AN1891 APPLICATION NOTE
APPLICATION IDEAS: DRIVING LEDS USING L497X, L597X, L692X DC-DC CONVERTERS FAMILIES
This application note, describes the main applications and driving methods for LEDs. After this, several application ideas using ST DC-DC converters are shown.
1 S U MM A R Y
1 2 3 4 SUMMARY . . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . ..... 1 INTRODUCTION . . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . . 2 DRIVING LEDS . . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . ... 2 APPLICATION IDEAS . . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . ... 4 4.1 DC BUS SUPPLY. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . . 4 4.1.1 L5970D APPLICATION IDEA. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . ... 5 4.1.2 L4973D APPLICATION IDEA. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . ... 6 4.1.3 L6902D APPLICATION IDEA. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . ... 6 4.2 BATTERY POWERED APPLICATION . . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . ... 7 4.2.1 L6920D APPLICATION IDEA. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . ... 7 4.2.2 L6926D BOOST TOPOLOGY APPLICATION IDEA. . .. . .. . .. . .. . .. . .. . .. . ... 8 4.2.3 L6926D BUCK-BOOST TOPOLOGY APPLICATION IDEA. . .. . .. . .. . .. . .. . .. 9 4.2.4 L6926D BUCK TOPOLOGY APPLICATION IDEA . . .. . .. . .. . .. . .. . .. . .. . ... 10 CONCLUSION . . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. 10
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AN1891 APPLICATION NOTE
2 INTRODUCTION
LED (Light Emitting Diode) is a Light Emitted p-n junction Diode, under forward bias. If a forward bias is applied between the p-n layers, electrons and holes are meeting in the active layer, and recombine themselves, emitting photons. If a reverse bias is applied, they don't move in the active layer, and consequentially, no light is emitted. LEDs generate practically a monochromatic light, with a focused beam of a single wavelength. Therefore, there is not wasted energy, and no filters are required to obtain different colors. The emitted radiation wavelength depends on the dopants. Due to their construction technology, LEDs are not subjected to shock, vibrations and heat, and this makes their life very long, measured in order of years (Typically 100.000 hours with an operating temperature comprised between -40C and +85C). Thanks to all these characteristics, LEDs are widely used in particular color-specific, power-crucial and high reliability application: M obile appliances Mobile phone screen and keyboard backlight PDAs, MP3 players and digital camera backlight Signs and displays Advertising signs Traffic variable messages signs Autom otive applications Interior application: lights for instrumental panel and dashboard Exterior lighting stop/turn/tail lights Signals Traffic signals Arrows and pedestrian signals Illumination Flashlights Architectural and design lighting Emergency lighting Water features and pools
3 DRIVING LEDS
LEDs are devices where the light intensity (brightness), measured in millicandelas, or in Lumens, is proportional to the forward current flowing through them. There are two main categories of LEDs: the white-blue LEDs, with a typical voltage drop of 3-4V, and the green-redyellow ones, with a typical voltage drop, which is about 2V. It is possible to make another distinction based on the forward current: Low current LEDs, from 15mA to 50mA, mainly used in the portable market for backlight and signaling applications. High current LEDs, with forward current between 350mA to 1000mA, typically used in lighting applications. Moreover, applying the same forward voltage to different LEDs of the same type, the current flowing through them can change significantly. This can be seen in Figure 1, showing the V-I characteristic of different LEDs of the same type.
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AN1891 APPLICATION NOTE Figure 1. V-I characteristic of 8 white LEDs of the same type
This is why, in order to have a brightness matching between different LEDs, they must be driven by matched constant current sources. Based on these considerations, it is clear that, in principle, the easiest and cheapest way to drive LEDs is to connect them in series as shown in Figure 2. However, this implies that the LED driver must provide an output voltage that is the sum of the LEDs forward voltage. Figure 2. Basic schematic of series driven LEDs
ILED Constant Current Source
So, in other cases, the preferred solution is to drive LEDs in parallel, matching the different current sources, as shown in Figure 3. Figure 3. Basic schematic of parallel driven LEDs
ILED1 Constant Current Sources ILED2 ILED3
A typical way to realize a constant current source to drive LEDs is to use a DC-DC converter, as shown in Figure 4. Since the voltage control loop of the device regulates the voltage at the FB pin, a constant current source can be obtained simply connecting a resistor between this pin and GND.
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AN1891 APPLICATION NOTE Figure 4. Basic schematic of a DC-DC driving LEDs
ILED= ILED DC-DC Driver FB RSENSE VLED VOUT VSENSE V FB RSENSE
The efficiency of the system is given by the general formula:
Po u Eff = ----------t Pi n Eq 1
In case of LEDs, the useful power (that is POUT), is the LEDs power, given by:
n
PLE D =
V
i=1
LEDi
IL E D i
Eq 2
So, the efficiency of the system is:
PL E E f f = -----------D -PIN Eq 3
The power dissipated on the RSENSE resistor is obviously not contributing to the output power and so it should be minimized. This implies that the FB pin voltage should be as low as possible.
