Oct 2007 - Low Dropout Regulator Can Be Directly Paralleled to Spread The Heat

LINEAR TECHNOLOGY
OCTOBER 2007
IN THIS ISSUE…
Cover Article
Low Dropout Regulator Can Be Directly
Paralleled to Spread The Heat.............1
Robert Dobkin
Linear in the News…............................2
Design Features
16-Channel LED Driver Drives up to 160
White LEDs with 5000:1 PWM Dimming
............................................................6
Keith Szolusha
2-Phase Synchronous Buck Controller
Delivers Maximum Features in
Minimum Footprint............................10
Eric Gu and Theo Phillips
Measure Microamps to Amps or Reduce
Power Dissipation by 99%, You Decide!
..........................................................13
Brendan J. Whelan
Pushbutton On/Off Controller Provides
µProcessor Reset Monitor and Input
Supply Monitoring..............................17
Victor Fleury
LED Driver Yields 3000:1 True Color
PWM Dimming with Any Buck, Boost
or Buck-Boost Topology from a Wide
3V–40V Input Range...........................20
Xin Qi
White LED Driver and OLED Driver
with Integrated Schottkys and Output
Disconnect in 3mm × 2mm DFN..........23
Alan Wei
Light Up 12 LEDs from a Single-Cell
Li-Ion Battery via Highly Integrated
3mm × 2mm Dual-LED-String Driver
..........................................................25
Ben Chan
Low Offset 2-Wire Bus Buffer Provides
Capacitance Buffering, Stuck Bus
Recovery, and Tolerates High VOL . ....28
John Ziegler
DESIGN IDEAS
.....................................................32–40
(complete list on page 32)
New Device Cameos............................41
Design Tools.......................................43
VOLUME XVII NUMBER 3
Low Dropout
Regulator Can Be
Directly Paralleled
to Spread The Heat
Introduction
by Robert Dobkin
The 3-terminal adjustable linear regu- output current, all-surface-mount aplator has been around since 1976, but plications where only a limited amount
since then, little has changed in its of heat can be dissipated in any single
essential architecture. A 1.2V refer- spot on a board—applications that
ence is boosted to generate a regulated previously demanded a switching
output somewhere above a minimum regulator.
1.2V. What if, however, you throw
When regulators are surface mountaway the voltage
ed on a system
reference and reboard, conducplace it with a
tive dissipation
The LT3080 is the first
precision current
and air -cooling
adjustable linear regulator
source? The result
limits the amount
that can be directly
is a giant leap
of power that can
paralleled to spread the
forward in linear
be dissipated in
current load and thus
regulator capabileach chip. With
ity, performance
a typical board,
spread dissipated heat.
and versatility.
allowing a max
This makes it possible to
The LT3080 is the
use linear regulators in high operating temfirst adjustable
perature of 60°C
output current, all-surfacelinear regulator to
to 70°C, a linmount applications where
do just that. This
ear regulator can
deceptively simsafely dissipate
only a limited amount of
ple architectural
approximately 1W
heat can be dissipated in
change allows this
any single spot on a board— to 2W. This numnew regulator to
ber depends on
applications that previously
be directly paralthe ability of the
demanded a switching
leled to spread the
board to spread
regulator.
current load and
the heat and airthus spread dissiflow across the
pated heat among
board. If high powthe ICs. Spreading the heat makes it er requirements cause the regulator
possible to use linear regulators in high to generate more heat than the board
continued on page Sales Offices......................................44
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companies that manufacture the products.
DESIGN FEATURES L
LT3080, continued from page Internal Precision Current
Source Makes it Possible to
Parallel the LT3080
A precision “zero” TC 10µA internal
current source is connected to the noninverting input of a power operational
amplifier. The power op amp provides
a low impedance buffered output from
the voltage on the non-inverting input.
A single resistor from the non-inverting
input to ground sets the output voltage
and if this resistor is set to zero, zero
output results. As can be seen, any
output voltage can be obtained from
zero up to the maximum defined by
the input power supply.
What is not so obvious from this
architecture is the benefits of using a true internal current source
as the reference as opposed to the
bootstrapped voltage reference of older
regulators. A true current source alLinear Technology Magazine • October 2007
2.5
10.20
10.15
SET PIN CURRENT (µA)
2
VIN – VOUT (V)
can dissipate, the regulator must be
mounted separately on a heat sink. In
all-surface-mount systems, this is not
an option, so the limitation of power
dissipation (1W for example) limits the
output current.
