USER GUIDE AND DESIGN GUIDELINE Dimmable MR16 Development Kit Atmel ATtiny24A Introduction The MR16 Development Kit demonstrates a dimmable MR16 LED bulb driver module. The board uses an ES version of the Atmel® ATtiny24A to decode dimming information from leading/trailing edge dimmer and uses it to change LED brightness accordingly. The ATtiny24A incorporates a two-stage topology consisting of a boost converter followed by a constant current buck converter to power the LEDs. The board is designed to induce halogen lamp electronic transformers to fire and operate reliably at 10W. The board is designed to work from the following power sources: • • • 10 to 15VDC input capable of producing 15W or more 11 to 12VAC 50/60Hz magnetic transformer capable of producing 15W or more 11 to 12VAC, 20kHz to 100kHz electronic transformer with 50/60Hz envelope, capable of producing 13W or more Features • • • • • • • Compatible with 12VAC electronic transformers from leading suppliers *) Compatible with leading and trailing edge dimmers from leading suppliers *) Supports DC, magnetic transformer and electronic transformer inputs 10% to 100% dimming performance Up to 80% efficiency Preconfigured to generate 10W output for four LEDs Scalable power rating and LED configuration Figure 1. MR16 Development Kit. *) Compatibility varies for ETs and dimmers and needs to be verified. 42137A−LED−12/2013 Table of Contents 1. Kit Contents ....................................................................................... 3 2. Connecting and Operating the MR16 Evaluation Board ..................... 4 3. Block Diagram.................................................................................... 5 4. Schematic .......................................................................................... 6 5. Design Equations ............................................................................... 7 5.1 5.2 Buck Stage........................................................................................................ 7 5.1.2 Buck Sense Resistor Reference Voltage ............................................ 9 Boost Stage ...................................................................................................... 9 5.2.1 Value of VBUS and CBUS ....................................................................... 9 5.2.2 Boost Inductor................................................................................... 10 5.2.3 Damping and Noise Rejection Network ............................................ 10 5.2.4 Boost Current-Sense Resistor .......................................................... 11 5.2.5 Boost Switching Frequency .............................................................. 11 5.2.6 Low conduction angle dimming behavior .......................................... 11 6. Performance Characterization for Four LEDs 10W Solution ............. 13 6.1 6.2 DC INPUT ....................................................................................................... 13 AC INPUT ....................................................................................................... 15 7. HATCH RS12-60M ELECTRONIC TRANSFORMER INPUT WITH LUTRON DIVA DVELV-300P DIMMER............................................ 18 7.1 7.2 7.3 Non-dimmed Waveforms ................................................................................ 18 50% Dimmed Waveforms ............................................................................... 20 Fully Dimmed Waveforms ............................................................................... 22 8. Bill of Material .................................................................................. 24 9. Revision History ............................................................................... 