4 APPLICATION IDEAS
The best device for a particular application can be selected based on its input voltage range, output current capability, output voltage range, as in standard DC-DC choice. However, some particularity of the application emphasizes the importance of other parameters. For example, a very low FB voltage, in order to minimize the power dissipation by the sense resistor, is very important. Beside this, the possibility to change the voltage across the sense resistor, in order to easily adjust the LEDs brightness, is often required. The following paragraphs show some LEDs driving solutions, distinguishing two main application classes: DC bus supplied applications Battery powered applications 4.1 DC BUS SUPPLY Most of automotive and lighting applications belong to this category. The most common input voltages are 12V, 18V, 24V and 48V. For these applications L497xD, L597xD and the L6902D families of step-down monolithic DC-DC converters are suggested. Table 1, summarizes the devices characteristics (devices are grouped by family and sorted by output current):
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AN1891 APPLICATION NOTE Table 1. DC-DC converters with DC bus supply
Device L4976D L4971D L4978D L4973Dx3.3 L4973Dx5.1 L6902D L5970D L5972D L5973A D L5973D Iout (A) 1 1.5 2 3.5 3.5 1 1 1.5 1 .5 2 Vout (V) 0.5 to 50 3.3 to 50 3.3 to 50 0.5 to 50 5.1 to 50 0.5 to 35 0.5 to 35 1.23 to 35 0.5 to 35 0.5 to 35 Vin (V) 8 to 55 8 to 55 8 to 55 8 to 55 8 to 55 8 to 36 4.4 to 36 4.4 to 36 4.4 to 36 4.4 to 36 Fsw (KHz) Up to 300 Up to 300 Up to 300 Up to 300 Up to 300 250 250 250 500 250 3.3V Vref 3.3V Vref Constant current control, 3.3V Vref 3.3V Vref 5.1V Vref Additional Features 5.1V Vref Packages Minidip/SO16W Minidip/SO16W Minidip/SO16W DIP18/SO20 DIP18/SO20 SO 8 SO 8 SO 8 HSOP8 HSOP8
4.1.1 L5970D APPLICATION IDEA The L5970D is a monolithic step-down power switching regulator, capable to deliver up to 1A of continuous output current. The input voltage range is comprised between 4.4V to 36V. The feedback voltage is 1.23V, but with a resistor divider connected with the internal reference of 3.3V, is possible to reduce the sense resistor voltage. This helps in order to reduce the wasted power and to adjust the LEDs brightness. Figure 5. L5970D driving up to 6 high current white LEDs
VIN 12V to 24V
VCC COM P OUT VREF
15 H Vref-Vfb
STPS2L25U
ILED 330F 25V
Up to 6 LEDs
10F 25V 220pF
22nF
L5970D
GND INH FB
40K R1
R2 15K
Vled
4.7K
Vfb -Vsense
Rsense
1.3
Vsense =0.45V
ILED=350 mA
Figure 5 shows how to drive 6 LEDs in series. The VSENSE is reduced down to 0.45V, in order to minimize the wasted power. The formula used to reduce it is:
( V R E F VF B ) V S E N S E = V F B ----------------------------------- R 2 R1 Eq 4
The table below shows the efficiency of the application.
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AN1891 APPLICATION NOTE Table 2. Efficiency results of the L5970D driving series LEDs
Efficiency Vin=12V Iout=350mA Vin=18V Iout=350mA Vin=24V Iout=350mA 86.5% 90% 94.5% 97.5% 83.5% 90% 95% 90% 76% 89% 93.5% 1 LED 2 LEDs 3 LEDS 4 LEDs 5LEDS 6LEDs
4.1.2 L4973D APPLICATION IDEA This application is really closed to the previous one. The substantial different is in the input voltage, up to 55V. The device can deliver up to 3.5A of continuous output current. Thus, it is possible to drive more white LEDs connected in series. Figure 6 shown the schematic circuit of L4973D supplied with a 48V bus. It is driving up to 12 high current white LEDs. Even in this case, the voltage drop across the sense resistor is minimized for efficiency reasons, by the same trick used for the L5970D. Figure 6. L4973D driving up to 12 high current white LEDs
Vin=48V
Vref-Vfb
50K R1 20K
7,8 Vcc 1 Osc 17 SS 18 Sync 16 5.1V 12 Vfb
Vfb-Vsense
220nF 63V 2.7nF 470nF
L4973V3.3
11 Comp 10 INH
9 Boot
4,5,6 13,14,15 2.3 GND Out
220nF 78K R2 63V 150H
Iled
D1 3x100F 40V
Up to 12 LEDs
220nF
Vled
9.1K
22nF
Rsense 1.4
Iled=350mA
Vsense 0.5V
The efficiency of this application is given by the following table. Table 3. Efficiency of L4973D driving up to 12 LEDs
Efficiency Vin=48V Iout=350mA 95% 96% 98% 8 LEDs 10 LEDs 12 LEDs
4.1.3 L6902D APPLICATION IDEA The L6902D is a simple and complete step down switching regulator, with adjustable current limit. This device is a good solution for LEDs lighting applications, thanks to its inner current loop that allows regulating a constant current at the output with a minimum voltage drop across the external sense resistor (100mV). Figure 7 shows a schematic application of L6902D, driving up to 6 high current white LEDs. The device works in current limit mode, set to 350mA; each LED current is 350mA. The losses on RSENSE are only 70mW.
