20A LED Driver with Accurate ±3% Full Scale Current Sensing Adapts to Multitude of Applications

May 2016
I N
T H I S
I S S U E
multi-output clock
synthesizer with integrated
VCO and low jitter 12
negative current-reference
linear regulator 20
Volume 26 Number 2
20A LED Driver with Accurate ±3%
Full Scale Current Sensing Adapts to
Multitude of Applications
Josh Caldwell and Walker Bai
load sharing for three or
four supplies with unequal
voltages 26
monolithic SEPIC/boost
regulators with wide VIN
range, high efficiency,
and power-on reset and
watchdog timers 28
Rapidly evolving LED lighting applications are replacing nearly
all traditional forms of illumination. As this transformation
accelerates, power requirements for LED drivers increase,
with higher currents making it more challenging to maintain
current sensing accuracy without sacrificing efficiency. LED
drivers must do this while managing current delivery to
multiple independent LED loads at high speeds, and be able
to connect parallel drivers with accurate current sharing.
Some high power LEDs have unique mechanical and
electrical considerations, where the anode is electrically
tied to the thermally conducting backtab. In a traditional
LED driver with a step-down regulator configuration,
where thermal management is achieved by cooling the
chassis, the anode connection to the backtab creates a
mechanical-electrical design challenge. The backtab must
have good thermal conductivity to the heat sink, but
also be electrically isolated from it if the voltage at the
tab is different from the chassis. Since is it difficult for
LED manufacturers to change processing or packaging,
the LED driver itself must meet this design challenge.
The LTC4125 5W AutoResonant wireless power transmitter features foreign object
detection and completes linear wireless charging solutions (see page 31).
w w w. li n ea r.com
One option is to use a 4-switch positive buck-boost LED
driver, but the additional switching MOSFETs add system
complexity and cost. An inverting buck-boost topology
uses only one set of switching power MOSFETs, and allows
the anode heat sink to be tied directly­—electrically and
(continued on page 4)
To meet high performance demands, the LT3744 can be configured
as a synchronous step-down or inverting buck-boost controller
to drive LED loads at continuous currents exceeding 20A. The
supply input for the LT3744 is designed to handle 3.3V to 36V.
(LT3744, continued from page 1)
Full-range analog current regulation
accuracy is 3%, and even at 1/20th scale,
it is better than ±30%. The LT3744 has
three independent analog and digital
control inputs with three compensation
and gate drive outputs for a wide range
of LED configurations. By separating
the inductor current sense from the LED
current sense, the LT3744 can be configured as a buck or inverting buck-boost.
For ease of system design, all input
signals are referenced to board ground
(SGND, signal ground), eliminating the
need for complex discrete level-shifters.
In the inverting buck-boost configuration, the total LED forward voltage
can be higher than the input supply
voltage, allowing high voltage LED
strings to be driven from low voltage
supplies. When PCB power density calls
for spreading the component power
4 | May 2016 : LT Journal of Analog Innovation
100
380 TYPICAL UNITS
VCTRL1 = 0V
90
80
NUMBER OF UNITS
To meet high performance demands, the
LT®3744 can be configured as a synchronous step-down or inverting buck-boost
controller to drive LED loads at continuous currents exceeding 20A. The supply
input for the LT3744 is designed to handle
3.3V to 36V. As a step-down converter,
it regulates LED current from 0V up
to the supply voltage. As an inverting
buck-boost converter, the LT3744 can
accurately regulate LED currents with
output voltages from 0V down to −20V.
125°C
25°C
–45°C
250
NUMBER OF UNITS
mechanically—to the chassis ground,
eliminating the need for electrical isolators on the heat sink, and simplifying
the mechanical design of the system.
300
200
150
100
125°C
25°C
–45°C
380 TYPICAL UNITS
VCTRL1 = 1.5V
70
60
50
40
30
20
50
10
0
–300 –200 –100
0
100
200
300
REGULATED VLED_ISP - VLED_ISN VOLTAGE (µV)
0
59
59.4
59.8
60.2
60.6
61
REGULATED VLED_ISP - VLED_ISN VOLTAGE (mV)
Figure 1. The LED current regulation amplifier
in the LT3744 has a typical offset of ±300µV with
VCTRL = 0V.
