High brightness LED dimming using the STM32F334 Discovery kit
Contents
1. og AN4885 Y A Application note High brightness LED dimming using the STM32F334 Discovery kit Introduction This application note illustrates the high brightness LED dimming feature embedded within the STM32F334 Discovery kit 32F3348DISCOVERY This demo application has been built with the STM32F334xx Cortex M4 microcontroller designed for digital power conversion applications It aims to demonstrate the usage of the various on chip peripherals the brand new 10 channel high resolution timer HRTIM the digital to analog converter DAC the ultra fast comparator COMP the direct memory access controller DMA This high level of integration inside the microcontroller is reducing the number of external required components generally included in LED dimming analog components based solutions The direct hardware connections between DAC and COMP COMP and HRTIM HRTIM and DMA are also used in order to decrease the CPU load and especially eliminating any latency in the system An additional description related to the triangle waveform generation from the high resolution timer is included to this application note The STM32F334 Discovery kit also includes a buck boost DC DC converter that is described separately to this application note This application does not require any additional hardware or software Once the STM32F334 Discovery kit is powered up through a mini B USB cable connected to the host PC the user can access to the high b2. PRODUCTS ARE NOT DESIGNED OR AUTHORIZED FOR USE IN A SAFETY CRITICAL APPLICATIONS SUCH AS LIFE SUPPORTING ACTIVE IMPLANTED DEVICES OR SYSTEMS WITH PRODUCT FUNCTIONAL SAFETY REQUIREMENTS B AERONAUTIC APPLICATIONS C AUTOMOTIVE APPLICATIONS OR ENVIRONMENTS AND OR D AEROSPACE APPLICATIONS OR ENVIRONMENTS WHERE ST PRODUCTS ARE NOT DESIGNED FOR SUCH USE THE PURCHASER SHALL USE PRODUCTS AT PURCHASER S SOLE RISK EVEN IF ST HAS BEEN INFORMED IN WRITING OF SUCH USAGE UNLESS A PRODUCT IS EXPRESSLY DESIGNATED BY ST AS BEING INTENDED FOR AUTOMOTIVE AUTOMOTIVE SAFETY OR MEDICAL INDUSTRY DOMAINS ACCORDING TO ST PRODUCT DESIGN SPECIFICATIONS PRODUCTS FORMALLY ESCC QML OR JAN QUALIFIED ARE DEEMED SUITABLE FOR USE IN AEROSPACE BY THE CORRESPONDING GOVERNMENTAL AGENCY Resale of ST products with provisions different from the statements and or technical features set forth in this document shall immediately void any warranty granted by ST for the ST product or service described herein and shall not create or extend in any manner whatsoever any liability of ST ST and the ST logo are trademarks or registered trademarks of ST in various countries Information in this document supersedes and replaces all information previously supplied The ST logo is a registered trademark of STMicroelectronics All other names are the property of their respective owners 2014 STMicroelectronics All rights reserved STMicroelectronics group of companies Au
3. STM32F334xx peripherals with the settings described below GPIOs some of the GPIOs are needed to connect the User push button B1 and the 4 signal LEDs used for different features in demonstration modes display e PB6 to PB9 set as output GPIOs to drive the signal LEDs e PAO set as input GPIO and connected to the User push button B1 DMA The STM32F334xx DMA channel 5 is configured to update the DAC output and generate the slope compensation curve on the internal comparator COMP4 minus input as described in section 1 4 4 The DMA requests are generated by HRTIM following events REP repetition period CMP1 Compare event 1 CMP2 CMP3 and CMP4 The curve is built from 5 points stored in the CurrentSenseTab array in RAM A DMA transfer from the RAM to the DAC 12 bit right aligned data holding register is triggered on each HRTIM event In order to compensate the DAC output settling time the top of the curve is synchronized with the CMP4 event instead of the PER event DMA Channel 5 is configured to interact with DAC channel 1 peripheral It provides high speed transfer between the memory and the DAC peripheral Base address of the memory is set to CurrentSense Tab array Its buffer size is 5 word long and contains the values of current thresholds that will be used for linear ramp generation slope compensation The five DMA parameters are set into HRTIM DMA configuration such as REP repetition period CMP1 to CMP4 Compare events 1
