DN386 - Ideal Diode Controller Eliminates Energy Wasting Diodes in Power OR-ing Applications

Ideal Diode Controller Eliminates Energy Wasting Diodes in
Power OR-ing Applications – Design Note 386
David Laude
Introduction
Many modern electronic devices need a means to automatically switch between power sources when prompted
by the insertion or removal of any source. The LTC®4412
simplifies PowerPathTM management and control by
providing a low loss, near ideal diode controller function. Any circuit that could otherwise use a diode OR
to switch between power sources can benefit from the
LTC4412. The forward voltage drop of an LTC4412 ideal
diode is far less than that of a conventional diode, and
the reverse current leakage can be smaller for the ideal
diode as well (see Figure 1). The tiny forward voltage
drop reduces power losses and self-heating, resulting
in extended battery life. Features include:
• Wide supply operating range of 2.5V to 28V (36V
absolute maximum)
• Voltage drop across the controlled external MOSFET
of only 20mV (typical)
Applications include anything that takes power from
two or more inputs:
• Low component count helps keep overall system
cost low
• Cellular phones
• 6-pin ThinSOTTM package permits a compact design
solution
• PDAs
1
• Protection of MOSFET from excessive gate-to-source
voltage with VGS limiter
• Low quiescent current of 11µA with a 3.6V supply,
independent of the load current
• A status pin that can be used to enable an auxiliary MOSFET power switch or to indicate to a
microcontroller that an auxiliary supply, such as a
wall adapter, is present
• A control input pin for external control, such as from
a microcontroller
• Portable computers
• MP3 players and electronic video and still cameras
CURRENT (A)
• USB peripherals
CONSTANT
RON
(VGS LIMITED)
• Wire-ORed multipowered equipment
LTC4412
• Uninterruptible power supplies for alarm and emergency systems
• Systems with standby capabilities
CONSTANT
VOLTAGE
(VGS VARIABLE)
• Systems that use load sharing between two or more
batteries
SCHOTTKY
DIODE
• Multibattery charging from a single charger
0
0.02
0.5
• Logic controlled power switches
FORWARD VOLTAGE (V)
DN386 F01
Figure 1. LTC4412 Ideal Diode Controller vs
Schottky Diode Characteristics
03/06/386_conv
, LTC and LT are registered trademarks of Linear Technology Corporation.
PowerPath and ThinSOT are trademarks of Linear Technology Corporation.
Automatic Power Switching Between
Two Power Sources
Figure 2 illustrates an application circuit for the automatic switchover of load between two power sources,
in this example a wall adapter and a battery. While the
wall adapter is absent, the LTC4412 controls the gate
of Q1 to regulate the voltage drop across the MOSFET
to 20mV, thus wasting negligible battery power. The
STAT pin is an open circuit while the battery provides
power. When a wall adapter or other supply connected
to the auxiliary input is applied, the SENSE pin voltage
rises. As the SENSE pin voltage rises above VIN – 20mV,
the LTC4412 pulls the GATE voltage up to turn off the
P-channel MOSFET. When the voltage on SENSE exceeds
VIN + 20mV, the STAT pin sinks 10µA of current to indicate that an AC wall adapter is present. The system
is now in the reverse turn-off mode, where power to
the load is delivered through the external diode and no
current is drawn from the battery. The external diode
is used to protect the battery against some auxiliary
input faults such as a short to ground. Note that the
external MOSFET is wired so that the drain to source
diode will reverse bias and not deliver current to the
battery when a wall adapter input is applied.
Load Sharing
Figure 3 shows a dual battery load sharing application
with automatic switchover of power between the batteries and a wall adapter. In this example, the battery
with the higher voltage supplies all of the power until
it has discharged to the voltage of the other battery.
Once both batteries have the same voltage, they share
the load with the battery with the higher capacity providing proportionally higher current to the load. In this
way, the batteries discharge at a relatively equal rate,
maximizing battery run time.
When a wall adapter input is applied, both MOSFETs turn
off and no load current is drawn from the batteries. The
LTC4412’s STAT pins provide information as to which
input is supplying the load current. The ganging of the
LTC4412s can be applied to as many power inputs as
are needed.
Conclusion
The LTC4412 provides a simple means to implement a
low loss ideal diode controller that extends battery life
and reduces self heating. The low external parts count
results in low implementation cost and with its ThinSOT
6-pin package, a compact design as well. Its versatility
is useful in a variety of applications (see the LTC4412
data sheet for additional applications).
1N5819
WALL
ADAPTER
INPUT
Q1
*
TO LOAD
BAT1
COUT
LTC4412
1
6
VIN SENSE
2
5
GND GATE
3
4
CTL STAT
VCC
470k
STATUS IS HIGH
WHEN BAT1 IS
SUPPLYING
LOAD CURRENT
1N5819
WALL
ADAPTER
INPUT
*
Q2
*
TO LOAD
BATTERY
CELL(S)
COUT
Q1
LTC4412
6
VIN SENSE
2
5
GND GATE
3
4
CTL STAT
1
BAT2
470k
DN386 F02
LTC4412
6
VIN SENSE
2
5
GND GATE
3
4
CTL STAT
1
VCC
STATUS OUTPUT LOW
WHEN WALL ADAPTER IS
SUPPLYING LOAD CURRENT
*PARASITIC DRAIN-SOURCE DIODE OF MOSFET
Q1: FAIRCHILD SEMICONDUCTOR FDN306P (408) 822-2126
Figure 2. Automatic Power Switching Between
a Battery and a Wall Adapter
Data Sheet Download
www.linear.com
Linear Technology Corporation
WHEN BOTH STATUS LINES ARE
HIGH, THEN BOTH BATTERIES ARE
SUPPLYING LOAD CURRENTS. WHEN
BOTH STATUS LINES ARE LOW, THEN
WALL ADAPTER IS PRESENT AND
SUPPLYING FULL LOAD CURRENT
VCC
470k
DN386 F03
*PARASITIC DRAIN-SOURCE DIODE OF MOSFET
STATUS IS HIGH
WHEN BAT2 IS
SUPPLYING
LOAD CURRENT
Q1, Q2: SILICONIX Si4953DY (800) 554-5565
Figure 3. Dual Battery Load Sharing with Automatic
Switchover of Power from Batteries to Wall Adapter
For applications help,
call (408) 432-1900
dn386f_conv LT/TP 0306 409K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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© LINEAR TECHNOLOGY CORPORATION 2006
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