Figure 2. At full current, the LED current regulation
loop has a typical accuracy of ±1.7% with VCTRL =
1.5V.
dissipation, the LT3744 can be easily
paralleled with other LT3744s to drive
high pulsed or DC currents in LED loads.
to 1/20th of the total current control
range. This is critical in applications
where the total digital PWM dimming
range is limited—or in applications where
very high dimming range is required.
As an example, with a 100Hz PWM
dimming frequency and a 1MHz switching frequency, the LT3744 is capable of
1250:1 PWM dimming, which can be
combined with 20:1 analog dimming to
extend the total diming range to 25,000:1.
HIGH ACCURACY CURRENT
SENSING
The LT3744 features a high accuracy
current regulation error amplifier, which
achieves accurate analog dimming down
PWM1
5V/DIV
SW
10V/DIV
ILED
1.67A/DIV
IL
20A/DIV
1µs/DIV, 5-MINUTE PERSISTENCE
Figure 3. The LT3744 features flicker-free LED
dimming.
Figure 1 shows the production consistency
of the LT3744 with regard to offset voltage
over temperature, in this case 380 typical
units when the analog control input is
at 0V. With the low offset of the error
amplifier, the control loop is capable of
a typical accuracy of ±10% at 1/20th
scale analog dimming. The distribution
of the regulated voltage across the LED
current sensing pins with the control
input equal to 1.5V is shown in Figure 2.
The accuracy at full range is better than
design features
In projection systems, reducing the turn-on time of the light source reduces timing
constraints. With a reduction in timing constraints, the image refresh rate can
increase, allowing higher resolution images and a reduction in the rainbow effect
from fast-moving white objects. The LT3744 is capable of transitioning between
the different output current states in less than three switching cycles.
Figure 4. The LT3744 is capable of driving
a single LED with three different current
levels.
EN/UVLO
VIN
EN/UVLO
PWM1
PWM2
PWM3
CTRL1
CTRL2
CTRL3
1µF
TG
220nF
BOOST
SW
LT3744
2V
VREF
2.2µF
RHOT
45.3k
L1
1.2µH
INTVCC
RNTC
680k
BG
CTRLT
VEE
ISP
ISN
SGND
RS
3mΩ
M2
SS
RT
82.5k
VEE
VC1
287k
C2
330µF
M4
D2
D3
PWM_OUT2
VFNEG
PWM_OUT3
M3
20A MAXIMUM
C3
330µF
M6
D1
SYNC PWM_OUT1
10nF
L1: IHLP-5050FD-ER1R2M01
RS: WSL28163L000D
RSLED: WSL28163L000J
M1: BSC050NE2LS
M2: SiR438DP
M3, M4, M5, M6, M7, M8: Si7234DP
D1, D2. D3, D4: BAT54A
C1, C2, C3: 10T4B330M
C1
330µF
22µF
100k
FAULT
M1
VIN
56µF 24V
×4
BLUE
51k
M8
M5
10µF
M7
D4
LED_ISP
LED_ISN
FB
VC2
VC3
287k
10nF
RSLED
3mΩ
287k
10nF
10k
10nF
±3%, which corresponds to ±1.8mV on
the 60mV full-scale regulation voltage.
FLICKER-FREE PERFORMANCE
One of the most important metrics in LED
driver performance is in the recovery of
the LED current during PWM dimming.
The quality of the end product is highly
dependent on the behavior of the driver
in the first few switching cycles after the
rising edge of the PWM turn-on signal.
The LT3744 uses proprietary PWM,
compensation and clock synchronization
technology to provide flicker-free performance—even when driving LEDs to 20A.
Figure 3 shows a 5-minute capture of the
LED current recovery with a 12V supply
delivering 20A to a red LED. The switching frequency is 550kHz , the inductor
is 1µ H, the PWM dimming frequency
is 100Hz with an on-time of 10µsec
(1000:1 PWM dimming). Roughly 30,000
dimming cycles are shown, with no
jitter in the switching waveform—every
recovery switching cycle is identical.
HIGH SPEED DIMMING BETWEEN
THREE DIFFERENT REGULATED
CURRENTS
In projection systems, reducing the
turn-on time of the light source reduces
timing constraints. With a reduction in
timing constraints, the image refresh rate
can increase, allowing higher resolution
images and a reduction in the rainbow
effect from fast-moving white objects.