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AN1891 APPLICATION NOTE Figure 7. L6902D driving up to 6 high current white LEDs
VIN 12V-24V
22 H Vcc COMP 22nF OUT CS+ CSFB Vref S TPS340U
100mV
Up to 6 LEDs
100 F 25v R1
L6902D
10 F 25v 220pF 5.1K G ND
R2
The additional benefit of this device is that its voltage loop can be used to perform an over voltage protection as indicated in figure 7. The table 4 indicates the efficiency values of this application. Table 4. Efficiency of L6902D driving up to 6 LEDs
Efficiency Vin=12V Iout=350mA Vin=18V Iout=350mA Vin=24V Iout=350mA 84% 88% 90% 94% 97.5% 86% 89% 96.5% 75% 89% 1 LED 2 LEDs 3 LEDS 4 LEDs 5LEDS 6LEDs
4.2 BATTERY POWERED APPLICATION Battery powered applications are characterized by lower input voltages compared to the applications shown in the previous pages. In these applications, the input voltage changes with the battery discharge profile. The battery types for this kind of applications are: Li-Ion cell, with the voltage range comprised between 4.2V and 2.7V or less; Ni-MH cell, with the voltage range between 1.5V to 0.9V. As a consequence, the Buck topology is not the commonest one. Boost or Buck-Boost topologies are often required. We will focus our application examples on L692xD family, including a step-up converter (L6920D) and step-down converters (L6925D-L6926D) that can be used in both Boost and in Buck-Boost topology. Some application ideas are show below, in order to explain how to use these devices in LEDs applications, using external references and dimming. Table 5. DC-DC converters for battery powered applications
Device L6920D L6925D L6926D Iout (A) 1 0.8 0.8 Vout (V) 2 to 5.2 0.6 to Vin 0.6 to Vin Vin (V) 0.6 to 5.5 2.7 to 5.5 2 to 5.5 Fsw (KHz) Not fixed Up to1400 Up to 1400 Vfb (V) 1 .23 0.6 0.6 Additional Features Packages TSSOP8 MSOP8 MSOP8
4.2.1 L6920D APPLICATION IDEA The L6920D is a high efficiency Step-up converter. The start up is guaranteed at 1V, but the operating input voltage can goes down to 0.6V. With a maximum voltage of 5.5V the device
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AN1891 APPLICATION NOTE can be supplied with many battery types: from 1 Li-Ion cell, or 1 to 3 Ni-MH-Alkaline cells. In figure 8 is shown the typical application schematic of the device. The voltage Feedback is 1.23V. With the resistor divider, connected to the external reference, it is possible to reduce the sense voltage down to 0.4V, or less, in order to maximize the efficiency. In table 6 are reported the efficiency result of this application. Figure 8. L6920D with external reference
Lx 7 LBI 2 8 VOUT
L3.3 H
47F
GND 6
L6920D
3
____ LBO
10F D1
VREF
______ SHDN 4 1 FB 5
R2
5K
Iout=350mA
100nF
Vfb-Vsense
R sense 12.5K
1.15
R1
1F
External Reference 3.3V
Vsense= 0.4V
Vext -Vfb
Table 6. Efficiency of L6920D driving a 350mA LED
Vin (V) Eff (%) 2 88 2 .5 88 3 90 3 .5 86 3.7 84
4.2.2 L6926D BOOST TOPOLOGY APPLICATION IDEA The L6926D is a high efficiency monolithic synchronous Step-down regulator, with an operating input voltage range comprised between 2V to 5.5V. The feedback voltage is equal to 0.6V; this allows low losses on the sense resistor. The device can be used in different topologies. In particular when the input voltage is lower than the typical LED voltage drop, a Boost topology can be implemented, as shown in Figure 9. This topology is possible only because the supply is a battery, not referred to Ground. As typical in Boost topology, this configuration doesn't have an effective short circuit protection. A battery discharge can be generated, when the device is turned off. The brightness control can be done by using a PWM signal. Varying the duty cycle of the signal is possible to change the medium current value, in order to adjust the brightness. In this case, the maximum intensity is achieved when the duty cycle of the PWM signal is equal to 0%. The minimum intensity is when the duty cycle reaches the 100%.