4. capability There are several existing dimming techniques and especially the PWM dimming based shown here with the STM32F334 Discovery kit This method prevents any change of the color in the LED during dimming and allows various brightness levels This example around the STM32F334xx advanced ARM based 32 bit MCUs shows one of the various capabilities that such a microcontroller can offer with a full set of peripherals and plethora of settings The high resolution timer is a model of the variety of available high performance peripherals It includes many configuration settings that make this application example possible with a minimum of external hardware and enhanced possibilities These configurations have mainly been anticipated especially for application using power converters topology and their requirements This demonstration example is a starting point for other applications and highlights how the high resolution timer can be configured in combination with other internal product resources It can be easily reused for further developments and includes all minimum software sets to control the high resolution timer interacting with associated peripherals DoclD026292 Rev 1 17 19 Revision history AN4885 2 Revision history Table 1 Document revision history owe eer wee 19 Jun 2014 Initial release 7 18 19 DoclD026292 Rev 1 AN4885 Please Read Carefully Information in this document is provided solely in connection wi
5. dimmer is active for decreasing the LED brightness and LD6 blue is turned ON When the brightness reaches the minimum value then LD6 toggles to show minimum brightness achieved To exit the manual dimmer quickly press the user button and the application returns to high brightness LED OFF state The figure below describes and summarizes all possible sequences DoclD026292 Rev 1 11 19 Application description AN4885 1 4 7 12 19 Figure 8 Application state diagram User short press User long press Automatic Dimmer Manual Dimmer Up Down LED Brightness MS33853V1 Triangle waveform generation The triangle waveform generation is an example performed in parallel of the high brightness LED demonstration that uses features of the high resolution timer Anyway there is no interaction between these two examples except that the triangle waveform generation can be observed at a specific state of the execution software If the demonstration software remains in STATE OFF a triangular waveform is generated on TP3 The difference between the positive slope and the negative slope of the triangle aims to show the benefit of the HRTIM high resolution refer to Figure 9 In this example the objective is to generate a triangular waveform with a frequency of 25Hz and an amplitude of around 600mV from Vmin 400mV to Vmax 1V The HRTIM channel D1 is programmed to output a 2MHz PWM on the STM32F334xx PB14 port This PWM is
6. interrupt and defines if an extra pulse must be applied to the buck converter or not The compare value of the HRTIM burst mode controller is updated accordingly These data are rolling circularly every 8 burst periods to cover one full dither sequence For instance the dither pattern to apply is 10100100 Then each data rolls at every interrupt 1 gt 0 gt 1 gt 0 gt 0 gt 1 gt 0 gt 0 and loops back to the first value 1 gt 0 gt 1 gt etc A pattern duration or dither period is then 20 ms DocID026292 Rev 1 15 19 Ordering and product information AN4885 3 3 1 16 19 Ordering and product information The STM32F334 Discovery kit is a low cost and easy to use development kit to quickly evaluate and start a development with an STM32F3 series microcontroller Before installing and using the product please accept the Evaluation Product License Agreement from http www st com epla For more information on the STM32F334 Discovery board and for demonstration software visit www st com stm32f3discovery Order code To order the STM32F334 Discovery kit use the user code STM32F3348 DISCO DoclD026292 Rev 1 yy AN4885 4 3 Conclusion Conclusion LED lighting applications are overtaking classic incandescent or halogen lamp solutions as they require less and less energy with constantly increasing performance For indoor as well as outdoor lighting solutions they re now frequently used with full range dimming
7. of the LED is proportional with the increase in forward conduction current As soon as the comparator threshold has been reached the high brightness LED current is regulated at the average value corresponding to Ep typical value The dimming is performed at constant current CC also called current mode control CMC and the LED is directly driven by the power supply This technique prevents for any DoclD026292 Rev 1 7 19 Application description AN4885 change in color temperature that can be observed with variations in the LED forward current and is a key point for dimming applications Figure 4 LED dimming signals generation HRTIM output F burst 400Hz PB12 A COMP input V RsENSE l LED A Lep typ MS33849V1 1 4 3 Anti flicker with dithering techniques In this demonstration an automatic dimmer mode is available where the high brightness LED is driven from the minimum brightness till its maximum This allows scanning the entire brightness range of the LED This mode is performed by using some advanced techniques such as the dither sequences As the high brightness LED is driven using a current mode control the PWM duty cycle is controlled automatically so that the high resolution timer output and consequently the LED itself shuts down only since the LED current level goes over the internal comparator threshold As soon as the high brightness LED is turned ON the high resolution timer output remains high