The LT3744 is capable of transitioning
between the different output current
states in less than three switching cycles.
The LT3744 features three regulated
current states, allowing color-mixing
system designers to sculpt the color
temperature of each LED. Color mixing
delivers high color accuracy, corrects
inaccurate LED colors, and eliminates
variations in production systems. While
the LT3743 has low and high current states,
the LT3744 features three current states so
that all three RGB LED colors can be mixed
with each other at their own light outputs
to independently correct the other colors.
Figure 4 shows a 24V input/20A output,
single LED driver with three different regulated currents—determined by the analog
voltages on the CTRL and the digital
state of the PWM pins. Note that since
RS is only used for peak inductor current
and absolute overcurrent protection,
May 2016 : LT Journal of Analog Innovation | 5
Within miniature “pocket” or smartphone projection systems, total solution space
and cost are paramount. The LT3744 combines switched output capacitor
technology with a floating gate driver to create a complete RGB solution from
a single LED driver, a significant space savings over multi-IC drivers.
A COMPLETE RGB LED SOLUTION
FOR POCKET OR SMARTPHONE
PROJECTORS
it does not need to be a high accuracy
resistor—which reduces system cost.
PWM dimming between the three differ-
Within miniature “pocket” or smartphone
projection systems, total solution space
and cost are paramount. In these applications, PCB space is extremely limited and
the total volume of the driver solution
(including component height) must be
minimized. Using only one LED driver
for all three LEDs drastically reduces
space—allowing use of larger batteries or higher power LEDs for improved
battery lifetime and projected lumens.
ent current states is shown in Figures
5 and 6. In Figure 5, the PWM signals
are sequentially turned on and off.
PWM3 has the highest priority and
PWM1 has the lowest. This allows
rapid, single input signal transitions to
change the output current. As shown
in Figure 6, there can be any arbitrary
interval between the PWM input signals.
Figure 7. The LT3744 is capable of driving
all three color component (R, G and B)
LEDs in a pocket or smartphone projector
from a single Li-ion battery.
EN/UVLO
EN/UVLO
PWM1
PWM2
PWM3
CTRL1
CTRL2
CTRL3
VIN
2V
VREF
RHOT
45.3k
L1
6.8µH
FAULT
CTRLT
VEE
ISP
ISN
20A MAXIMUM
330µF
M2
D1
VEE
VC1
B
G
M5
D4
VFNEG
M8
107k
M7
LED_ISP
LED_ISN
FB
VC2
VC3
RSLED
12mΩ
10k
6.8nF
6.8nF
6 | May 2016 : LT Journal of Analog Innovation
G
M6
D3
PWM_OUT2
6.8nF
D5
330µF
M3
D2
PWM_OUT3
40.2k
2.2µF
330µF
M4
10nF
RT
RS
6mΩ
22µF
SYNC PWM_OUT1
SS
Each LED can be turned on sequentially,
with a time delay in between, or with any
L1: MSS1048-682NL
RS: WSPL08056L000FEA18
RSLED: WSLP1206R0120D
M1: BSC010NE2LSI
M2: SiR438DP
M3, M4, M5, M6, M7, M8: Si7234DP
D1, D2. D3, D4: BAT54A
D5: PMEG4010
M1
220nF
INTVCC
BG
SGND
VIN
3.3V
VEE
TG
100k
RNTC
680k
47µF
20µF
BOOST
SW
LT3744
2.2µF
The LT3744 combines switched output
capacitor technology with a floating gate
driver to create a complete RGB solution
from a single LED driver. The LT3744
uses a unique gate driver for the PWM
output pins. The negative rail of the driver
floats on the VFNEG pin, allowing it to
pull down the gates of all of the switches
that are off to negative voltages. This
ensures that the switches in-series with the
output capacitors do not turn on in any
condition. This driver allows up to a 15V
difference between any string of LEDs.
VEE
R
design features
In addition, with the three independent analog control inputs, each
LED can operate at a different regulated current. When the LT3744 is
configured as an inverting buck-boost, a single lithium-ion battery can
drive three independent LED strings using only a single controller.