Figure 1 shows the maximum output current at different input-output
differentials that can be obtained for
a regulator with both 1W and 2W
dissipation. 2W dissipation is a reasonable limitation on a single regulator.
Paralleling LT3080s increases the
maximum total output current by
spreading the heat, helping to maintain low peak temperatures.
The LT3080 is also especially well
suited to applications needing multiple
rails. The new architecture adjusts
down to zero with a single resistor, handling modern low voltage digital ICs.
Adjusting to zero output makes it possible to shut off the powered circuitry
when the input is preregulated—such
as a 5V or 3.3V input supply. External
resistors in series with IN can help
spread the heat, keeping the system
all surface mount.
Finally, the new regulator is made
in a 40V bipolar process. This allows
high input voltage as well low operating voltage, since bipolar transistors
turn on at 0.6V.
POWER DISSIPATION = 2W
1.5
POWER DISSIPATION = 1W
POWER DISSIPATION = 0.5W
1
0.5
10.10
10.05
10.00
9.95
9.90
9.85
0
0
1
2
3 4 5 6 7
LOAD CURRENT (A)
8
9
9.80
–50 –25
10
Figure 1. The available output current as
a function of input-output differential and
allowable power dissipation. At 2W, 1A
output currents are possible even with 1V
to 2V input-to-output differential.
lows the regulator to have gain and
frequency response independent of
the output voltage since the loop
gain does not change. Traditional
adjustable regulators, such as the
LT1086, have a change in loop gain
and bandwidth with output voltage
as well as bandwidth changes when
the set/adjustment pin is bypassed
0
25 50 75 100 125 150
TEMPERATURE (°C)
Figure 2. Temperature performance of
the LT3080’s precision current source.
to ground. With the LT3080, however,
the loop gain remains unchanged with
changing output voltage or bypassing.
Output regulation is no longer fixed
at a percentage of the output voltage
but rather a fixed number of millivolts.
With a true current source, all the gain
in the buffer amplifier provides regulation; none of it is needed to amplify the
reference to a higher output voltage.
This, and the LT3080’s precise DC
Table 1. Comparison of the LT3080 to traditional 1A regulators
LT317
LT1086
LT3080
Dropout (V)
3V
1.5V
1.3V or 300mV
Min Load (mA)
10
10
0.3
Min Output (V)
1.2
1.2
0
IOUT (A)
1.5
1.5
1.1
Parallel Operation
—
—
L
External Resistors
2
2
1
Table 2. Some key specifications for the LT3080
Parameter
Value
Load Regulation, IOUT = 10mA to 1.1A
<1mV
Line Regulation, IN = 2V to 40V
<1mV
SET Pin Current
10µA ±1%
Min Load Current
0.3mA
SET to OUTPUT Offset
1mV
Operating Temp Range
–55°C to 125°C
Dropout (3-Terminal) 1.1A
1.3V
Dropout (4-Terminal) 1.1A
0.3V
Ripple Rejection (120Hz)
75 dB
L DESIGN FEATURES
characteristics, makes it possible to
easily parallel regulators (see below:
“It is Easy to Parallel the LT3080”).
VCONTROL
10µA
High Performance
No sacrifices were made in regulator
performance for the LT3080. Line
and load regulation are excellent over
temperature. Its low dropout and a
new architecture make it extremely
versatile. On chip trimming keeps the
accuracy of the reference current below
one percent, and the offset voltage
between the SET pin and the output
to under 2mV.
Line regulation is virtually immeasurable, a few nanoamps, since the
internal circuitry double-regulates the
current source section. The temperature performance of the reference is
shown in Figure 2 and is nearly flat
from –55°C to 150°C. Thermal limiting is set at about 160°C. Quiescent
current is only about 300µA, allowing
this device to be used in light load
and battery-powered applications.
High frequency ripple rejection is also
excellent, making the LT3080 a good
fit as a post regulator to switching
regulators when low output ripple is
needed.
+
–
SET
VCONTROL Pin Offers Additional
Ways to Spread the Heat
Clearly, one of the driving design objectives for this new regulator was to
enable the thermal design for surface
mount boards—notably eliminating
the need for heat sinks. Paralleling
LT3080s makes a significant difference, but another feature also helps.
The collector of the output transistor
is available at the VCONTROL pin (see
Figure 3). This can decrease peak
temperatures in two ways.
First, the dropout on the collector
is 400mV (IN pin) so it can take a
lower voltage supply than is used for
the LT3080’s control circuitry (1.3V
OUT
Figure 3. Block diagram of the LT3080.
Four terminals are available from the
package to allow the device to be used
in a low dropout mode with only 300mV
input-to-output dropout.
