AN374 Application Note Considerations for Light Engine Selection for a CS1630/31 2-Channel TRIAC Dimmable Circuit 1 Overview of the CS1630 The CS1630/31 is a high-performance offline AC to DC LED controller for dimmable and high color rendering index (CRI) LED replacement lamps and luminaires. It features Cirrus Logic's proprietary digital dimmer compatibility control technology and digital correlated color temperature (CCT) control system that enables two-channel LED color mixing.
AN374 IMPORTANT SAFETY INSTRUCTIONS Read and follow all safety instructions prior to using this demonstration board. This Engineering Evaluation Unit or Demonstration Board must only be used for assessing IC performance in a laboratory setting. This product is not intended for any other use or incorporation into products for sale. This product must only be used by qualified technicians or professionals who are trained in the safety procedures associated with the use of demonstration boards.
AN374 TABLE OF CONTENTS 1 OVERVIEW OF THE CS1630 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1 Definition of Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.
AN374 2 Introduction This application note provides a guide to designing a solid-state lighting (SSL) LED lamp circuit using Cirrus Logic's CS1630/31. The first half of the document presents an introduction to the CS1630/31 color control system and the criterion for selecting a compatible light engine. The second half of the document supports the design effort by detailing the requirements to curve fit the polynomial gain equations for a robust color correlation temperature solution. 2.
AN374 2.
AN374 3 Introduction to the Color System The CS1630/31 is a two-channel TRIAC dimmable LED driver IC designed to change the color temperature of the light output by independently varying the gains of the two different color LED strings to establish levels of color mixing. This feature can be used to make the color temperature versus dim characteristics of the light similar to that of an incandescent light bulb.
AN374 4 Light Engine Choices This section examines key design considerations for a complete lamp design using the CS1630/31 two-channel TRIAC dimmable driver IC. It provides some of the appropriate design choices based on a comprehensive understanding of the CS1630/31 driver and digital control algorithms. 4.
AN374 IRed, NTC, and dim variables, the curve fitter will have to compute gains across a larger number of points, which creates the risk of increased error. Failure to compensate for these points may cause a color shift during startup until the LED strings reach thermal equilibrium. Constraint 4: Red currents increase with an increase in ambient temperature The current IRed in the red channel should increase monotonically with ambient temperature at a given dim value.
AN374 Constraint 3: Ratio of peak currents between the two channels must be less than 4 The DAC reference on the current sense comparator has a 1.4V full-scale threshold. There is a minimum peak current IPK(FB) at full brightness threshold that is usually set at approximately 25%, so the theoretical maximum ratio of the peak currents is 5.6. However, a range is required to dim down to the lower dimming settings. Hence a ratio of 1.5 is considered optimum. Any peak ratio beyond 2.
AN374 Step 1) LED string configuration Determine if a series configuration or a parallel configuration is a viable solution for the identified light engine. Figure 2a illustrates a series light configuration. The two LED strings are arranged in series so that current passes through either one or both LED strings. A MOSFET is used to shunt current around one string on alternating switching cycles. In this configuration, one string is required to have a larger current than the other string.
AN374 Bit STRING in register Config3 at address 7 selects the second stage output channel configuration. When bit STRING is set to ‘1’ a series configuration is selected. Figure 3 illustrates the process used to select the second stage flyback mode using the CS163X system design application. Step 2a: Select Series vs. Parallel Step 2b: Select Flyback, Buck, or Tapped Buck Figure 3.
AN374 VB S T VB S T R12 R26 D10 D9 V CC D3 U2 R3 D9 C10 R2 U2 Q3 D R25 GND FBAUX GND 12 CS1630 /31 Q5 IGND FBAUX R22 FBSENSE Q4 13 15 R22 11 R23 R21 GND 12 R23 Channel 2 LED (Red) L3 GD 11 R21 R27 D C16 Q5 15 D11 _ Q C14 IGND FBSENSE Z3 GND Channel 2 LED (Red) L3 GD V CC D3 D11 _ Q C15 13 Channel 1 LED (White) C15 C13 D9 C14 CS1630 /31 D10 Channel 1 LED (White) C8 GND GND Figure 4a. Tapped Buck Series Output Model Figure 4b.