Summary of Linear’s high power LED driver-controller family
LT3741
LT3743
LT3744
LT3763
LT3791
V IN range
6V–36V
6V–36V
3.3V–36V
6V–60V
4.7V–60V
LED output range
0V–34V
0V–34V
−20V–36V
0V–55V
0V–52V
Topology
buck
buck
buck and inverting
buck-boost
buck
buck-boost
LED current regulation accuracy
±6%
±6%
±3%
±6%
±6%
⁄ 10 scale LED current accuracy
±60%
±60%
±17%
±60%
±35%
50mV
50mV
60mV
50mV
100mV
1
Full-scale LED current sense
Common anode connection for LEDs
L
LED fault indication
L
L
L
Low side LED PWM gate driver(s)
0
2
3
1
1
Individual LED current states
1
2
3
1
1
pattern input into the PWM digital inputs.
In addition, with the three independent
analog control inputs, each LED can
operate at a different regulated current.
When the LT3744 is configured as an
inverting buck-boost, a single lithiumion battery can drive three independent
LED strings using only a single controller. Figure 7 shows a 3.3V/5A inverting
tri-color buck-boost LED driver designed
specifically for RGB pocket projectors.
PWM1
5V/DIV
PWM1
5V/DIV
PWM2
5V/DIV
PWM3
5V/DIV
PWM2
5V/DIV
PWM3
5V/DIV
ILED
6.67A/DIV
ILED
6.67A/DIV
25µsec/DIV
Figure 5. The LT3744 transitions between any of
three regulated current states and off in less than
three switching cycles.
25µsec/DIV
Figure 6. The different current states can be turned
on at any time—with or without time in between
each state.
324W 2-LED DRIVER USING TWO
PARALLEL LT3744 LED DRIVERS
A significant limiting factor in any high
power/high current controller design is
power density in the PCB. PCB power
density is limited to roughly 50W⁄cm2
to prevent excessive temperature rise
within the power path components.
In extreme cases, where an LED load
requires more power than a single driver
can support (while remaining within
power density limits), multiple converters can be paralleled to spread the load.
An efficient high current LED drivercontroller, with modern power MOSFETs,
can provide roughly 200W (at a solution
size of approximately 4cm2) and limit all
power path component temperatures to
under 80ºC. For LED loads higher than
200W, the LT3744 can be paralleled with
other LT3744s to limit the temperature rise
May 2016 : LT Journal of Analog Innovation | 7
100k
PWM1
EN/UVLO
PWM1
PWM2
PWM2
22µF
1µF
D1
VIN
FAULT
U1
LT3744
CTRLT
100k
M2
470µF
2.43k
ISP
D5
D6
FB
SYNC
PWM_OUT1
RT
PWM_OUT2
SS
VFNEG
10nF
604Ω
1nF
LED_ISP
SGND VEE VC1
VC2
LED_ISN
M5
M9
M6
M10
M13
M15
1nF
226k
2mΩ
ISN
CTRL2
100k
0.22µF
BG
CTRL1
100k
2.2µF
L1
0.82µH
470µF
100k
100k
47µF
×2
M1
SW
VREF
1nF
VIN
12V
BOOST
TG
82.5k
56µF
×2
INTVCC
PWM3
100k
10µF
×4
25.5k
1nF
226k
25.5k
10nF
10nF
D3
2mΩ
VIN
100k
1µF
22µF
EN/UVLO
PWM1
VIN
BOOST
PWM3
TG
82.5k
1nF
FAULT
VREF
U2
LT3744
BG
D7
PWM_OUT2
VFNEG
10nF
1nF
LED_ISP
SGND VEE VC1
226k
1nF
470µF
2.43k
D8
PWM_OUT1
SS
226k
M4
FB
SYNC
RT
1nF
2mΩ
ISN
CTRL2
100k
0.22µF
ISP
CTRL1
100k
2.2µF
47µF
×2
L2
0.82µH
470µF
CTRLT
100k
M3
D1, D2: NXP PMEG4002EB
D3–D8: BAT54A
L1, L2: VISHAY IHLP-5050FD-ERR82
M1, M3: BSC032NE2LS
M2, M4: BSC010NE2LS
M5–M12: Si7234DP
M13–M16: BSC010NE2LS
SW
100k
100k
56µF
×2
INTVCC
PWM2
100k
10µF
×4
D2
VC2
25.5k
10nF
LED_ISN
604Ω
M7
M11
M8
M12
M14
25.5k
10nF
M16
D4
2mΩ
Figure 8. A 57A/324W 2-LED driver
8 | May 2016 : LT Journal of Analog Innovation
design features
Figure 11. Parallel board
temperatures at 100% duty
cycle delivering 324W to the
LED
INDUCTOR
SWITCHING
MOSFETS
CHANNEL 1
CHANNEL 2
9A/DIV
LT3744
10ms/DIV
VIN = 12V
VOUT = 4V
IOUT = 54A
fSW = 400kHz
100% DUTY CYCLE
Figure 9. LED current sharing during start-up
controllers in this design produces 27A—
for a total of 54A at 6V. By tying the
corresponding compensation outputs
together, both controllers behave in
unison to provide a smooth, well behaved
start-up and accurate DC regulation.