8. until the necessary LED current is achieved If the high brightness LED must be controlled with the minimum luminous flux only 1 pulse is applied and repeated each 2 5ms t 1 400 at every burst period However this only pulse can be perceived by the human eye and the minimum amount of brightness cannot be reduced in that case Therefore a method has been introduced in this demonstration that makes use of the dither generation scheme Instead of setting ON the high brightness LED at each burst sequence the command can vary according to the amount of desired luminous flux and add some extra ON states during the periodic LED ignitions This technique consists of different sequences and patterns that are scheduled in each of the 8 repetitive burst periods This design flexibility can be achieved with the features of the 8 19 DocIDO26292 Rev 1 yy AN4885 1 4 4 q HRTIM output Application description embedded high resolution timer of STM32F334xx device The diagram below shows how the brightness can be set gradually according to different dither patterns This method is used for the lowest brightness range but also during the automatic dimming mode In this first operating range this allows dimming for the LED at the minimum brightness and for all other cases to eliminate any flickering effect during the fast dimming of the LED where transitions can be visible The burst frequency is 400Hz If the dither patterns are repeated for
9. ED much more efficiently By looking at the yellow first pulse emitted during a single burst period it takes some time t2 for the high brightness LED current to achieve its target value as it first starts from zero This time t2 is relatively longer than the high resolution timer base time and can be evaluated approximately to 2 or 3 timer periods maximum This means that if the LED has to be driven with only one single pulse its necessary to wait until the current rises up to the regulation value corresponding to t2 time The soft start current regulation shortens the time required to establish the current regulation by progressively setting the current thresholds with the DAC peripheral across the period and triggering the PWM off at start x value For instance for the first 250 burst periods the current threshold values may start from zero and increment until they reach the typical current value The result is a lower amount of global light energy during the start of the LED ON sequence until the thresholds achieve their nominal values corresponding to typical for the high brightness LED forward current This is explained on the figure below 7 DocIDO26292 Rev 1 AN4885 1 4 6 3 Application description Figure 7 Soft start current regulation Burst period Brightness EE Er increases Pd 7 a lt lt t2 start x p MS33852V1 Getting started with the application The ap
10. M generation once the current level is reached through the R SENSE resistor level voltage Figure 3 LED dimming function block diagram 5V or VEXT STM32F3348 MCU a Ig Brightness LED p POWER CONTROL 0 6 6 6 8 6 6 PEST 6 6 8 6 6 6 6 6 6 6 6 6 6 6 6 6 6 MS33877V1 The high resolution timer is set to have a PWM frequency of 250kHz then the high brightness LED is driven for a maximum period of 4 us At the LED ignition this time is lengthened for the first pulse as the current starts flowing When the high resolution timer switches the high brightness LED ON via the power control transistor the forward current increases through the LED until it reaches its maximum value of 250 mA detected on the comparator by the way of R SENSE resistor level The high brightness LED is then turned OFF The LED brightness is adjusted proportionally to the number of pulses delivered by the high resolution timer The dimming function is achieved by alternating pulses of 250mA current controlled 250kHz PWM and sequences with no current the MOSFET is no more driven To prevent any flickering effect that the human eye would be able to detect the driven pulses are repeated in a burst mode at a frequency equal to 400 Hz Figure 4 shows the waveforms output by the high resolution timer and the R SENSE voltage feedback to the microcontroller The current across the high brightness LED is also represented The brightness