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AN1891 APPLICATION NOTE Figure 9. L6926D in boost topology with digital adjustable brightness control
K L=10 H
+ -
1 Vbatt
Iled 350mA
D1
Sync
7
Vcc RUN
10F 6.3V
5 L6926D 2
COMP
1F
LX PGOOD VFB
5K 1 22F 6.3V 00
6 1
8 3
4
GND
1F 1.7
3.3 V PWM Signal 100%=LED Off 0%=LED On
2.2K
1 22K
nF
Table 7. L6926D in boost topology with digital adjustable brightness control
Vin (V) Eff (%) 2.1 76 2 .2 79 2.3 80 2 .4 82 2.5 83 2 .6 85 2.7 86 2 .8 86 2.9 87 3 .0 88 3.1 89 3 .2 89 3.3 90 3 .4 91
4.2.3 L6926D BUCK-BOOST TOPOLOGY APPLICATION IDEA If a single Li-Ion cell is used to supply the device, the Buck-Boost topology is usually required. In fact, the voltage drop across a white LED is comparable with the Li-Ion cell voltage. The advantage of this application is that the input voltage range can be higher or lower than the output ones and so the battery can be used over the full load range. Moreover, when the device is turned off, there isn't current flowing from the input to the output. An actual shutdown and short circuit protection are allowed. The typical application schematic is shown in figure 10. Figure 10. L6926D in Buck-Boost topology
VIN=2.7V 4.2V 4.7uH Sync Vcc C1 10uF 6.3V Ceramic RUN 7 6 1 2 5 LX PGOOD STPS1L30M C2 1uF 6.3V
L6926D 8
4 COMP GND
3 VFB
Iled 350mA 1.7
330pF
STT5NF20V
The efficiency of this system is reported in the table below: Table 8. Efficiency of L6926D driving a 350mA LED in Buck-Boost topology
Vin (V) Eff (%) 2.9 58 3 .0 59 3.1 60 3 .2 61 3.3 61 3 .4 62 3.5 62 3 .6 63 3.7 63 3 .8 64 3.9 64 4 .0 64 4.1 64 4 .2 65
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AN1891 APPLICATION NOTE 4.2.4 L6926D BUCK TOPOLOGY APPLICATION IDEA An USB port gives a voltage range between 4.75V to 5.25V, and provides a maximum current of 500mA. This is a good solution to drive a single high current white LED, in Buck topology. Infact, the USB voltage is always higher than the output voltage required driving the LED. The output voltage is given by the sum of the LED drop and the feedback voltage. The Figure 11 shows the application schematic while the Table 9 reports the efficiency results. Figure 11. L6926D supplied by USB port
Vin=5V L=6.8 H Sync 7 Vcc RUN 10F 6.3V 6 1 2 COMP 5 LX PGOOD VFB 10F 6.3V 1.7 Iled 350mA
L6926D
4
8 3
C3 220pF R1 10K
GND
Table 9. Efficiency of L6926D driving a 350mA LED in Buck topology
Vin (V) Eff (%) 5 85 5 .5 84 5.96 84
All the battery powered application ideas shown in this paper, are performed with a white LED, which has a voltage drop of approximately 3.3V, and a forward current of 350mA.
5 C O N C L U S I ON
The aim of this paper is to show how monolithic DC-DC converters can be used to drive LEDs. The right choice between all the ICs belonging to L497xD, L597xD and L692xD families depends on the final application requirements (power source, number of LEDs, additional functions etc...). This paper shows only few application ideas about LEDs driving: it is possible to find much more details on the specific ICs in the dedicated Datasheet and Application Notes available on ST web site.
Table 10. Revision History
Dat e March 2004 June 2004 Revision 1 2 First Issue Add. summary, changed any textes, changed style look. Description of Changes
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AN1891 APPLICATION NOTE
The present note which is for guidance only, aims at providing customers with information regarding their products in order for them to save time. As a result, STMicroelectronics shall not be held liable for any direct, indirect or consequential damages with respect to any claims arising from the content of such a note and/or the use made by customers of the information contained herein in connection with their products.
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. The ST logo is a registered trademark of STMicroelectronics. All other names are the property of their respective owners 2004 STMicroelectronics - All rights reserved STMicroelectronics GROUP OF COMPANIES Australia - Belgium - Brazil - Canada - China - Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States www.st.com
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