LED Electrical Driving Consideration

APPLICATION NOTES:
LED Electrical Driving Consideration
Introduction:
THE NEEDS OF A CONTROLLED CURRENT SOURCE
The IV curve for LED is similar to the IV curve for normal diode, except that it has steeper
slope of If versus Vf in the high current region. As an example from the figure 1 below,
variation of 0.1V supply voltage will cause the forward current differs by approximately
50mA. Since the luminous intensity of LED varies with forward current, this causes the
relative intensity to vary by >10%. Therefore, LED circuitry should be designed to drive by
controlled current source rather than controlled voltage source. With rapid advancement in
semiconductor technology, various IC component suppliers are capable of producing
constant current LED driver with output current accuracy +-3.5% between drivers and
current matching smaller than 0.5% within driver ( for LED driver with more than 1 output
channel drive).
Figure 1– SPNova InGaN White 350mA If vs Vf and Iv vs If Characteristic
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LED Electrical Driving Consideration
GENERAL LED CIRCUIT DESIGN OVERVIEW
Current Limiting
Method
Resistor ( most
common in existing
automotive circuit
design )
Advantage
•
•
Easy to design.
Inexpensive.
Disadvantage
•
•
•
Linear Current
Regulator
•
•
Switching Regulator
Control
•
•
•
•
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Allow dimming
through current
control.
Linear control-loop
circuitry accurately
controlled the LED
current.
Efficient operation
over wide range of
input voltage.
Able to regulate LED
current precisely.
Allow dimming by
PWM.
Lower power
dissipation.
-2-
•
•
•
•
•
•
It cannot control current
accurately.
Current varies to some
extent with supply voltage.
High power dissipation in
the resistor especially on
high power LED.
More
expensive
than
simple resistive current
limiter.
High power dissipation.
Might require heat sink for
active pass device.
More complex design.
More expensive compare
to linear current supply.
Need to design for
electromagnetic
compatibility.
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LED Electrical Driving Consideration
Current Limiting Method : Resistor
Series LED Drive Configuration :
Pros :
• Matching guaranteed with well regulated LED current
• Easy implementation with inductive boost
• Only 1 ballast element, most power efficient, ballast
loss= VBallast x ILED
Cons :
• High output voltage drive required for long string of
series LED. For low input voltage application, boost type
regulator might be required
• A single defective LED would cause the whole string to
fail
Parallel LED Drive Configuration :
Pros :
• Single defective LED will not cause all
LED network to fail
• Lower output voltage drive required
Cons :
• Require regulated current source to
ensure good matching, 1 for each LED
• More power loss compare to series
implementation.
Total ballast loss = N x VBallast x ILED
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LED Electrical Driving Consideration
Parallel Strings of LEDs Configuration :
Pros :
• Good for driving lots of LEDs
• Reduce the output voltage drive requirement compare to
the series implementation for the same LED count.
• Lower power loss ( ballast loss ) compare to pure parallel
implementation for the same LED count.
Cons :
• Only the 1st string has regulated current. Diode voltages set
the current for all secondary strings.
• LED voltage mismatches would lead to brightness
difference from one string to another.
Current Limiting Method : Linear Current Regulator
Figure 2, a typical LED circuit design using linear current regulator NUD4001 produce by
On Semiconductor.
Vin
Iout
1
8
Iout
4 Ohm
Boost
Rext
2
NUD4001
7
Iout
3
6
4
5
GND
Iout
ILED=175mA
12 V
SPNova 0.5W
LED1 - LED4
Features :
• Up to 30V operating range
• Adjustable output current
level up to 500mA
• Minimum component count
• Suitable for automotive
application in tail light,
CHMSL and internal
lighting application
Figure 2 – On Semiconductor NUD4001 linear current regulator
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LED Electrical Driving Consideration
Current Limiting Method: Switching Regulator Control
A. SWITCHING POWER SUPPLY CURRENT CONTROL DRIVER
With ever demanding for higher brightness LED and higher power efficiency especially for
portable power application, there is trend for major IC component manufacturer to
introduce more advance switching LED current control driver with various functionality and
better current matching/regulation.
