ST STEVAL-ILL009V1 Instrucciones de instalación y funcionamiento

September 2008 Rev 2 1/40

AN2531

Application note

Generating multicolor light using RGB LEDs

Introduction

The new high power and brightness RGB LEDs are going to be used in many different

lighting applications as backlighting, general lighting systems, traffic signals, automotive

lighting, advertising signs, etc. They are becoming popular mainly because it is possible to

generate an easy multicolor light with special lighting effects and their brightness can be

easy changed. On top of this, their long lifetime and small size make them the light source of

the future.

This document describes how to drive RGB LEDs, how to calculate a power dissipation,

how to design an over temperature protection, how to use a software PWM modulation and

why over voltage protection should be implemented for this kind of application.

STEVAL-ILL009V1 reference board shown in Figure 1 was developed in order to

demonstrate this design concept. This board was designed for driving super high brightness

multicolor RGB LEDs with current up to 700 mA per LED. The LED brightness and color can

be very easy changed by potentiometers and an automatic color change mode continuously

modulates the color of the LED to generate multicolor light. The LED over temperature

protection is designed on this board and therefore the power delivered to the LED can be

automatically limited to prevent LED overheating.

The STEVAL-ILL009V1 is a mother board assembled without LEDs. To evaluate light effect

features, it is necessary to order a load board (additional board with assembled RGB LEDs).

Two load boards are available for easy performance evaluation. The first one with the

OSTAR

Projection Module (refer to Chapter 12, point 1) has ordering code STEVAL-

ILL009V3 and the second one with the Golden DRAGON

LEDs (refer to Chapter 12,

point 2) has ordering code STEVAL-ILL009V4. All technical information about these

reference boards such as bill of materials, schematics, software, temperature protection and

so on are described in the sections below.

Note: A new reference board STEVAL-ILL009V5 was designed in order to replace the former

STEVAL-ILL009V1. The main reason why the new board was developed is to demonstrate

a new DC/DC converter capabilities using the ST1S10 and new improved LED drivers

STP04CM05 and STP08CP05. Thanks to the ST1S10 the size of the inductor is extremely

decreased, efficiency improved and board size significantly reduced. The STEVAL-

ILL009V5 reference design is described in Chapter 9.

Figure 1. STEVAL-ILL009V1 reference board

www.st.com

Contents AN2531

2/40

Contents

1 Driving concept for RGB LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2 How to drive many LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3 How to set high current for LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4 Color control - software modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

5 Power dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

6 Over voltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

6.1 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

6.2 Type of solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

7 LED temperature protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

8 STEVAL-ILL009V1 reference board . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

8.1 General description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

8.2 Getting started . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

8.3 Schematic description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

8.4 Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

8.5 Design calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

8.5.1 LED supply voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

8.5.2 Temperature protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

8.5.3 SW PWM frequency calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

8.6 Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

9 STEVAL-ILL009V5 reference board . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

9.1 STEVAL-ILL009V5 schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . 31

9.2 Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

10 STEVAL-ILL009V3 load board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

10.1 Schematic description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

10.2 Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

AN2531 Contents

3/40

11 STEVAL-ILL009V4 load board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

11.1 Schematic description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

11.2 Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

12 Reference and related materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

13 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

List of tables AN2531

4/40

List of tables

Table 1. BOM - STEVAL-ILL009V1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Table 2. Temperature limit setting using STLM20 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Table 3. Temperature limit setting using NTC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Table 4. STEVAL-ILL009V5 bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Table 5. STEVAL-ILL009V3 bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Table 6. STEVAL-ILL009V4 bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Table 7. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

AN2531 List of figures

5/40

List of figures

Figure 1. STEVAL-ILL009V1 reference board. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Figure 2. Driving concept for RGB LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 3. LED driver connection - serial configuration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 4. LED driver connection - parallel configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 5. Common drain configuration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 6. Software brightness modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 7. RGB LED configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 8. Over voltage on STP04CM596. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 9. Possible over voltage protections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 10. Temperature protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 11. Components position on the STEVAL-ILL009V1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 12. STEVAL-ILL009V1 schematics - LED drivers part. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Figure 13. STEVAL-ILL009V1 power sources schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Figure 14. Send data time diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Figure 15. Main program flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Figure 16. Blink function flowchart - first part. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Figure 17. Blink function flowchart - second part. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Figure 18. Manual color modulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Figure 19. Blink function flowchart - third part . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Figure 20. STEVAL-ILL009V5 reference board. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Figure 21. STEVAL-ILL009V5 power sources schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Figure 22. STEVAL-ILL009V3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Figure 23. STEVAL-ILL009V3 schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Figure 24. STEVAL-ILL009V4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Figure 25. STEVAL-ILL009V4 schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Driving concept for RGB LEDs AN2531

6/40

1 Driving concept for RGB LEDs

RGB refers to the three primary colors, red, green, and blue. Different colors can be

generated by controlling the power to each LED. In this application, the microcontroller

provides three software PWM signals (principle is described below in Chapter 4) for LED

drivers STP04CM596 so the color can be regulated.

The STP04CM596 is a high-power LED driver with 4-bit shift register designed for power

LED applications. In the output stage, four regulated current sources provide 80-500 mA

constant current to drive high power LEDs.

Figure 2 shows the driving concept for RGB LEDs using an STP04CM596 LED driver. The

LED supply voltage is connected to anodes of RGB LED and LEDs cathodes are connected

to the ground through constant current sources. The supply voltage value is very important

due to the power dissipation on drivers (detail explanation is described in Chapter 5).