CHANNEL 1
9A/DIV
CHANNEL 2
9A/DIV
20µs/DIV
Figure 10. DC LED current sharing at full load—
showing very little variation between the two parallel
drivers
in any particular component. All compensation outputs should be paralleled, allowing current sharing between each regulator.
Figure 8 shows a 324W converter using
two Linear DC2339A demo boards
connected in parallel. Each of the parallel
Figure 9 shows the LED current start-up
behavior of each board. Note that the
regulated current provided by each board
is identical throughout the entire startup sequence. In DC regulation, without
PWM dimming, Figure 10 shows excellent current sharing between the two
application boards (the waveforms are
directly on top of each other). Figure 11
shows that the temperature rise above
ambient of the board at 100% duty
cycle is about 55ºC. Component L1 is the
Figure 13. Parallel board
temperatures at 50% PWM
dimming delivering 54A
pulses to the LED
PWM
2V/DIV
inductor, Q1 and Q3 are the switching
power FETs, R5 is the inductor current
sense resistor, R32 is the LED current
sense resistor, and U1 is the LT3744.
In this application, two independent LED
strings can be PWM dimmed at the full
54A. When PWM dimming, Figure 12
shows that the LED current is completely
shared between the two drivers. In this
test, the rise time of the current in the LED
from 0A to 54A is 6.6µs. The electrical
connection from the output of each driver
to the LED must be carefully balanced to
avoid added inductance in either path—
which reduces the effective rise time.
Figure 13 shows the temperature
rise in each demo board with a 50%
PWM-dimmed LED current of 54A. To
INDUCTOR
SWITCHING
MOSFETS
CHANNEL 1
CHANNEL 2
9A/DIV
LT3744
20µs/DIV
Figure 12. The LT3744 features excellent LED current
sharing between parallel drivers during PWM
dimming.
VIN = 12V
VOUT = 4V
IOUT = 54A
fSW = 400kHz
50% DUTY CYCLE
May 2016 : LT Journal of Analog Innovation | 9
10µF
56µF
22µF
1µF
100k
VIN
EN/UVLO
PWM1
PWM1
BOOST
PWM3
100k
SYNC
TG
SYNC
82.5k
1nF
VEE
INTVCC
PWM2
100k
10µF
×2
D1
VEE
FAULT
VREF
U1
LT3744
CTRLT
L1
1.3µH
0.22µF
1mΩ
10µF
M2
BG
470µF
ISN
CTRL2
VEE
4.02k
VEE
ISP
CTRL1
100k
M1
SW
100k
100k
Figure 14. This parallel inverting
application delivers 120W to a
chassis tied common-anode LED.