When choosing the switching power supply LED current control driver, consideration
should be taken on the supply input voltage level versus the output LED load voltage
level. Basically the are 3 type of topologies for switching power supply :
1. Buck Regulator
It can function if the input voltage always exceeds the sum of the maximum forward
voltages of every LED in a string. It can reduce the output voltage to a lower level
to minimize power loss.
2. Boost Regulator
When the sum of all the forward voltages drop in a string will always exceed the
maximum input voltage, a boost regulator is needed to amplify the output voltage to
the required range in order to drive the LED network.
3. Buck/Boost Regulator
When the input voltage might swing above or below the sum of all the forward
voltage drop in a string, then a buck/boost regulator should be use to drive the LED
network.
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LED Electrical Driving Consideration
Various LED driving circuitry is depicted as below, please refer to respective IC
manufacturer for detail design consideration when using their LED driver.
Switching Buck Regulator
Figure 3, a high power LED driving circuit design using National Semiconductor LM3402
0.5A constant current buck regulator. Vin range from 6V to 42V ( LM3402 ).
Figure 3 – LM3402, an example of buck regulator
Figure 4, another LED circuit design using Texas Instrument bq24105. The bqSWITHER
series can achieve 10% current regulation accuracy for 100mV to 200mV sensing resistor
voltage.
Figure 4 – bq24105, an example of buck regulator
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LED Electrical Driving Consideration
A list of market commonly available buck regulator is as tabulated below :
Type
Buck
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Supplier
Part Number
Vin (DC)
Exar
SP6652
2.7 to 5.5
Vout(max, DC)
5
Iout (max, mA)
1000
Exar
SP6655
2.7 to 5.5
5
400
STMicroelectronics
Melexis
National Semiconductor
National Semiconductor
National Semiconductor
National Semiconductor
National Semiconductor
National Semiconductor
National Semiconductor
National Semiconductor
National Semiconductor
National Semiconductor
Sipex
STMicroelectronics
Zetex
Zetex
Zetex
Zetex
Zetex
Zetex
Zetex
Zetex
National Semiconductor
Sipex
Catalyst
L6926
2 to 5.5
5
800
MLX10803
6 to 32
32
External
LM3401
4.5 to 35
35
External
LM3407
4.5 to 30
27
350
LM3402
6 to 42
41
500
LM3402HV
6 to 75
74
500
LM3404
6 to 42
41
1200
LM3406
6 to 42
37
1500
LM3406HV
6 to 75
67
1500
LM3405A
3 to 22
21
1000
LM3404HV
6 to 75
74
1500
LM3489
4.5 to 35
Adjustable
External
SP6137
3 to 20
3 to 15
External
L6902
8 to 36
34
1000
ZXLD1350E5
9 to 30
30
350
ZXLD1350
7 to 30
30
350
ZXLD1360
7 to 30
30
1000
ZXLD1362
6 to 60
60
1000
ZXLD1320
4 to 18
18
1500
ZXLD1352
7 to 30
30
350
ZXLD1356
6 to 60
60
550
ZXLD1366
6 to 60
60
1000
LM2734
3 to 20
18
1000
SP7601
4.5 to 29
29
External
CAT4201
6.5 to 20
32
350
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LED Electrical Driving Consideration
Switching Boost Regulator
Figure 5 below shows a typical switching boost regulator circuit design using Linear
Technology LT3466-1, white LED driver and boost converter. It can drive up to 10 white
LEDs from a 3.6V supply. It allows +-4% current programming accuracy.