The SET pin is very high impedance
and the output voltage is set by the
10µA current times an external resistor. Even a 0.1µF capacitor is large
enough to bypass the SET pin at 60Hz,
allowing for reduction of output noise
and pickup into the SET pin.
With a capacitor on the SET pin,
output noise is 40µVRMS—about the
VIN
3.3V
≥ VOUT +1.3V
Operation of the LT3080
Figure 3 shows a block diagram of the
LT3080. The simplest application, as
a 3-terminal adjustable regulator, is
shown in Figure 4. The VCONTROL and IN
pins are tied together. (These two pins
can connect to different supplies for
additional thermal benefits, described
below.) The only added components
are input and output capacitors and
a resistor to set the output voltage. In
this case, the output is set to 1.8V,
which at 1.3V dropout works with a
3.3V input. Input and output capacitors are required for stability—they can
be ceramic, tantalum, or electrolytic
capacitors. Unlike older 3-terminal
regulators, the minimum load current
is guaranteed at only 1mA for this
device. By making the adjustment
resistor zero or tying the SET pin to
the ground with a switch, the output
goes to zero, turning off connected
circuitry. Typically, the quiescent
current is under 300µA.
same as many low noise regulators. In
other applications, the SET pin can be
driven with an amplifier or a reference
voltage to be used as a power buffer.
With multiple regulators, the SET pins
and outputs can be tied together for
paralleling the regulator (described
below). Grounding the SET pin brings
the output to zero.
LT3080
IN
LT3080
IN
VCONTROL
+
–
1µF
OUT
SET
1µF
VOUT
1.8V
1µF
180k
Figure 4. Basic hookup for the LT3080 regulator. The IN and VCONTROL pins are tied
together and a single resistor sets the output voltage. A 1µF output capacitor ensures
stability. If the adjustment resistor is adjusted to zero, the output is zero.
RD
2.9Ω
VIN
5V
(4.7V MIN)
LT3080
IN
VCONTROL
+
–
1µF
SET
180k
RD =
OUT
VOUT
1.8V
1µF
VIN(MIN) − ( VOUT + 0.4V )
IOUT(MAX)
Figure 5. Adding a resistor in series with the collector of the output device to
remove some of the power dissipation from the regulator. This disperses heat
around the surface mount board rather concentrating it at the regulator.
Linear Technology Magazine • October 2007
DESIGN FEATURES L
VIN
LT3080
Table 3. Trace resistance for
ballast resistors in mΩ/in
VCONTROL
+
–
10mil Width 20mil Width
OUT 10mΩ
SET
VIN
4.5V TO 30V
VIN
LT3080
VCONTROL
+
–
1µF
SET
OUT 10mΩ
VOUT
3.3V
2A
100µF
165k
Figure 6. Paralleling of two regulators. Need more current? Add more regulators.
Current sharing is assured by the 10mΩ PC board traces, which act as ballast resistors.
dropout). Lowering the input-to-output voltage on the power transistor
increases efficiency and thus reduces
dissipation.
Second, a resistor can be inserted
in series with the collector. Adding
this resistor splits power dissipation
between the internal power transistor and an external resistor so that
some of the heat from the IC can be
moved to elsewhere on the PC board.
Figure 5 shows such a design using a
2.9Ω resistor. The dropout voltage for
the output transistor is only 400mV,
so several volts can be dropped across
the external resistor, minimizing the
heating of the IC. At full load, the
external resistor drops approximately
3V and dissipates 3W. To minimize
peak temperatures on a PC board,
this resistor can be split into several
1Ω resistors and thus further spread
dissipated heat. The power dissipation
in the LT3080 peaks at about 750mW
when the power dissipation in the
resistor and the power dissipation in
the transistor are equal. The copper
planes in the PC board can easily
handle this power.
Of course the LT3080 can be operated in 3-terminal mode by simply
connecting the VCONTROL pin to the
power input pin, but this limits the
input to the 1.3V dropout of the
regulator. Alternately, by tying the IN
Linear Technology Magazine • October 2007
pin to a lower voltage than VCONTROL,
it is possible to produce a 1.1A, 2.5V
to 1.8V or 1.8V to 1.2V regulator with
low dissipation—likewise for other low
IN – OUT differentials. To achieve the
same peak operating temperatures,
the dissipation constrained design
current must be lower for higher IN
– OUT differentials, such as 5V to 3.3V
or 3.3V to 1.5V.