11. e kare o eee as 16 4 CONCIUSION ARE ER nen ee N EO 17 5 Revision history ER EE N EE EE N EE N 18 2 19 DocIDO26292 Rev 1 yy AN4885 List of tables List of tables Table 1 Document revision history SE ES EE ss SEE eee eens 18 ky DoclD026292 Rev 1 3 19 List of figures AN4885 List of figures Figure 1 Overview of STM32F334 Discovery Kit EES ESE SE SS ee eee 5 Figure 2 High brightness LED buck converter SES SS SE SE eens 6 Figure 3 LED dimming function block diagram a n naaa aaaea SE ee ees 7 Figure 4 LED dimming signals generation 0 cc eee ees 8 Figure 5 DUNEHNG ieehndies is ss Me oi Be he See eee eee we WE ee a ee ee ee 9 Figure 6 DAC slope compensation aaa aeaea ee eee eens 10 Figure 7 Soft start current regulation 0 Es SE SE Es ES ee eens 11 Figure 8 Application state diagram ESE SS ES ES SS ee Se ee ee ee 12 Figure 9 Triangle waveform ss ss SE es es eee eee ees ee 13 AN4885 1 1 1 Note 1 2 1 3 Application description Application description Hardware required This application uses STM32F334 Discovery kit on board high brightness LED LD7 4 signal LEDs LD3 to LD6 and the User push button B1 No additional components are required This example and its hardware are totally independent of buck boost DC DC converter described in another document Hardware settings of the STM32F334 Discovery kit The Idd j
12. each of the 8 burst periods a new dither sequence can be applied every 20ms and repeated indefinitely for constant current settings This 20ms value is still suitable to prevent any eye flicker perception Figure 5 Dithering techniques Dither sequences through BURST periods VOODOO 9 OG 1 burst perjod 2 5ms 400z l l i 00000000 10000000 10001000 A 10100100 MT A 10101010 A 11011010 l A 1 LI 1 tot 11101110 11111110 MS33850V1 Slope compensation By using a current mode buck converter topology the energy applied into the inductor at every PWM pulse is converted through the R SENSE resistor and fed back to the microcontroller as a current regulation control loop This voltage level is directly connected to one comparator input when the other input is fed by the DAC peripheral which is able to deliver a linear ramp which creates slope compensation Drawback of the current mode convert is subharmonic oscillation that can be observed if the PWM duty cycles applied in the switching frequency are grea
13. filtered externally through a RC filter R32 and C23 to get the 25Hz triangular signal In order to observe the high resolution benefit the positive slope of the triangle is generated without using the high resolution of the HRTIM this corresponds to what could be achieved with the TIM1 timer for example while the negative slope is generated using the high resolution The HRTIM is fed with a 144MHz clock but the analog delays embedded in the HRTIM allow to reach an equivalent clock frequency of 144MHz x 32 4 608Ghz Considering a PWM at 2MHz with a duty cycle range from 0 to 100 this gives 72 levels without the high resolution equivalent to a 6 bit resolution Digital to Analog converter and 2304 levels with the high resolution equivalent to a 11 bit Digital to Analog converter DoclD026292 Rev 1 yy AN4885 Application description In our example as the supply is 3 3V full duty cycle range of the PWM the ramp from 400mV to 1V is generated from 13 steps without the high resolution and 418 steps with the high resolution Figure 9 Triangle waveform File Yertical Tirmebase Trigger Display Cursors Measure Math Analysis Utilities Help rigger 5 00 msidivj Auto BOO m 250 M5 50 EI EL Positive AAAA014 6 23 39 PM DocIDO26292 Rev 1 13 19 q Software description AN4885 2 2 1 Note 14 19 Software description STM32F334xx peripherals used by the application This application example uses the following
14. has been limited by software to 250 mA and an optical cube shaped protection has been placed over the LED Do not override this current limitation and do not remove the optical protection while the LED is operating These precautions must be taken due to the high luminous flux emitted by the high brightness LED Do not expose your eyes to direct light High brightness LED dimming Figure 3 shows the high brightness LED dimming block diagram and the different microcontroller resources used in this application One of the high resolution timer outputs is connected to the power control MOSFET driving the high brightness LED This unit uses an inverter buck converter topology and includes an inductor connected serially between the MOSFET drain pin and the high brightness LED A sense resistor R SENSE is connected to the MOSFET source pin and this voltage level is fed back to the internal comparator COMP4 PBO The electrical scheme is shown in Figure 2 Figure 2 High brightness LED buck converter R SENSE 7 DocIDO26292 Rev 1 AN4885 q Application description This voltage is the image of the high brightness LED forward current The second input of COMP4 is driven by the DAC peripheral that provides the threshold level controlling the LED forward current A DMA channel is used to service all DAC channel DMA requests Finally the comparator output is then connected to the high resolution timer to trigger and turns off the PW