Figure 5—LT3466-1, an example of boost regulator
Figure 6, another switching boost regulator design using On Semiconductor NCP5010,
500mW boost converter for white LED. It featuring 2.7V to 5.5V input voltage range. 84%
Efficiency for 5 LEDs at 30mA and Vin =4.2V
Figure 6—NCP5010, an example of boost regulator
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LED Electrical Driving Consideration
A list of market commonly available boost regulator is as tabulated below :
Type
Boost
Supplier
ON Semiconductor
ON Semiconductor
Exar
Exar
Exar
Zetex
Zetex
Zetex
National Semiconductor
National Semiconductor
National Semiconductor
National Semiconductor
STMicroelectronics
Part Number
Vin (DC)
Vout(max, DC)
NCP1422
1.0 to 5.0
5
Iout (max, mA)
800
NCP1450A
0.8 to 6.0
6
1000
SP6641B
1.0 to 5.0
6
500
SP6648
0.7 to 4.5
4
400
SP7648
2.7 to 4.5
2.7 to 5.5
800
ZXSC310E5
0.8 to 8.0
8
1100
ZXSC400E6
8
1100
ZXLD1321
1.8 to 8.0
1.2 to
12.0
18
1000
LM2623A
0.8 to 14
14
1000
LM2700
2.2 to 12
17.5
1000
LM3551
2.7 to 5.5
11
1000
LM3553
2.7 to 5.5
19
1200
L6920
0.6 to 5.5
8
500
Switching Buck/Boost Regulator
Figure 7 below shows a typical switching buck/boost regulator circuit design using Linear
Technology LTC3453, Synchronous Buck Boost High Power White LED Driver. It is
optimized to drive up to 4 white LEDs at a combined current of 500mA from a single Li-Ion
battery input. The regulator operates in either buck, boost or buck-boost mode, depending
on the input voltage and LED maximum forward voltage.
Figure 7-- LTC3453, an example of buck/boost topology regulator
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LED Electrical Driving Consideration
Figure 8, another switching buck-boost regulator circuit design using MAXIM MAX1759,
Buck-Boost Charge Pump Regulator. In the design below, the regulator biases white
LEDs with 15mA current from a wide 1.6V to 5.5V voltage range.
Figure 8—MAX1759, an example of buck/boost topology regulator
A list of market commonly available buck/boost regulator is as tabulated below:
Type
Buck/Boost
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Supplier
Part Number
Vin (DC)
Vout(max, DC)
Iout (max,
mA)
STMicroelectronics
STMicroelectronics
STCF02
2.7 to 5.5
2.5 to 5.3
600
STCF03
2.5 to 5.3
800
Zetex
ON Semiconductor
ON Semiconductor
ON Semiconductor
National
Semiconductor
National
Semiconductor
Zetex
ZXLD1322
2.7 to 5.5
2.5 to
15.0
15
700
NCP5030
2.7 to 5.5
5.5
1200
NCP3163
2.5 to 40
40
3000
NCP3065
3.0 to 40
40
1500
LM3423
4.5 to 75
75
External
LM3421
4.5 to 75
75
External
ZXLD1322
2.5 to 15
15
700
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LED Electrical Driving Consideration
B. ADVANCE LIGHT MANAGEMENT UNIT (ALMU)
With the increasing popularity of LED application in various fields, especially in mobile
handheld application and display backlighting, several major integrated circuit supplier
have already starts to produce the advance light management IC. This ALMU not only
regulate the LED current, but also provide various functionality to manage the brightness,
sequence, and to synchronize the LEDs with audio input. The ALMU usually will come
with an I2C or SPI interface. This ALMU will further reduce the space required for LED
control circuit and will simplify the circuit design effort. Several examples are depicted
below. Please consult respective IC manufacturer’s website for detail information.
National Semiconductor, LP3954, Advance Light Management Unit
Features :
•
•
•
•
•
•
Advance Light Management
Unit for handheld product
Audio synchronization for
color/RGB LEDs
High current driver for flash
LED with built-in timing
Six constant current white
LED
driver
with
8-bit
programmable adjustment
SPI/I2C compatible interface
Possibility for external PWM
dimming control
Figure 9—LP3954 Advance Light Management Unit
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LED Electrical Driving Consideration
MAXIM MAX1707, Light Management Unit
Figure 10 – MAX1707 Light Management Unit
Features :
•
•
•
•
•
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The MAX1707 provides complete light management for main display
backlight, sub-display backlight and white LED camera flash with
regulated constant current up to total 610mA.
An I2C serial port is used for on/off control and setting the LED currents
in 32 linear steps.
0.3% LED current accuracy and matching.
The I2C port provides 32k colors and programmable ramp up/down
rates.
The camera flash may be turned on/off by the I2C port
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LED Electrical Driving Consideration
LED CIRCUIT PRACTICAL DESIGN GUIDE
For automotive/general lighting application, the most common input supply voltage would
be 12V & 24V DC. The most common LED drive current level would be 120mA -150mA
for SPNova 0.5Watt and 320mA - 350mA for SPNova 1Watt.