26 Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 2 1. Kit Contents • • • • MR16 main development and evaluation board LED load board with heat-sink attached (+ LEDs are in series on the load board) LED load board connection wires The following items are required but not included in the kit • A compatible power source – maybe 12VDC, 12V magnetic transformer or 12V electronic transformer (recommendation: Hatch RS12-80MGN) • Leading or trailing edge dimmer (recommendation: Lutron Diva DVELV-300P) Figure 1-1. Input and Output Connectors. VLED+ VLED- VIN Note: The ATtiny24A functions as the LED driver IC and comes pre-coded with firmware to perform the power control, dimming control, and housekeeping functions. The firmware is available upon request by signing a royality-free licensing agreement. Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 3 2. Connecting and Operating the MR16 Evaluation Board a) b) c) Attach wires between output connector X2 on the main board to connector J1 on the LED load board. 1. Ensure the heat-sink is attached to the back of the LED load board. 2. If the heat-sink has fallen off in shipping, reattach it to the back of the LED load board by apply gentle and even pressure. Connect the input connector X1 to your power source. The board is designed to work from any of the following power sources: 1. 10 to 15VDC input capable of producing 15W or more. 2. 11 to 12VAC 50/60Hz magnetic transformer capable of producing 15W or more. 3. 11 to 12VAC 40kHz electronic transformer with 50/60Hz envelope, capable of producing 15W or more. To dim the MR16 bulb, connect an AC dimmer compatible to your chosen transformer prior to the transformer shown in Figure 2-1. Refer to the electronic transformer manufacturer’s datasheet for the dimmer compatibility. Specific dimmer and transformer combinations might require higher load. For such combinations more than one lamp will be needed for stable operation. Figure 2-1. Connection Diagram. Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 4 3. Block Diagram Figure 3-1. Circuit Block Diagram. MR16 Evaluation Board ATtiny24A-MU Figure 3-1 shows the block diagram of the scheme used in the implementation of the MR16 lamp. The input voltage is processed by the boost converter. A digital block inside the Atmel ATtiny24A implements the PI controller transfer function and adjusts the conditions in the circuit such that the desired boost bus voltage is achieved. A constant off-time floating buck topology is used. The floating buck is advantageous in cases where the load (the LEDs) does not have to be grounded, because it uses a low-side MOSFET, which is easy to drive. Constant off-time control allows for accurate control of the LED current and is simple to implement. The average LED and inductor currents are equal. Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 5 4. Schematic Figure 4-1. Schematic. Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 6 5. Design Equations 5.1 Buck Stage The MR16 evaluation boards consist of boost converter followed by a constant current floating buck power stage as shown in Figure 4-1. It is a constant off-time floating buck topology. The output bus voltage of the boost section acts as input to the buck converter to regulate the LED string current. The buck is operated in the peak-current control mode with buck inductor current in continuous conduction mode. There is an internal delay inside the controller in detecting the buck inductor peak-current and it restricts the minimum on time TonMIN of the MOSFET Q2 to 2µs. With the MOSFET off time having a fixed value of Toff, it follows that the maximum switching frequency for the buck converter is, Note: 𝑓𝑏𝑢𝑐𝑘𝑀𝐴𝑋 = 1 = 200 𝑘𝐻𝑧 𝑇𝑜𝑛𝑀𝐼𝑁 + 𝑇𝑜𝑓𝑓 Constant off-time operation of the buck converter is particularly attractive for LED drivers because the ripple current, and hence the average LED current, is insensitive to the changes in the buck stage input voltage, as long as the LED voltage is relatively constant. The switching frequency adjusts to keep the ripple and average current constant as the input voltage