1oz Weight
54.3
27.1
2oz Weight
27.1
13.6
It is Easy to Parallel the LT3080
The architecture of the LT3080 allows
direct paralleling unlike any other type
of regulator. Parallel linear regulators
distribute the current load and distribute power dissipation around the
system board. Need more power but
can’t afford more spot heating? Add
more regulators. Even paralleling 5–10
devices is reasonable.
Practical current sharing by parallel
LT3080s is made possible by internal
trimming, which keeps the offset voltage between the adjustment pin and
the output under 2mV. Figure 6 shows
how easy it is to parallel LT3080s.
Simply tie the SET pins of the LT3080s
together, and do the same for the IN
pins. This is the same whether it’s in
3-terminal mode or has a separate IN
supply. The outputs are also connected
in common but with a small piece of
PC trace in series with each OUTPUT
continued on page 27
Figure 7. Thermograph shows two regulators, each dissipating 0.7W
from a 0.7V input-to-output differential at 2A total load. The result
is a 28°C rise over ambient at each IC on a two sided PC board.
DESIGN FEATURES L
in Figure 1, the circuit works from a
single Li-Ion (3V) battery or 5V wall
adapter. Figure 2 shows efficiency with
a 3.6V input.
Li-Ion to a 2-LED and
6-LED Display
Figure 8 (Buck-Boost/Boost configuration) shows a white LED driver used
to backlight two displays: a 6-LED
main and a 2-LED sub display. This
design generates a constant 20mA in
each white LED string from a Li-Ion
(3V~4.2V) or 5V adapter input. Two
independent dimming and shutdown
controls (CTRL1 and CTRL2) simplify
power management and extend battery life. Figure 9 shows the efficiency
of the circuit.
Conclusion
The LT3497 is a dual channel white
LED driver capable of driving up to 12
white LEDs from a single cell Li-Ion
input. The device features 35V internal power switches, internal Schottky
diodes, DC or PWM dimming control,
open LED protection and optimized
internal compensation. The LT3497
is an ideal solution for a wide range of
applications including multipanel LCD
backlighting, camera flash or space
constrained portable applications
such as cellular phones, MP3 players,
PDAs and digital cameras. L
LT3080, continued from page pin serving as ballast to equalize the
currents. PC trace resistance in milliohms/inch is shown in Table 3. Only
a tiny area is needed for ballasting.
Figure 6 shows two devices with a
small 10mΩ ballast resistor, which at
full output current gives better than
80% equalized sharing of the current.
The external resistance of 10mΩ (5mΩ
for the two devices in parallel) only
adds about 10mV of output regulation
drop at an output of 2A. Even with the
1V output, this only adds 1% to the
regulation.
Thermal Performance
Two LT3080 3mm × 3mm QFN devices
are mounted on a double sided PC
board. They are placed approximately
1.5 inches apart and the board is
mounted vertically for convection cooling. Two tests were set up to measure
the cooling performance and current
sharing of these devices.
The first test was done with approximately 0.7V input-to-output
differential and a 1A load per device.
This setup produced 700mW dissipation in each device and a 2A output
current. The temperature rise above
ambient is approximately 28°C and
both devices were within ±1°C of each
other. Both the thermal and electrical
sharing of these devices is excellent.
The thermograph in Figure 7 shows
the temperature distribution between
these devices, where the PC board
reaches ambient within about 0.5in
from the devices.
Figure 8 shows what happens when
the power is increased to 1.7V across
each device. This produces 1.7W disLinear Technology Magazine • October 2007
Figure 8. Thermograph shows a 65°C rise for two regulators, each
dissipating 1.7W from a 1.7V input-to-output differential at 2A load.
sipation in each device and a device
temperature of about 90°C, about 65°C
above ambient. Again, the temperature matching between the devices is
within 2°C, showing excellent tracking
between the devices. The board temperature drops to about 40°C within
0.75 inches of each device.
While 95°C is an acceptable operating temperature for these devices, this
rise is in a 25°C ambient environment.
For higher ambient temperatures, the
temperature rise must be controlled
to prevent the device temperature
from exceeding 125°C. A 3-meterper-second airflow across the devices
decreases the device temperature
by about 20°C, providing a margin
for higher operating ambient temperatures. Also, this example is for a
2-layer board. A 4-layer board would
provide better power dissipation.
Conclusion
The LT3080’s breakthrough design and
high performance DC characteristics
allows it to be paralleled for high current all-surface-mount applications.
It is also adjustable to zero output,
an impossible feat with a traditional
3-terminal adjustable linear regulator.
It is optimized for new circuit applications and all-surface-mount system
assembly techniques—especially high
performance, high density circuit
boards. L
27