9

LTC4412HV

sn4412hv 4412hvfs

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 LTC4412HV 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 LTC4412HV 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 MOSFET.

Once the auxiliary MOSFET has turned on the voltage drop

across it can be very low depending on the MOSFET’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

charger 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 LTC4412HV’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

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

TYPICAL APPLICATIO S

GND

CTL

SENSE

GATE

STAT

1

2

3

6

5

4

LTC4412HV

PRIMARY

P-CHANNEL

MOSFET

TO LOAD

STATUS OUTPUT

DROPS WHEN A

WALL ADAPTER

IS PRESENT

470k

4412HV F02

BATTERY

CELL(S)

WALL

ADAPTER

INPUT

*

*

AUXILIARY

P-CHANNEL

MOSFET

*DRAIN-SOURCE DIODE OF MOSFET

Figure 2. Automatic Switchover of Load Between a Battery and a

Wall Adapter with Auxiliary P-Channel MOSFET for Lowest Loss

GND

CTL

SENSE

GATE

STAT

1

2

3

6

5

4

LTC4412HV

BATTERY

CHARGER

P-CHANNEL

MOSFET

TO LOAD

STATUS OUTPUT

IS LOW WHEN A

WALL ADAPTER

IS PRESENT

470k

*DRAIN-SOURCE DIODE OF MOSFET

4412HV F03

BATTERY

CELL(S)

*

WALL

ADAPTER

INPUT

Figure 3. Automatic Switchover of Load Between

a Battery and a Wall Adapter in Comparator Mode