10
LTC4412
4412fa
TYPICAL APPLICATIO S
U
This is due to the 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 COUT 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 avail-
able).
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 VIN falls below VLOAD, it will turn on the
auxiliary MOSFET if CTL is low, but VLOAD must stay up
long enough for the MOSFET to turn on. At a minimum,
COUT capacitance must be sized to hold up VLOAD until the
transistion between the sets of MOSFETs is complete.
Sufficient capacitance on the load and low or no capaci-
tance on VIN will help ensure this. If desired, this can be
avoided by use of a capacitor on VIN to ensure that VIN
falls more slowly than VLOAD.
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 supply-
ing the load current. This concept can be expanded to
more power inputs.
VIN
GND
CTL
SENSE
GATE
STAT
1
2
3
6
5
4
LTC4412
COUT
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
VCC
BAT1
WALL
ADAPTER
INPUT
VIN
GND
CTL
SENSE
GATE
STAT
1
2
3
6
5
4
LTC4412
470k
VCC
BAT2
*
*DRAIN-SOURCE DIODE OF MOSFET
Figure 5. Dual Battery Load Sharing with Automatic
Switchover of Load from Batteries to Wall Adapter
VIN
GND
CTL
SENSE
GATE
STAT
1
2
3
6
5
4
LTC4412
*DRAIN-SOURCE DIODE OF MOSFET
PRIMARY
P-CHANNEL MOSFETS
COUT
TO LOAD
4412 F04
AUXILIARY POWER
SOURCE INPUT
*
PRIMARY
POWER
SOURCE INPUT
AUXILIARY
P-CHANNEL MOSFETS
470k
MICROCONTROLLER
0.1
μF
Figure 4. Microcontroller Monitoring and Control
of Two Power Sources