15. plication needs to be connected to a PC thanks to a mini B USB connector on the STM32F334 Discovery kit side As soon as the application is power supplied all signal LEDs LD3 to LD6 are lit one by one and the high brightness LED is not controlled OFF state In that state the triangle waveform generation is performed see the description below As soon as the user button B1 is pressed the application enters automatic dimmer mode that covers the entire brightness range with the features explained above The red LED LD3 is ON when the brightness increases whereas the blue LED LD6 is ON when the brightness decreases If the user button B1 is pressed again then the application enters flash mode Leds LD4 and LD5 orange and green are toggling at the flash rate The high brightness LED is switched ON and OFF at the frequency of 10Hz and the brightness is set at its maximum By pressing again B1 the application comes back to the OFF state with the high brightness LED OFF and LD3 to LD6 periodically lit At any time the user can switch the application into manual dimmer where the LED brightness can be set manually by maintaining the user button pressed for more than one second The LED brightness first starts to increase from zero to maximum when LD3 red is turned ON When the brightness reaches the maximum value then LD3 toggles to show maximum brightness achieved If a second and long push onto the user button is performed then the manual
16. rightness LED dimming demonstration software This example comes preloaded with your STM32F334 Discovery kit Reference documents e Getting started with STM32F334 Discovery kit UM1733 e Discovery kit for STM32F334 microcontrollers DB2343 e Discovery kit for STM32F3 series with STM32F334C8 MCU User Manual UM1735 The above documents are available from STMicroelectronics web site http www st com June 2014 DocID026292 Rev 1 1 19 www st com Contents AN4885 Contents 1 Application description 0 0 cc ees 5 1 1 Hardware required SS SS ES ee ee ee eee 5 1 2 Hardware settings of the STM32F334 Discovery kit 5 1 3 Application schematics EES a ee ee 5 1 4 Application principles EES ESE ee se ee 6 1 4 1 OVEIVICW mo area od na BETER Gn ROER AD ED eN Be A 6 1 4 2 High brightness LED dimming 0 0 00 cece eee 6 1 4 3 Anti flicker with dithering techniques 0 00000 cee eens 8 1 4 4 Slope compensation ESE SE SE SS eee 9 1 4 5 High brightness LED current regulation soft start 10 1 4 6 Getting started with the application 0 0 0 0 00 ce eee 11 1 4 7 Triangle waveform generation aaa aaaea eee 12 2 Software description 0 ce es 14 2 1 STM32F334xx peripherals used by the application 14 3 Ordering and product information ss EE EE EE eee 16 3 1 IIE COG AA ES eee shee ne eee eae aw a
17. stralia Belgium Brazil Canada China Czech Republic Finland France Germany Hong Kong India Israel Italy Japan Malaysia Malta Morocco Philippines Singapore Spain Sweden Switzerland United Kingdom United States of America www st com ky DoclD026292 Rev 1 19 19
18. ter than 50 per cent Indeed the waveform expected for the steady state of the inductor current is perfectly triangular Actually the necessary duty cycle controlled by the PWM to energize the inductor can exceed 50 per cent A way to limit subharmonic oscillation is then to introduce slope compensation using the DAC peripheral here dedicated to this task The DAC output signal DoclD026292 Rev 1 9 19 Application description AN4885 1 4 5 10 19 delivers a linear ramp set synchronously with the high resolution timer events at different scheduled times These points are represented on the figure below as repetition period event REP and compare events CMP1 to CMP4 By injecting slope compensation in the current loop the stability is recovered for all values of the duty cycles The subharmonic oscillation may result in a PWM switching frequency degradation that can reach one half of the initial switching frequency and would interfere with LED current regulation Figure 6 DAC slope compensation Expected Inductor Current Inductor Current Real Inductor Current DAC slope compensation current PWM periods MS33851V1 High brightness LED current regulation soft start Another technique implemented in this automatic dimmer demonstration is to start and stop the LED with a progressive current threshold adjustment in the regulation control loop This allows control of the LED with the very minimum forward current thereby dimming the L
