller
Figure 4 illustrates an application circuit for microcon-
troller monitoring and control
of two power sources. The
microcontroller’s analog inputs, perhaps with the aid of a
resistor voltage divider, monitors each supply input and
commands the LTC4412 through the CTL input. Back-to-
back MOSFETs are used so that the drain-source diode will
not power the load when the MOSFET is turned off (dual
MOSFETs in one package are commercially available).
With a logical low input on the CTL pin, the primary input
supplies power to the load regardless of the auxiliary
voltage. When CTL is switched high, the auxiliary input
will power the load whether or not it is higher or lower
than the primary power voltage. Once the auxiliary is
on, the primary power can be removed and the auxiliary
will continue to power the load. Only when the primary
voltage
is higher than the auxiliary voltage will taking
CTL low switch
back to the primary power, otherwise
the auxiliary stays connected. When the primary power
is disconnected and V
IN
falls below V
LOAD
, it will turn
on the auxiliary MOSFET if CTL is low, but V
LOAD
must
stay up long enough for the MOSFET to turn on. At a
minimum, C
OUT
capacitance must be sized to hold up
V
LOAD
until the transition between the sets of MOSFETs
is complete. Sufficient capacitance on the load and low
or no capacitance on V
IN
will help ensure this. If desired,
this can be avoided by use of a capacitor on V
IN
to ensure
that V
IN
falls more slowly than V
LOAD
.
Load Sharing
Figure 5 illustrates an application circuit for dual battery
load sharing with automatic switchover of load from
batteries to wall adapter. Whichever battery can supply
the higher voltage will provide the load current until it is
discharged to the voltage of the other battery. The load will
then be shared between the two batteries according to the
capacity of each battery. The higher capacity battery will
provide proportionally higher current to the load. When
a wall adapter input is
applied, both MOSFETs will turn
off and no load current will be drawn from the batteries.
The STAT pins provide information as to which input is
supplying the load current. This concept can be expanded
to more power inputs.
Figure 4. Microcontroller Monitoring and Control
of Tw o Power Sources
Figure 5. Dual Battery Load Sharing with Automatic
Switchover of Load from Batteries to Wall Adapter
V
IN
GND
CTL
SENSE
GATE
STAT
1
2
3
6
5
4
LTC4412
*DRAIN-SOURCE DIODE OF MOSFET
PRIMARY
P-CHANNEL MOSFETS
C
OUT
TO LOAD
4412 F04
AUXILIARY POWER
SOURCE INPUT
*
*
*
*
PRIMARY
POWER
SOURCE INPUT
AUXILIARY
P-CHANNEL MOSFETS
470k
MICROCONTROLLER
0.1µF
V
IN
GND
CTL
SENSE
GATE
STAT
1
2
3
6
5
4
LTC4412
C
OUT
TO LOAD
STATUS IS HIGH
WHEN BAT1 IS
SUPPLYING
LOAD CURRENT
WHEN BOTH STATUS LINES ARE
HIGH, THEN BOTH BATTERIES ARE
SUPPLYING LOAD CURRENTS. WHEN
BOTH STATUS LINES ARE LOW THEN
WALL ADAPTER IS PRESENT
STATUS IS HIGH
WHEN BAT2 IS
SUPPLYING
LOAD CURRENT
470k
4412 F05
V
CC
BAT1
WALL
ADAPTER
INPUT
V
IN
GND
CTL
SENSE
GATE
STAT
1
2
3
6
5
4
LTC4412
470k
V
CC
BAT2
*
*
*DRAIN-SOURCE DIODE OF MOSFET
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