varies. Figure 5-1. Buck Stage Plots. Continuous conduction operation is assured when the peak-to-peak ripple current in the inductor, ∆iL, is less than twice the average LED current, i.e., where IAVE is the average LED string current. ∆𝑖𝐿 ≤ 2 𝐼𝐴𝑉𝐸 Assuming the valley buck inductor current = IV then the average LED string current or average buck inductor current IAVE can be written as: 𝐼𝐴𝑉𝐸 = 𝐼𝑃𝐸𝐴𝐾 + 𝐼𝑉 2 If the fraction of average buck inductor current allowed as ripple in the buck inductor is β (a design parameter). Then ∆𝑖𝐿 = 𝛽𝐼𝐴𝑉𝐸 Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 7 Figure 5-2. Current in the Presence of the Comparator Propagation Delay and Digital Control Loop Delay. Calculations are needed to accurately determine the value of sense resistor for the buck MOSFET. Propagation delays in the current sense comparator and in the control block, results in higher peak buck inductor current. This makes the LED current higher than desired, necessitating an increase in the sense resistor value. If the total propagation delay tdelay from the instant that the inputs of the comparator become equal to when the MOSFET switches-off, is known, and assuming that the input offset voltage of the comparator is negligible in comparison with the comparator’s reference voltage, the correct sense resistor value can be computed as follows: The total peak-to-peak ripple current ∆iL in the inductor is set by the duty ratio D ∆𝑖𝐿 = ∆𝑖𝐿_𝑖𝑑𝑒𝑎𝑙 + ∆𝑖𝐿_𝑑𝑒𝑙𝑎𝑦 = (𝑉𝐵𝑈𝑆 − 𝑉𝐿𝐸𝐷 )𝐷 𝐿2 𝑓𝑏𝑢𝑐𝑘 From Figure 5-2 two equations can be written to express the starting current and, 𝐼𝑠 = 𝐼𝑅𝐸𝐹 − = VLED VBUS . Is (𝑉𝐵𝑈𝑆 − 𝑉𝐿𝐸𝐷 ) 𝐷 𝑉𝑅𝐸𝐹 (𝑉𝐵𝑈𝑆 − 𝑉𝐿𝐸𝐷 ) 𝐷 � − 𝑡𝑑𝑒𝑙𝑎𝑦 � = − � − 𝑡𝑑𝑒𝑙𝑎𝑦 � 𝑅11 𝑓𝑏𝑢𝑐𝑘 𝑓𝑏𝑢𝑐𝑘 𝐿2 𝐿2 𝐼𝑠 = 𝐼𝐴𝑉𝐸 − where IAVE is the desired LED average current. (𝑉𝐵𝑈𝑆 − 𝑉𝐿𝐸𝐷 ) 𝐷 � � 𝐿2 2𝑓𝑏𝑢𝑐𝑘 Equating these two expressions gives: 𝑅11 = 𝐼𝐴𝑉𝐸 𝑉𝑅𝐸𝐹 (𝑉𝐵𝑈𝑆 − 𝑉𝐿𝐸𝐷 ) 𝐷 + � − 𝑡𝑑𝑒𝑙𝑎𝑦 � 𝐿2 2𝑓𝑏𝑢𝑐𝑘 This equation is accurate if the input to the system is DC and the bus voltage is ripple-free. In the case of ac-input the ripple current in the inductor is dependent on the bus ripple voltage. For the AC input, R11 will need to be increased slightly and it can be achieved empirically. Now we need to have a second design equation to decide the inductance of the buck inductor. Equation for the MOSFET in off-state can be written as: 𝐿2 = 𝑉𝐿𝐸𝐷 = 𝐿 ∆𝑖𝐿 𝑇𝑜𝑓𝑓 𝑉𝐿𝐸𝐷 𝑇𝑜𝑓𝑓 𝑉𝐿𝐸𝐷 𝑇𝑜𝑓𝑓 = ∆𝑖𝐿 𝛽𝐼𝐴𝑉𝐸 Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 8 Finally, two buck-stage design equations can be summarized as listed below. 𝑅11 = 𝐼𝐴𝑉𝐸 𝐿2 = 𝑉𝑅𝐸𝐹 (𝑉 − 𝑉𝐿𝐸𝐷 ) 𝐷 + 𝐵𝑈𝑆 � − 𝑡𝑑𝑒𝑙𝑎𝑦 � 𝐿2 2𝑓𝑏𝑢𝑐𝑘 𝑉𝐿𝐸𝐷 𝑇𝑜𝑓𝑓 𝛽𝐼𝐴𝑉𝐸 , Where Toff = 3µs. Buck inductor saturation current should be more than twice the peak inductor current IPEAK flowing through the inductor in order to avoid any un-intentional core saturation because of the current overshoots or overtemperature. 5.1.2 5.2 5.2.1 Buck Sense Resistor Reference Voltage The reference voltage is compared to that across sense resistor R11 to determine when to turn off the buck FET Q2. If this voltage is too low, it becomes comparable to the input offset voltage of the comparator and current sensing accuracy suffers. If it is too high, there is excessive dissipation in the sense resistor R11, and the voltage across this resistor subtracts from the gate drive voltage of Q2. The reference voltage corresponding to peak current should be set between 200 and 500mV. The reference voltage is generated by the dimming decoding circuit in the controller. A PWM signal with amplitude of 5V and a duty ratio proportional to the dimming level required is output at pin 5 of the controller. When no dimming is required the output is steady at VCC = 5V and the peak reference voltage is given by, 𝑅19 (𝑉𝑏𝑢𝑐𝑘𝐹𝐵 )𝑃𝐸𝐴𝐾 = × 5𝑉 = (𝐼𝑏𝑢𝑐𝑘 )𝑃𝐸𝐴𝐾 × 𝑅11 = 200𝑚𝑉 𝑡𝑜 500𝑚𝑉 𝑅17 + 𝑅19 Boost Stage Value of VBUS and CBUS Let us consider switching frequency of the buck converter is fBuck = 150kHz. Hence, 𝑇𝑂𝑁 + 𝑇𝑂𝐹𝐹 = 1000 𝑓𝐵𝑢𝑐𝑘 , where TON and TOFF are in µs and the value of TOFF is 3µs. Therefore, the value of TON is given by the following equation. 