VIN
12V
D5
FB
2.2µF
PWM_OUT1
1nF
PWM_OUT2
1nF
226k
226k
1nF
VFNEG
1k
M5
LED_ISP
SGND VEE RT
VEE
VC1 LED_ISN
SS
M6
D3
33nF
143k
10nF
VEE
VEE
3mΩ
VEE
VEE
VIN
10µF
56µF
1µF
100k
22µF
VEE
EN/UVLO
PWM1
100k
100k
VIN
1nF
PWM2
TG
FAULT
M3
L2
1.3µH
0.22µF
1mΩ
SW
VREF
U2
LT3744
CTRLT
BG
ISP
CTRL1
100k
D1, D2: NXP PMEG4002EB
D3–D6: BAT54A
L1, L2: WÜRTH 7443551130
M1, M3: BSC026N04LS
M2, M4: BSC018N04LS
M5–M8: Si7234DP
BOOST
PWM3
100k
100k
VEE
INTVCC
SYNC
82.5k
10µF
×2
D2
10µF
M4
470µF
ISN
CTRL2
VEE
4.02k
VEE
D6
FB
2.2µF
PWM_OUT1
1nF
226k
PWM_OUT2
1nF
1nF
VFNEG
226k
M7
LED_ISP
SGND VEE RT
SS
143k
VEE
VEE
VC1 LED_ISN
M8
D4
33nF
10nF
1k
VEE
VEE
3mΩ
VEE
10 | May 2016 : LT Journal of Analog Innovation
design features
By regulating the LED current directly and level-shifting all input signals, the
LT3744 is capable of producing negative voltages, allowing low voltage battery
operated systems to drive multi-LED strings with a simple 2-switch solution.
minimize the inductance from each of the
demo boards to the LED, the parallel LED
driver boards were mounted directly on
top of each other. A more optimized layout
would feature both drivers mounted on
a single board, with the driver layouts
mirroring each other, reflected across their
mutual connection to the LED. Whenever
designing the conduction path from a
LED driver to a high current LED, careful
attention should be placed on the total
inductance. Since inductance is a function of wire length, the longer the wire,
the longer the current recovery in the
LED—no matter how fast the driver.
INVERTING BUCK-BOOST, 120W
LED DRIVER WITH TWO PARALLEL
LT3744s
Inverting buck-boost applications have the
same thermal concerns as non-inverting
converters, with the additional design
challenge of increased inductor current.
For low input voltages and high LED voltages, the average current in the inductor
could be very high. For example, if the
input is 3.3V and the output is one green
LED—which has a forward voltage of 6V
at 20A—the peak inductor current is 70A.
The inductor used in the design should
have a saturation current at least 20%
higher—in this case, greater than 80A.
Since this current flows in the switching
MOSFETs, the MOSFETs must be rated
for greater than 80A. By placing two
LT3744 inverting buck-boost converters
in parallel, the peak switched current is
cut in half, reducing the requirements
of the power path components.
Figure 15. Parallel inverting
board temperatures
delivering 120W to the LED
INDUCTOR
SWITCHING
MOSFETS
LT3744
VIN = 12V
VOUT = −4V
IOUT = 30A
fSW = 350kHz
In the inverting buck-boost topology, the
inductor current is delivered to the load
only during the synchronous FET conduction time. If the two parallel converters
are allowed to run at their free-running
frequencies, there is noticeable beat
frequency apparent in the LED current
ripple resulting from the slight switching
frequency differences. To avoid this, each
converter uses the same RT resistor value,
but they are synchronized using an external clock. In the application in Figure 14,
the converters are designed to run at a
non-synchronized frequency of 300kHz —
with a 350kHz synchronizing clock.
Figure 15 shows the component temperature rise when delivering 30A to the LED in
a parallel inverting buck-boost application.
CONCLUSION
With features including high current
regulation accuracy, a floating PWM gate
driver, and level shifted input signals, the
LT3744 can be configured to drive LEDs
in a wide range of applications. The
LT3744 has the capability to be used as the
single driver in an RGB projection system,
drastically reducing total solution space—
making it possible to produce high lumen
video projection from a smartphone.
Through the use of three current regulation states, the LT3744 gives system
designers freedom to sculpt LED color,
producing more faithful video images. By
regulating the LED current directly and
level-shifting all input signals, the LT3744
has the capability to produce negative
voltages, allowing low voltage battery
operated systems to drive multi-LED
strings with a simple 2-switch solution.
The LT3744 can be easily paralleled
with other LT3744s to efficiently deliver
extremely high current to an LED, while
maintaining current accuracy and sharing
even when PWM dimming. Paralleling the
LT3744 lowers board temperatures, reduces
inductor currents and expands supported
LED power to hundreds of watts. n
May 2016 : LT Journal of Analog Innovation | 11
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