Two examples of reference design to drive a series of LEDs at Vin =12V, using On
Semiconductor NUD4001, High Current LED Driver is depicted below. Please refer to
NUD4001 datasheet for further technical details.
Reference Design 1 : Driving 4 Series Connected SPNova 0.5Watt At Forward Current
of 150mA , Supply Voltage, Vin = 12V
Vin
Iout
1
8
2
7
Iout
4 Ohm
Boost
4.7 ohm
Rext
NUD4001
Iout
3
6
4
5
GND
Iout
ILED =175mA
ILED =150mA
12 V
SPNova 0.5W
LED1 - LED4
Bill Of Materials:
Part
Manufacturer
Part Number
Rext
Vishay
CRA06E0803472JRT1 4.7 Ohm resistor
NUD4001
On Semiconductor
NUD4001DR2G
High Current
LED Driver
LED1-LED4
Dominant
Semiconductor
NPR-SSS-XY2-1
SPNova 0.5W
Red
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Attribute
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LED Electrical Driving Consideration
Several key parameters for NUD4001 are as follow :
1. Vsense = 0.7 V at Tj= 25 ºC
2. P D_control, Power dissipation for internal circuitry of NUD4001 is 0.055W at 12V
input, 0.25W at 24V input.
3. Maximum power dissipation of NUD4001 is 1.13W.
4. It supplies constant LED current for varying input voltage up to 30V max
5. Output current level is defined by the resistor, Rext
Electrical Design Guide:
1. LED current required, ILED is 150mA
2. Resistor value for Rext = Vsense / ILED = 0.7V/0.150 Ohm = 4.7 Ohm
Thermal Design Guide :
Base on Dominant Semiconductor datasheet for SPNova, for example NPY-SSS-XY2, the
minimum forward Voltage Vfmin =1.9V, typical forward voltage Vftyp= 2.2V. The power
dissipation in the current regulator IC should be calculated for Vfmin, where the voltage
drop across the current driver would be maximum. This will prevent the NUD4001 from
being killed due to overheat.
Vfmin = 1.9V
Vftyp = 2.2V
Total voltage across LED,
VLED = 1.9V x 4 = 7.6V
VLED = 2.2V x 4 = 8.8V
Voltage drop across NUD4001,
Vdrop = Vin – Vsense- VLED
= 12V – 0.7V -7.6V
Vdrop = 12V -0.7V – 8.8V
= 3.7V
= 2.5V
Power dissipation across NUD4001,
PD_driver = Vdrop x Iout
= 3.7V x 0.175A
PD_driver = 2.5V x 0.175A
= 0.4375W
= 0.6475W
Total power dissipation,
PD_total = PD_driver + PD_control
= 0.6475W + 0.055W
PD_total = 0.4375W + 0.055W
= 0.7025W
= 0.4925W
Base on both case of Vf above, the total power dissipation is < 1.13W, which means the
circuit above can be operated without overheating the NUD4001.
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LED Electrical Driving Consideration
Reference Design 2 : Driving 3 Series Connected SPNova 1 Watt At Forward Current of
350mA , Supply Voltage, Vin = 12V
Vin
Iout
1
8
Iout
2 Ohm
Boost
Rext
2
7
NUD4001
Iout
3
6
4
5
GND
Iout
ILED =350mA
12 V
SPNova 1W
LED1 - LED3
Bill Of Materials:
Part
Manufacturer
Part Number
Attribute
Rext
Vishay
CRCW12062R00FRT1E3 2 Ohm resistor
NUD4001
On Semiconductor
NUD4001DR2G
High Current
LED Driver
LED1-LED3
Dominant
Semiconductor
NPW-USD-AD-1
SPNova 1W
White
Several key parameters for NUD4001 are as follow :
1. Vsense = 0.7 V at Tj= 25 Deg C
2. P D_control, Power dissipation for internal circuitry of NUD4001 is 0.055W at 12V
input, 0.25W at 24V input.