𝑇𝑂𝑁 = � 1000 − 𝑇𝑂𝐹𝐹 � 𝜇𝑠 𝑓𝐵𝑢𝑐𝑘 In addition, the buck MOSFET on-time duration equation can be written as, Hence, 𝑉𝐵𝑈𝑆 − 𝑉𝐿𝐸𝐷 = 𝐿2 𝐼𝑃𝐸𝐴𝐾 − 𝐼𝑉 𝑇𝑂𝑁 𝑉𝐵𝑈𝑆 = 𝑉𝐿𝐸𝐷 + 𝐿2 𝛽𝐼𝐴𝑉𝐸 𝑇𝑂𝑁 Potential divider resistors need to be sized correctly in order to achieve the desired boost stage bus voltage. A digital PI controller implements the feedback transfer function, adjusting the conditions in the circuit such that the output voltage of the gain stage sits at a voltage equal to the digital reference number for the ADC, nominally 2.56V. Values of the potential divider resistors R4 and R9 for the bus voltage can be selected using following equation. 𝑉𝐵𝑈𝑆 × 𝑅9 = 2.56 𝑉 𝑅4 + 𝑅9 Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 9 The bus voltage has slightly different average values for DC and for AC operation. During AC operation the bus voltage has a significant ripple across it and is sampled once in each line half cycle. Its DC value will depend on what point it is sampled at in the ripple cycle. The difference in the averaged values is of little consequence because the buck converter corrects for it and gives the right LED current. Because of the limited space in MR16 application, the largest through-hole boost converter electrolytic capacitor package that will fit in this fashion in the MR16 form factor is 10mm diameter by 12mm long (or 0.4in diameter by 0.5in long). The bus voltage capacitor should be as large as possible, because this minimizes the capacitor’s physical size. The largest capacitor value in this size should be used. For example, in four LED, 10W design, the best design becomes one where VBUS = 24V for VLED = 14V (for four LEDs). Then for PLED = 10W the best capacitor value is CBUS = 220µF, 35V. The input current is shaped to have a constant value during all times while the input voltage is being applied to the circuit. For the current shape the peak-to-peak ripple voltage with a non-dimmed magnetic transformer is 5V. If the circuit were power factor corrected then the ripplevoltage would be 8V. In both cases the ripple voltage is inversely proportional to the bus capacitance for a fixed bus voltage. The bus voltage should not be lowered below about 20V even if less than four LEDs are powered. Otherwise, the bus capacitor can become too large or the ripple will be too large, unless the attendant problems are acceptable. If more than four LEDs are used a good rule of thumb is to make the bus voltage approximately 1.5 to 2 times greater than the LED voltage. If the power level is lowered below 10W, these cautions can be relaxed, since the relative ripple voltage in the bus capacitor is less. 5.2.2 Boost Inductor The boost inductor value is determined by the need to fit in the MR16 form factor, and to ensure boost continuous conduction (CCM) operation. The largest practical inductor value is 10mm x 10mm. For reasons related to the need to ensure proper firing of electronic transformers under all dimming conditions, the worst operating condition for the inductor is going to be observed with 12VDC input. If the boost current can be kept continuous with the DC input, it will be continuous with the AC input as well. The switching frequency of the boost converter was picked to be 350kHz so that the design could fit in the space available. Then it becomes easy to pick a minimum inductor value to ensure CCM operation. The minimum inductor value LBST_MIN which will give critical conduction operation, where the peak inductor current is twice the average input current. Ignoring power losses, the peak inductor current ∆iL_BST in critical conduction is, ∆iL _ BST =2 I IN =2 PLED VIN The duty ratio of the boost converter is, Hence the minimum inductor value is, 𝐷𝐵𝑆𝑇 = 1 − 𝐿𝐵𝑆𝑇_𝑀𝐼𝑁 = 𝑉𝐼𝑁 𝑉𝐵𝑈𝑆 𝑉𝐼𝑁 𝐷𝐵𝑆𝑇 ∆𝑖𝐿_𝐵𝑆𝑇 𝑓𝐵𝑆𝑇 A good operating point is with ripple current equal to input current, that is, with half the ripple current obtained with critical conduction. This gives adequate operating margin. 