Dual Monolithic Ideal Diode Extends Battery Run Time and Prioritizes Power Sources with Glitch-Free Switchover

Dual Monolithic Ideal Diode Extends Battery Run Time and
Prioritizes Power Sources with Glitch-Free Switchover
Joshua Yee
Mobile devices increasingly
rely on several power
sources that must be
interchangeable on the fly,
such as a wall adapter and
a backup battery, where
switchover between sources
must be transparent and
immediate. The simplest
scheme for switching
between two power sources
is a simple diode-OR, but
at higher current levels,
power losses in the diode
are a problem. To minimize
power losses and maximize
battery run time, replace the
Schottkys in a traditional
diode-OR with the LTC4415
dual monolithic ideal diode.
IN1
100k
21.5k
R1
R2
IN2
+
OUT1
4.7µF
D2
TO
LOAD
470k
470k
470k
470k
OUT2
GND
Figure 1. Automatic ideal diode switchover between wall adapter and battery
FEATURES
Figure 2 shows that the LTC4415 conducts with a regulated forward voltage
drop of 15mV when the load current
is below 500m A, an improvement of
>28mV compared to the LTC4413. Once the
load current climbs above that, LTC4415
operates with an on-resistance of 50mΩ, a
140mΩ improvement over the LTC4413 in
this region. At 4A, the LTC4415 produces
a mere 200mV forward drop, a reduction of over 50% compared to a typical
Schottky diode, which would produce
a forward drop greater than 400mV.
Because of its low forward voltage drop,
LTC4415 dissipates less than half the
power of a typical, low reverse leakage Schottky diode at 4A and less than
a quarter at 2A, as shown in Figure 3.
Another useful feature of the LTC4415
is short-circuit protection. The current
limit of LTC4415 can be adjusted up to
4A with R1 and R2. Eliminating R1 and
R2, and grounding CLIM1 and CLIM2,
2
CONSTANT CURRENT
1.8
ILIM
1.6
3
2
RON = 50mΩ
SCHOTTKY
DIODE
MBRS410E
CONSTANT
RESISTANCE
1
0
POWER LOSS (W)
LTC4415
1.4
1.2
1
SCHOTTKY
0.8
POWER
SAVINGS
0.6
0.4
CONSTANT
VOLTAGE
0
100
300
400
200
FORWARD VOLTAGE DROP (mV)
500
Figure 2. The LTC4415 I-V curve and regions of
operation vs a typical Schottky diode
34 | April 2012 : LT Journal of Analog Innovation
D1
LTC4415
EN1
CLIM1
STAT1
CLIM2
WARN1
WARN2
STAT2
EN2
SECONDAY
POWER
SOURCE
(BAT)
4
LOAD CURRENT (A)
The LTC4415 efficiently and intelligently switches between two
power sources in an input voltage
range of 1.7V–5.5V. When used in
the diode-OR application as shown
in Figure 1, LTC4415 delivers load
current through ideal diode D1 from
the wall adapter until its voltage
drops below the 4.5V switchover
threshold set by the resistor divider
on EN1/EN2. Once D1 is disabled, the
battery delivers load current through
D2 instead. STAT1 and STAT2 indicate
which ideal diode is conducting.
PRIMARY
POWER
SOURCE
(ADAPTER)
LTC4415
0.2
0
0
0.5
1
1.5
2 2.5
ILOAD (A)
3
3.5
4
Figure 3. The LTC4415 dissipates only 800mW at
4A, which is over 50% lower than the 1700mW
dissipated by a Schottky diode
4.5
design ideas
The LTC4415 is an easy-to-use, high performance
ideal-diode-OR solution for instantaneous power
supply switchover. It requires as few as four
external resistors and one output capacitor.
LTC4415
IOUTX
1000
CLIMX
R CLIMX = 1000 •
OUTX
INX
0 . 5V
I LIM
RCLIMX
124Ω
PX
UVLOX
ENX OUTX
GATE
DRIVER
UP TO 4A
CURRENT
LIMIT
TO WARNX
RLOAD
Figure 4. Current limit detection inside the
LTC4415 for each channel
instead triggers the internal limit at 6A.
Current limit is accomplished by a novel
approach that allows the LTC4415 to both
detect the load current and simultaneously produce a scaled analog voltage
for load current monitoring across the
same current limit resistor. Figure 4 shows
a simplified block diagram. This eliminates series losses, and saves board space
and BOM costs associated with a current
sense resistor and amplifier circuit.
LTC4415 P-channel MOSFETs are optimized for minimal on-resistance with
rapid switchover between sources
VIN2 = 4.6V
VIN1,2
1V/DIV
VIN1 = 3.6V
VIN2 = 2.6V
VOUT1,2
1V/DIV
STAT1
5V/DIV
STAT2
5V/DIV
without any appreciable load droop.
Figure 5 shows the LTC4415 switching
between input sources of different voltages, with only a 200mV transient dip
and recovery within 20µs. Note that
transient voltage spikes are usually
caused by inductive connections. This
can be reduced with short leads, proper
layout technique, and input and output
bypass capacitors with appropriate ESR.
For status monitoring purposes, the
active-low signals of STATx and WARNx
provide feedback to a digital controller/processor. STATx reflects conduction
VOUT
2V/DIV
OUTPUT SHORTED
TO GND
IOUT
2A/DIV
3.55V
20µs/DIV
COUT = 47µF
RLOAD = 3.6Ω
VOUT1 = VOUT2 (SHORTED)
Figure 5. Rapid path switchover with only 5%
transient voltage dip
STAT
5V/DIV
WARN
5V/DIV
THERMAL
SHUTDOWN
VIN = 3.6V
RCLIM = 124Ω
COUT = 4.7µF
RESTART DUE TO
THERMAL HYSTERESIS
10ms/DIV
Figure 6. Current limit warning and thermal
shutdown on output short circuit
status of a given channel. It can also be
used to detect failure of a source. WARNx
serves the dual purpose of indicating if
a path is in current limit—when STATx
is also low—or in thermal shutdown.
Thermal shutdown is triggered when
die temperature exceeds 160°C. Figure 6
shows how these two signals reflect the
system behavior when a path transitions
back and forth between current limit
and thermal shutdown. For about 25ms
after the output is shorted, the current
limit is active and WARNx stays low. Then
STATx goes high as thermal shutdown is
triggered. Restart occurs as the device
cools below 140°C, but shuts down
repeatedly due to the persistent short.
CONCLUSION
The LTC4415 is an easy-to-use, high
performance ideal-diode-OR solution for
instantaneous power supply switchover.
It requires as few as four external resistors and one output capacitor. The
low power loss and status monitoring makes LTC4415 an obvious choice
in applications requiring dual diodes
with built-in protection features.
LTC4415 is offered in both 3mm × 5mm
16-pin DFN and MSOP packages. n
April 2012 : LT Journal of Analog Innovation | 35
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