3. Maximum power dissipation of NUD4001 is 1.13W
4. It supplies constant LED current for varying input voltage up to 30V max
5. Output current level is defined by the resistor, Rext
Electrical Design Guide:
1. LED current required, ILED is 350mA
2. Resistor value for Rext = Vsense / ILED = 0.7V/0. 350 Ohm = 2 Ohm
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LED Electrical Driving Consideration
Thermal Design Guide :
Base on Dominant Semiconductor datasheet for SPNova, NPW-USD-AD-1, the minimum
forward Voltage Vfmin =3.0V, typical forward voltage Vftyp = 3.6V.
Vfmin = 3.0V
Vftyp = 3.6V
Total voltage across LED,
VLED = 3.0 V x 3 = 9.0V
VLED = 3.6V x 3 = 10.8V
Voltage drop across NUD4001,
Vdrop = Vin – Vsense- VLED
= 12V – 0.7V -9.0V
Vdrop = 12V –0 .7V – 10.8V
= 2.3V
= 0.5V
Power dissipation across NUD4001,
PD_driver = Vdrop x Iout
= 2.3V x 0.350A
PD_driver = 0.5V x 0.350A
= 0.805W
= 0.175W
Total power dissipation ,
PD_total = PD_driver + PD_control
= 0.805W + 0.25W
PD_total = 0.175W + 0.25W
= 1.055W
= 0.425W
Base on both case of Vf above, the total power dissipation is < 1.13W, which means the
circuit can be operated without overheating the NUD4001.
Please note that for the both reference design above, care should be taken to ensure
minimum input voltage, Vinmin > Vsense + VLED(Max) , in order to supply enough forward
voltage drive for the LEDs.
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LED Electrical Driving Consideration
Reference Design 3: Using National Semiconductor LM3402 ( 0.5A Constant Current
Buck Regulator ).
Driving 6 Series Connected SPNova 1 Watt at Forward Current of 330mA, Supply
Voltage, Vin = 24V ( Please refer to National Semiconductor LED Reference Design
Library : http://www.national.com/webench/ledrefdesigns.do for further details and circuit
simulations. )
ILED =330mA
Bill Of Materials:
Part
Cb
Cf
Cin
Co
D1
L1
Ron
Rsns
U1
LED1-LED6
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Manufacturer
Vishay
Vishay
TDK
TDK
Central Semiconductor
TDK
Vishay
Panasonic
National Semiconductor
Dominant Semiconductor
Part #
VJ0805Y103KXXAT
VJ0805Y104KXXAT
C3225X7R1H225M
C3225X7R1E475M
CMSH1-40M
SLF7045T-470MR75
CRCW08054873F
ERJ6BQFR56V
LM3402
NPW-TSD
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Value
10nF
100nF
2.2uF
4.7uF
0.5V
47uH, 0.15 Ohm
487k Ohm
0.56 Ohm
SuperNova 1 W
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LED Electrical Driving Consideration
LED DRIVER
The following is a list of integrated circuit suppliers that produce LED driver/LED
Management device or regulator that can be configured to drive LED circuits. Please refer
to their website for detail design consideration and specification. ( Note : The name of the
IC manufacturers below is directly hyperlink to their respective website )
IC Supplier
Analog Devices
Fairchild Semiconductor
Holtek Semiconductor Inc.
Infineon
Intersil Corporation
Linear Technology
Maxim Integrated Products
Micrel Semiconductor
MICRO ANALOG SYSTEMS
MicroChip
Microsemi Corp.
Mitsubishi Semiconductor
National Semiconductor
SIPEX
SUPERTEX
STMicroelectronics
Texas Instruments
ZETEX
With the rapid growth of LED industry, several premier analog IC suppliers have compiled
a comprehensive LED driver guide for their products that would provide the designer with
a vast choice of LED drivers based on their application needs. Four of the LED driver
guide selections can be downloaded from the link below:
National Semiconductor :
http://www.national.com/appinfo/power/files/lighting_solutions.pdf
Linear Technology :
http://www.linear.com/ then select Power Management For LEDs
Arrow Electronics :
http://www.arrownac.com/industry_solutions/lighting/selector_guide.html
Supertex :
http://www.supertex.com/products/selector_guides
SUMMARY :
To ensure good long-term reliability and brightness uniformity, the LED shall be driven by
constant current source. Since there are a wide range of LED drivers available in the
market from different electronic IC manufacturers, the implementation of constant current
drive becomes very simple without the need of extensive circuit design while minimizing
the total component count.
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