5.2.3 Damping and Noise Rejection Network Capacitor C9 and resistor R9 constitute a damping and noise reduction network. The capacitor is required to provide energy storage at the switching frequency (20kHz – 100kHz) of the electronic transformer. Without it the input voltage to the boost is a high frequency square wave with a line frequency sinusoidal envelope and is extremely noisy. The resistor is added to damp the tendency of some electronic transformers to oscillate waywardly with the input of the M16. The damper is extremely effective. Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 10 5.2.4 Boost Current-Sense Resistor The boost sense resistor R2 is picked to ensure continuous conduction operation, but also to minimize power dissipation. It is determined by the maximum peak input current and power dissipation desired. Referring to the schematic, the maximum reference (or current limit) voltage to which the peak current in R2 is compared, is given by, 𝑅10 𝑉𝐼_𝑙𝑖𝑚 = 𝑉𝐶𝐶 𝑅8 Where VCC = 5V is the supply voltage of the controller. The sense resistor is given by dividing this voltage by the maximum peak current at current limit. 5.2.5 Boost Switching Frequency In theory the boost converter’s switching frequency is supposed to be determined by the inductor, sense resistor and the components in the circuit around the comparator U4. However, the comparator used for U4 is a generic one, and has a long response time in the 1µs range. This reaction time significantly lowers the switching frequency. Including the effects of comparator response time and with respect to the components on the schematic, the peak-to-peak ripple current in the input inductor is given by, 𝑅12 𝑅10 𝑉𝐼𝑁 (𝑉𝐵𝑈𝑆− 𝑉𝐼𝑁 ) ∆𝑖𝐿 = 𝑉𝐶𝐶 �1 + �+ 𝑡𝑟_𝑜𝑛_𝑜𝑓𝑓 + 𝑡𝑟_𝑜𝑓𝑓_𝑜𝑛 𝑅2 (𝑅7 + 𝑅12 ) 𝑅8 𝐿1 𝐿1 where tr_on_off and tr_off_on are respectively the comparator response times when MOSFET Q1 is turning off and turning on, and are usually about the same and VCC = 5V is the supply voltage to the comparator. Therefore, the switching frequency is given by, 𝑓𝑏𝑠𝑡 = 1 𝑉𝐼𝑁 � � (𝑉𝐵𝑈𝑆 − 𝑉𝐼𝑁 ) 𝐿1 ∆𝑖𝐿 𝑉𝐵𝑈𝑆 The switching frequency for a DC input voltage of 12V and a bus voltage of 24V is about 350kHz, which allows the circuit to fit in the MR16 form factor at a good efficiency. The switching frequency will vary with the line for the AC input. Note: Negative (or cathode pin) of the LED string is floating and not connected to the ground of the driver. Therefore, make sure that the scope is isolated before taking voltage measurement of the LED string. Otherwise, use a differential voltage probe to measure the LED string voltage. 5.2.6 Low conduction angle dimming behavior If the circuit is dimmed to an input voltage conduction angle of about 10-20 degrees or less, the LEDs can flicker to a significant degree for the following reason. At such low angles the LED current is dominated by its ripple component, because the dc part has become very small. In this development kit a circuit has been implemented to turn off the buck converter at the lowest dimming angles, but to leave the LEDs on. This scheme, which we are using and is illustrated by the components Q5 and R3 in Figure 5-3, can be implemented by connecting a resistor from the cathode terminal of the bottom LED to ground through a MOSFET controlled by the MCU. Then, when the buck converter goes off, a constant current determined by the difference between the boost voltage and the LED voltage flows in the resistor and LEDs, shown in the equation below. If the resistor is chosen so that the current in it matches the minimum buck switching current the apparent flash is eliminated. In this scheme is that the power required for loading the electronic transformer and maintaining the proper shape of the input voltage is mostly dissipated in the LEDs. Hence, the resistor can be the same size as the others in the circuit. In the prototype the resistor has a value of about 50Ω and only 50mW power is dissipated in it, allowing it to have a 0603 footprint. Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 11 Figure 5-3. Preload circuit for low conduction angle dimming. Equation: ILED = VBUS – VLED R3 There are other ways of reading low conduction angle flicker. User can modify the circuit and code if necessary. Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 12 6. Performance Characterization for Four LEDs 10W Solution An MR16 prototype board designed to produce 10W was characterized for various input sources. The load was four series-connected LEDs running at a current of 730mA and a voltage of about 13.5V. 6.1 DC INPUT Table 6-1. Efficiency with DC Input. VIN [V] IIN [A] V_LED [V] I_LED [A] PIN [W] POUT [W] Efficiency 12.43 1.08 13.16 0.762 12.52 10.02 0.80 Figure 6-1. Start-up Waveforms: Ch1=input voltage; Ch2=boost input (inductor) current at 1.33A/div.; Ch3=buck_FET current at 1A/div.; Ch4=boost output voltage. Figure 6-1 illustrates that the system starts up gracefully without significant or dangerous voltage or current overshoots. Figure 6-2. Switching Waveforms: Ch1=boost FET gate drive voltage; Ch2=boost input (indicator) current at 1.33A/div.; Ch3=buck FET current at 1A/div.; Ch4=buck FET gate drive voltage. Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 13 Figure 6-3. Switching Waveforms: Ch1=boost FET drain voltage; Ch2=boost inductor current at 1.33A/div.; Ch3=buck FET current at 1A/div.; Ch4=buck FET drain voltage. Figure 6-2 and Figure 6-3 show that the switching waveforms are clean and have the expected shapes and values. Figure 6-4. Boost Converter Load Transient Response: Ch1=output voltage; Load current step=300mA to 350mA. Buck LED driver unloaded. Figure 6-4 illustrates that the digital PI controller in the boost converter confers excellent stability to the feedback loop. Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 14 Figure 6-5. Boost Converter Waveforms: Ch1=sensed input voltage at microcontroller pin; Ch2=sensed boost output voltage at microcontroller pin; Ch3=output of microcontroller PWM for boost at pin 7; Ch4=low pass filtered microcontroller PWM across C16. Figure 6-5 shows the output of the PWM controller and low-pass filtered analog output. 6.2 AC INPUT Figure 6-6. Start-up Waveforms: Ch1=rectified input voltage; Ch2=boost input (inductor) current at 1.33A/div; Ch3=buck FET current at 1A/div.; Ch4=boost output voltage. Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 15 Figure 6-7. Detail of Start-Up Waveforms: Ch1=rectified input voltage; Ch2=boost input (inductor) current at 1.33A/div; Ch3=buck FET current at 1A/div.; Ch4=boost output voltage. Figure 6-8. Finer Detail of Start-Up Waveforms: Ch1=rectified input voltage; Ch2=boost input (inductor) current at 1.33A/div; Ch3=buck FET current at 1A/div.; Ch4=boost output voltage. These figures illustrate that even with an AC input voltage, the circuit again starts up gracefully and safely without voltage or current overshoots. Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 16 Figure 6-9. Switching Waveforms: Ch1=rectified input voltage; Ch2=boost input (inductor) current at 1.33A/div.; Ch3=buck FET current at 1A/div.; Ch4=boost output voltage. Ideally the buck current should be insensitive to the boost output voltage ripple. In practice, because of finite propagation delays in the LED current comparison comparator, the peak current tends to follow the peak boost voltage somewhat. If this dependency is unacceptable, a faster current comparator can be used. Figure 6-10. Boost Converter Load Transient Response: Ch4=boost output voltage. Boost output current step 300mA to 350mA with LEDs disconnected. Figure 6-10 shows that the load transient response of the boost converter remains excellent with an ac input voltage. Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 17 7. HATCH RS12-60M ELECTRONIC TRANSFORMER INPUT WITH LUTRON DIVA DVELV-300P DIMMER 7.1 Non-dimmed Waveforms Figure 7-1. Undimmed Start-up Waveforms: Ch1=rectified input voltage; Ch2=boost input (inductor) current at 1.33A/div; Ch3=buck FET current at 1A/div.; Ch4=boost output voltage. This graph again shows excellent system start-up characteristics. Immediately their input voltage is applied, an inrush current control resistor is connected in the input path to limit the current. This resistor is bypassed just before the circuit begins to switch. The input inductor current shows a large inrush current spike. This spike is limited by design to a maximum value of the peak input voltage (always 17V or less) divided by the inrush current resistor (presently set at 2.5Ω), or to less than 6.8A in all cases. Figure 7-2. Detail of Undimmed Start-up Waveforms: Ch1=rectified input voltage; Ch2=boost input (inductor) current at current at 1.33A/div; Ch3=buck FET current at 1A/div.; Ch4=boost output voltage. Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 18 Figure 7-3. Finer Detail of Undimmed Start-up Waveforms: Ch1=rectified input voltage; Ch2=boost input (inductor) current at current at 1.33A/div; Ch3=buck FET current at 1A/div.; Ch4=boost output voltage. Figure 7-4. Undimmed Switching Waveforms: Ch1=rectified input voltage; Ch2=boost input (inductor) current at 1.33A/div.; Ch3=buck FET current at 1A/div.; Ch4=boost output voltage. This graph shows that even with the very dirty input voltage from a high frequency AC electronic transformer, the LED current is still well-regulated. The disturbances in the inductor current (blue) waveforms are not real, but are oscilloscope artifacts, as one zooms into the waveform, as Figure 7-5 shows. Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 19 Figure 7-5. Detail of Inductor Current Waveform (Ch2) at 1.33A/div. 7.2 50% Dimmed Waveforms Figure 7-6. Start-up Waveforms With 50% Dimming: Ch1=rectified input voltage; Ch2=boost input (inductor) current at 1.33A/div; Ch3=buck FET current at 1A/div.; Ch4=boost output voltage. Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 20 Figure 7-7. Detail of Start-up Waveforms With 50% Dimming: Ch1=rectified input voltage; Ch2=boost input (inductor) current at 1.33A/div; Ch3=buck FET current at 1A/div.; Ch4=boost output voltage. Figure 7-8. Finer Detail of Start-up Waveforms With 50% Dimming: Ch1=rectified input voltage; Ch2=boost input (inductor) current at 1.33A/div; Ch3=buck FET current at 1A/div.; Ch4=boost output voltage. Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 21 Figure 7-9. Switching Waveforms With 50% Dimming: Ch1=rectified input voltage; Ch2=boost input (inductor) current at 1.33A/div.; Ch3=buck FET current at 1A/div.; Ch4=boost output voltage. 7.3 Fully Dimmed Waveforms Figure 7-10. Fully Dimmed Start-up Waveforms: Ch1=rectified input voltage; Ch2=boost input (inductor) current at 1.33A/div; Ch3=buck FET current at 1A/div.; Ch4=boost output voltage. Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 22 Figure 7-11. Fully Dimmed Switching Waveforms: Ch1=rectified input voltage; Ch2=boost input (inductor) current at 1.33A/div; Ch3=buck FET current at 1A/div.; Ch4=boost output voltage. Figure 7-12. Detail of Fully Dimmed Switching Waveforms: Ch1=rectified input voltage; Ch2=boost input (inductor) current at 1.33A/div; Ch3=buck FET current at 1A/div.; Ch4=boost output voltage. Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 23 8. Bill of Material Bill of Material for Four LEDs and 10W Design Table 8-1. Bill of Material. Part Value Package Description Part # Manufacturer C1 100nF C0603 Ceramic capacitor C3 OPEN C2220K Ceramic capacitor C5 2.2µF/25V C0603 Ceramic capacitor TMK107ABJ225KA-T Taiyo Yuden C6 220µF/35V E5-10,5 Electrolytic capacitor ESC227M035AH1AA Kemet C7 47nF C0603 Ceramic capacitor C8 1µF C0603 Ceramic capacitor CC0603KRX5R5BB105 Yageo C9 2.2µF/25V C1210 Ceramic capacitor C3225X7R1E225KT5 TDK C10 100nF C0805K Ceramic capacitor C11 4.7µF/35V 153CLV-0405 