DN1003 - Ideal Diode Controller Eliminates Energy Wasting Diodes in Power OR-ing Applications

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Ideal Diode Controller Eliminates Energy Wasting Diodes in Power
OR-ing Applications – Design Note 1003
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)
• 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
• 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
TM
• 6-pin ThinSOT package permits a compact design
solution
1
• Uninterruptible power supplies for alarm and emergency systems
LTC4412
CURRENT (A)
• MP3 players and electronic video and still cameras
• Wire-ORed multipowered equipment
• Systems with standby capabilities
• Systems that use load sharing between two or more
batteries
SCHOTTKY
DIODE
• Multibattery charging from a single charger
0
0.02
0.5
FORWARD VOLTAGE (V)
DN1003 F01
Figure 1. LTC4412 Ideal Diode Controller vs
Schottky Diode Characteristics
12/03/1003
• PDAs
• USB peripherals
CONSTANT
RON
(VGS LIMITED)
CONSTANT
VOLTAGE
(VGS VARIABLE)
• Portable computers
• Logic controlled power switches
, 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
6
VIN SENSE
5
2
GND GATE
4
3
CTL STAT
1
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
1
VIN SENSE
5
2
GND GATE
4
3
CTL STAT
BAT2
VCC
470k
DN1003 F02
LTC4412
6
VIN SENSE
5
2
GND GATE
4
3
CTL STAT
1
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
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
DN1003 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
Data Sheet Download
http://www.linear.com
Linear Technology Corporation
For applications help,
call (408) 432-1900, Ext. 2593
dn1003f LT 1203 • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2003
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