The value of the constant current is set by only one external resistor for all the four driver

channels. The control unit in this application is a microcontroller, which sends data through

serial peripheral interface (SPI) to the shift registers inside STP04CM596. The data are

shifted bit by bit to the next drivers in a cascade with falling edge of the clock frequency (the

maximum communication frequency for this drivers is 25 MHz). When all data are

transmitted to the drivers through SPI, the micro sets latch input terminal (LE) pin “log 1" to

rewrite the data to the storage registers and to turn on or off the LEDs. More details on

timings and features are available in Application Note AN2141 (refer to Chapter 12, point 3)

and Datasheet of the STP04CM596 (refer to Chapter 12, point 4).

Temperature protection is designed in order to protect LEDs and increase their lifetime.

A sensor (STLM20) is assembled close to the RGB LEDs and informs the microcontroller

about RGB LED temperature. If the temperature is above its limit, the microcontroller

decreases LED brightness (LED power) through PWM signal.

An easy and user friendly hardware interface (potentiometers and buttons) was designed to

demonstrate features such as color set, brightness regulation, mode changes, etc.

Figure 2. Driving concept for RGB LEDs

AM00285

IC supply

voltage

CONTROL PANEL

MODE COLOR

Micro SPI

STP04CM596

Constant

current

Control

and

logic

part

I - reg.

LED supply

voltage Temperature sensor

Full color pixel

AN2531 How to drive many LEDs

7/40

2 How to drive many LEDs

In several applications not only one RGB LED, but many of them must be driven. There are

at least two possible ways to drive many RGB LEDs using the STP04CM596 LED driver,

depending on the specific lighting application.

If the request is to control each RGB LED independently, a serial configuration (drivers in

cascade connection) must be used as shown in Figure 3. The data are sent through all LED

drivers via the SPI and then latched to the outputs. The main advantage is that current in

each channel can be regulated by software PWM modulation, which in fact means color

control of each RGB LED. The disadvantage of this solution is lower PWM resolution for

a higher number of RGB LEDs, because it needs time to send data to all drivers. More

information about this principle is described in Chapter 4: Color control - software

modulation.

If the request is to build up a high power light with many LEDs of the same color, drivers can

be connected in parallel as shown in Figure 4. Main advantages are a simpler solution and

better PWM resolution, because only four bits are sent through the SPI and it takes a short

time. Color is also regulated by software PWM signals as described in Chapter 4.

Note: It is also possible to mix serial and parallel configurations in order to provide severaldifferent

colors with high lighting power. For example, two different colors using 10 RGB LEDs can be

implemented using two STP04CM596 connected in series and five such blocks connected

in parallel.

Figure 3. LED driver connection - serial configuration

AM00286

Micro

SPI

LED supply

voltage

STP04CM596

Control

and

logic

part

STP04CM596

Control

and

logic

part

Serial connection

SPI

How to drive many LEDs AN2531

8/40

Figure 4. LED driver connection - parallel configuration

AM00287

Micro

SPI

LED supply

voltage

STP04CM596

Control

and

logic

part

STP04CM596

Control

and

logic

part

Parallel connection

AN2531 How to set high current for LEDs

9/40

3 How to set high current for LEDs

The STP04CM596 is focused on driving high brightness and power LEDs and its output

constant current can be set between 80 and 500 mA. In case a LED with even higher

current is used, there is still a solution to control such LED using the STP04CM596. Thanks

to a common drain configuration, the outputs can be connected together as shown in

Figure 5. This increases the performance and current capability of this driver. This

configuration allows driving the whole range of HB LEDs available on the market. For

example, this principle is also used in the STEVAL-ILL009V1 presented in this application

note, because the board has maximum current capability of 700 mA (2 channels x 350 mA).

Figure 5. Common drain configuration

AM00288

STP04CM596 Rext

I-REG

Vf+ Vc

Color control - software modulation AN2531

10/40

4 Color control - software modulation

Software control modulation allows adjusting power to each channel of the STP04CM596

driver (i.e. LED brightness). Figure 6 explains the principle showing an example of how to

set an 8% duty cycle for red, 28% duty cycle for blue, 6% duty cycle for green and 98% duty

cycle for a fourth LED. For one complete dimming cycle, the microcontroller sends a certain

number of “0”s and “1”s to each LED. First, the microcontroller sends four bits in “logical 1"

(i.e. 1111b or Fh) to the driver in order to turn ON all the output channels. Then

microcontroller sends the same data (1111) until an output should be turned OFF

(depending on desired preset color). (Each bit of the 4-bit frame controlling its

corresponding output.) In this example, it is output 3 with green LED (6% duty cycle

required). From that moment, the microcontroller keeps sending 1101. In the next step the

output 1 with red LED (8% duty cycle) should be turned OFF and so data frame changes to

0101. This frame is sent until output 2 with blue LED (28% duty cycle) should be turned OFF

and when the frame 0001 is used. Finally, the output 4 with another LED (usually second

green LED) is turned OFF with 98% duty cycle, which means than 0000 is being sent until

maximum time for one cycle is reached. After that, the entire period for all outputs can start

again.

Figure 6. Software brightness modulation

The resolution of the LED dimming defines how many steps are possible to change the duty

cycle from 0% to 100% (e.g. 6-bit means 64 steps; 7-bit means 128 steps and so on). It is

obvious that it is preferred to design the control signal with a resolution as high as possible,

but several limitations should be taken into account. Limitations concern mainly the speed of

the serial communication interface inside the microcontroller (SPI) and the general

calculation power of the microcontroller. First, the general LED frequency should be

selected. This value is recommended to be above 100 Hz in order to avoid flickering as at

AM00289

DATA

Output 1

Output 2

Output 3

Output 4 98 % duty cycle

1111 1111 or new data1101 0101 0001 0000

T SW_PWM

LEVELS

6 % duty cycle

T SEND_DATA

8 % duty cycle

28 duty cycle

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