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20. to 4 DMA mode is circular and the values are automatically reloaded DAC DAC Channel 1 is configured as a 12 bit digital to analog converter Data from CurrentSense labl array are converted to deliver a variable voltage to the comparator 4 input COMP4 The comparator is used to make the comparison between the voltage present on the R SENSE resistor image of the high brightness LED current and the current thresholds set by the DAC peripheral in slope compensation The comparator COMP4 inverting input is connected to DAC1_CH1 while the COMP4 non inverting input is connected to PBO where R SENSE resistor is present The comparator output is not used externally but rerouted as an external event source 2 to the HRTIM high resolution timer The output polarity of COMP4 is not inverted 7 DocIDO26292 Rev 1 AN4885 Software description 3 HRTIM High resolution timer The high resolution timer is at the center of this application example as it has many interactions with other peripherals HRTIM configuration sets PB12 as HRTIM Timer C output This pin is configured as an alternate function A HRTIM DLL calibration is performed at the beginning of HRTIM initialization function The HRTIM Timer C period is set to 4us 250kHz This imposes the switching frequency of the current mode controlled buck converter The Timer C output 1 is configured to be set on the Timer C counter period event and to be reset on the HRTIM external e
21. umper JP3 must be fitted and all SBx solder bridges set in their initial factory configuration See STM32F334 Discovery kit user manual for further information related to hardware Application schematics Figure 1 shows the description of STM32F334 Discovery kit hardware Figure 1 Overview of STM32F334 Discovery kit Push buttons User Reset High brightness LED MS33847V1 DoclD026292 Rev 1 5 19 Application description AN4885 1 4 1 4 1 Note 1 4 2 6 19 Application principles Overview The STM32F334 Discovery kit embeds the minimum components required to implement the LED dimming function All the other needed functions are managed by the STM32F334xx microcontroller thanks to its embedded features the LED current sense measurement and comparison with the target current is done with the internal comparator The PWM generation to drive the power MOSFET and the current mode control is managed by internal high resolution timer There are several modes available driving the high brightness LED e Automatic dimmer This mode scans the entire brightness range e Flash mode The high brightness LED is turned ON with a 10Hz frequency at full power ton 20 ms torr 80 ms e Manual dimmer The brightness can be adjusted manually by continuing to hold down push button B1 and it can be increased or decreased successively For safety reasons the maximum high brightness LED forward current
22. vent number 2 which is configured to be connected to the comparator COMP4 output There are 5 main events used to interact with DMA and DAC that are REP repetition period and CMP1 to CMP4 events compare1 to compare 4 REP event is aligned with Timer C period event and the other CMP1 to CMP4 events are distributed respectively at 20 period 40 period 60 period and 80 period The DMA transactions are then synchronized with high resolution timer The HRTIM operates in continuous burst mode This means that one or more pulses are output followed by an idle period The burst mode frequency is set to 400Hz according to the eye flicker detection and the different dimming techniques detailed above Interrupts Some interrupts are managed in this application example as described below e Systick Handler Based on the system tick timer this 1ms interrupt manages the delays used by the software e EXTIO IROHandler This interrupt detects any activity present on the user push button B1 and determine if this key is pressed for a short or long time Following the cases statements the application will access different menus Each time the interrupt is serviced the different modes are changed accordingly as described in Figure 8 e HRTIM1_ Master IROHandler Interrupt manages the extra pulses determined by the dither sequences and occurs each burst period t 2 5ms A different set of data included in the dither table is scanned at each
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