ST AN1891 Application note
AN1891
APPLICATION NOTE
APPLICATION IDEAS: DRIVING LEDS USING L497X, L597X,
L692X DC- D C CONVERT ERS FAMILIES
This application note, describes th e m ai n applications and dr i ving m et hods for LEDs. After
this, several appli cat io n ide as usi ng ST D C-DC converters are shown.
1 SUMMARY
1 SUMMARY....................................................................................................................... 1
2 INTRODUCTION..............................................................................................................2
3 DRIVING LEDS ................................................................................................................ 2
4 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
5 CONCLUSION ...............................................................................................................10
AN1891/0604
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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 app lied between t he p -n la yers, elect ron s and hol es are meet ing i n the ac tive la yer, and recombine the ms elves, emitting photons. If a reverse bias i s applied, they don't move
in the active layer , and c onse quenti ally , no l ight is emit ted. LEDs gener at e pract ical ly a monochromatic light, with a focused beam of a single wavelength. Therefore, there is not wasted
energy, and no filt ers are re qui red to obt ain dif fere nt color s. The emitt ed radiat ion wavel en gth
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 -40°C and
+85°C). Thanks to all these characteristics, LEDs are widely used in particular color-specific,
power-crucial and high reliability application:
■ Mobile appliances
● Mobile phone screen and keyboard backlight
● PDAs, MP3 players and digital camera backlight
■ Signs and displays
● Advertising signs
● Traffic variable messages signs
■ Automotive applications
● Interior appli cation: lights fo r instrumental panel and dashboard
● Exterior lighti ng stop/turn/tail li ghts
■ 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 proport ional to t he f orwar d curr ent fl owing through th em. Ther e are t wo main c ate gories of LEDs: the white-blue LEDs, with a typical voltage drop of 3-4V, and the green-redyellow ones, wit h a typical voltag e drop, which is about 2V. It is possible to make another di stinction based on th e forward current:
■ Low current LEDs, fr om 15mA to 50mA, mai nly used in th e portable market for backl ight and
signaling applications.
■ High current LEDs, with forward current between 350mA to 1000mA, typically used in
lighting applicat ions.
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 chara cteristic of 8 white LEDs of the same type
This is why, in order to have a bri ghtness mat ching betwee n differ ent LEDs, the y 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 volta ge.
Figure 2. Basic schematic of series driven LEDs
I
LED
Constant
Current
Source
So, in other cases, the preferred solution is to drive LEDs in parallel, matching the different
current sourc es, as shown in Figure 3.
Figure 3. Basic schematic of parallel driven LEDs
I
LED1
Constant
Current
Sources
I
LED3
I
LED2
A typical way to rea li ze a constant current source to drive LEDs is t o use a DC-DC converter,
as shown in Fi gure 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
FB
SENS
Figure 4. Basic schematic of a DC-DC driving LEDs
I
LED
I
LED
=
V
R
E
DC-DC
Driver
FB
R
V
V
OUT
V
The efficiency of the system is given by the general formula:
P
Eff
out
-----------=
P
in
Eq 1
In case of LEDs, the useful power (that is POUT), is the LEDs power, given by:
n
P
LED
V
=
∑
i1=
⋅
LEDiILEDi
Eq 2
So, the efficiency of the system is:
P
--------------=
LED
P
Eq 3
IN
Eff
The power dissipated on the RSENSE resistor is obviously not contributing to the output power and
so it should be minimized. This impl ie s tha t t he FB pin voltage should be as low as possib le.
4 APPLICATION IDEAS
The best device for a particular application can be selected based on its input voltage range,
output current capabil ity , outpu t voltage r ang e, as in standar d DC-DC choi ce. Howev er, some
particularity of the application emphasizes the importance of other parameters. For example,
a very low FB volt age, in or der to minimi ze the p ower di ssipat ion by the sens e resi stor, is ve ry
important. Besi de thi s, the poss ibil ity to change the volta ge acros s th e sense r esist or, in or der
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 applic ati ons
– Battery powered app li cations
4.1 DC BUS SUPPLY
Most of automotive and lig hti ng applications belo ng to this category. The most common in put
voltages are 12V, 18V, 24V and 4 8V. For these applica tions L497 xD, L597xD and t he L6902 D
families of step-down monolithic DC-DC converters are suggested. Table 1, summarizes the
devices charac ter istics (devices are grouped by family and sor ted by output current):
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