Electrolytic capacitor VEJ-4R7M1VTR-0406 Lelon C12 OPEN C2220K Ceramic capacitor C13 100nF C0603 Ceramic capacitor C14 330pF C0603 Ceramic capacitor C15 100pF C0603 Ceramic capacitor C16 470nF C0603 Ceramic capacitor C17 100nF C0603 Ceramic capacitor C18 100nF C0603 Ceramic capacitor C19 10µF C0603 Ceramic capacitor C20 100nF C0603 Ceramic capacitor C21 OPEN C2220K Ceramic capacitor D1 PMEG4030ER SOD-123_MINI-SMA Schottky rectifier PMEG4030ER,115 NXP Semi D2 PMEG4030ER SOD-123_MINI-SMA Schottky rectifier PMEG4030ER,115 NXP Semi D3 PMEG4030ER SOD-123_MINI-SMA Schottky rectifier PMEG4030ER,115 NXP Semi D4 PMEG4030ER SOD-123_MINI-SMA Schottky rectifier PMEG4030ER,115 NXP Semi D5 PMEG4030ER SOD-123_MINI-SMA Schottky rectifier PMEG4030ER,115 NXP Semi D6 PMEG4030ER SOD-123_MINI-SMA Schottky rectifier PMEG4030ER,115 NXP Semi D7 BAT854C SOT23 Dual diode F1 5A slow blow 603 Fuse in 0603 package SF-0603S500-2 Bourns L1 22µH DR74 Shielded inductor DR74-220-R Cooper Bussmann L2 100µH 744 066 Shielded inductor 744 066 101 Würth Q1 Si2318CDS SOT23 N-CHANNEL MOS FET Si2318CDS Vishay Q2 Si2318CDS SOT23 N-CHANNEL MOS FET Si2318CDS Vishay Q3 Si2318CDS SOT23 N-CHANNEL MOS FET Si2318CDS Vishay Q4 MMBTA2222A SOT23-BEC NPN transistor MMBTA2222A Q5 Si2318CDS SOT23 N-CHANNEL MOS FET Si2318CDS Vishay R1 5.11Ω x 2, 1W ea. R2010 Total resistance=2.55Ω CRCW20105R11FKEFHP Vishay Dale R2 0.15Ω/0.5W R1206 Thick film resistor LRC-LR1206LF-01-R150-F TT Electronic/IRC NXP Semi Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 24 R3 49.9Ω R0603 Thick film resistor R4 51.1kΩ R0603 Thick film resistor R5 10kΩ R0603 Thick film resistor R6 1.00kΩ R0603 Thick film resistor R7 100kΩ R0603 Thick film resistor R8 100kΩ R0603 Thick film resistor R9 4.99kΩ R0603 Thick film resistor R10 4.99kΩ R0603 Thick film resistor R11 0.5Ω/0.5W R1210 Thick film resistor R12 499Ω R0603 Thick film resistor R13 51.1kΩ R0603 Thick film resistor R14 10.0kΩ R0603 Thick film resistor R15 1.0kΩ R0603 Thick film resistor R16 10kΩ R0603 Thick film resistor R17 1MΩ R0603 Thick film resistor R19 82.5kΩ R0603 Thick film resistor R20 4.99kΩ R0603 Thick film resistor R21 4.99kΩ R0603 Thick film resistor R22 100kΩ R0603 Thick film resistor R23 1Ω/500mW R1206 Thick film resistor R24 15kΩ R0603 Thick film resistor R25 1.00kΩ R0603 Thick film resistor R26 10.0kΩ R0603 Thick film resistor R27 1kΩ R0603 Thick film resistor R28 0.15Ω/0.5W R1206 Thick film resistor R29 0Ω R0603 Thick film resistor U1 LM3480-5.0 SOT23 30VIN, 100mA, Quasi LDO LM3480-5.0 U2 ATtiny24A SO14 U3 74LVC1G02 SOT23-5 U4 TL331 RL1210FR-070R5L Yageo CRCW12101R00FKEA Vishay Dale LRC-LR1206LF-01-R150-F TT Electronic/IRC National/TI ATtiny24A Atmel 2-input NOR gate 74LVC1G02 TI SOT23-5 Comparator TL331 TI X1 1751248 PCB terminal block 1751248 Phoenix Contact X2 1751248 PCB terminal block 1751248 Phoenix Contact C0603 Ceramic capacitor C1 100nF Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 25 9. Revision History Doc. Rev. Date Comments 42137A 12/2013 Initial document release Dimmable MR16 Development Kit [USER GUIDE AND DESIGN GUIDELINE] 42137A−LED−12/2013 26 Atmel Corporation Atmel Asia Limited Atmel Munich GmbH Atmel Japan G.K. 1600 Technology Drive Unit 01-5 & 16, 19F Business Campus 16F Shin-Osaki Kangyo Building San Jose, CA 95110 BEA Tower, Millennium City 5 Parkring 4 1-6-4 Osaki, Shinagawa-ku USA 418 Kwun Tong Road D-85748 Garching b. Munich Tokyo 141-0032 Tel: (+1)(408) 441-0311 Kwun Tong, Kowloon GERMANY JAPAN Fax: (+1)(408) 487-2600 HONG KONG Tel: (+49) 89-31970-0 Tel: (+81)(3) 6417-0300 www.atmel.com Tel: (+852) 2245-6100 Fax: (+49) 89-3194621 Fax: (+81)(3) 6417-0370 Fax: (+852) 2722-1369 © 2013 Atmel Corporation. All rights reserved. / Rev.: 42137A−LED−12/2013 Atmel®, Atmel logo and combinations thereof, Enabling Unlimited Possibilities®, and others are registered trademarks or trademarks of Atmel Corporation or its subsidiaries. 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