AN374 Figure 5 illustrates the process used to select the second stage flyback mode using the CS1630/31 system design utility. Figure 5. Selecting the Second Stage Mode Using the CS1630/31 System Design Utility The CS1630/31 has a minimum required T1 time that is dependent on leading edge blanking time TLEB. Blanking time TLEB is programmable from 150ns to 800ns and is used to effectively disable the peak current comparator from turning off the gate drive too early due to spurious switching noise.
AN374 In Mode 2 the phase synchronizer MOSFET is turned ‘ON’ and the output power PMODE2 is: P MODE2 = I CH1 – I CH2 V CH1 [Eq. 8] 2. In the parallel configuration, the voltages of the two strings should at least have a 20% difference between the two strings in order for the CS1630/31 to distinguish between the two strings.
AN374 Figure 6 illustrates the current comparison through the inductor. In this case, the buck is chosen to explain the point. The same logic can be extended to the flyback topology as well. Power Transferred in a Single String Controller i(t) T1 T2 T3 T1 T3 t T3CH 2 t T2 TT TT Power Transferred in a Two String Controller with Unequal Balance between Two Strings T1CH1 T2CH 1 T3CH1 T1CH2 TTCH 1 T2CH2 TTCH2 TT Figure 6.
AN374 The following are disclaimers for this analysis: a. This analysis works better for a non-isolated design. For isolated flyback designs, the size may be determined by safety requirements for either triple insulated wire or margin tape spacing. As a result, increased peak current may or may not translate into the size of the inductor easily. b. The explanation above shows a qualitative trend in understanding the tradeoffs of having power delivered to two strings unevenly.
AN374 4.3 Synchronizer Design Considerations The synchronizer circuit drives an external MOSFET, which may be ground referenced depending on the topology. Constraint 1: Non-isolated synchronizer circuit considerations In the case of a non-isolated flyback, the secondary ground of the flyback can be referenced to the primary ground thereby having the least expensive synchronous circuit solution consisting of the synchronous MOSFET and the diode.
AN374 5 Design Flowchart Figure 8 shows the flowchart for eliminating incompatible light engines.
AN374 Figure 9 shows a way for selecting the correct second stage string configuration. Start Not compatible Provide completed light engine design across dim and temperature Yes Voltage Same? Yes Currents cross at any dim? No No Parallel string configuration Voltage same? Yes Series string configuration No No hard limitations to using either string configuration The series string configuration has higher efficiency Yes Buck? No Flyback topology.
AN374 Figure 10 shows a method for selecting the correct power train topology. Start Flyback Yes Isolation required? No Flyback Yes Required VMODEx / VBST < 10% No Buck Figure 10.
AN374 6 Data Improvements for the Curve Fitting Process The curve-fitting process consists of translating a typical LED system specification into polynomial gain equations that the curve fitter can then calculate the required current for a given dim and NTC reading. The gains can be used in conjunction with the design of the power stage to complete a system design for a CS1630/31-based LED driver. 6.
AN374 Sum of Each Channel’s Lumens 3 K1, PRed + K2, PWhite Brightness (lm) Brightness (lm) 650 2 1 0.02 1.0 2 1 0.02 Ratio of the 2 Channel’s Lumens 2700 Approximated Using Current 3 Relative Brightness (lm) Step 2) Translate into LED current profile at room temperature The translation is as shown in Figure 13. 1.0 m1, IRed, m2, IWhite 2 1 0.02 Approximated Using Current K1, PRed Ratio Ratio 1.0 IRed / IWhite K2, PWhite CCT (K) 3 1600 0.02 0.02 1.0 Dim 1.0 0.02 Dim 1.
AN374 NTC Resistance (: Note that any variation of the white string should be compensated for by changing the currents in the other string so that the correct lumen and CCT are maintained across the entire temperature range of interest. At this point, both the currents of the white LED across dim and the data represented in Figures 14 and 15 should be available for the design. Tamb = 5°C Tamb = 25°C Tamb = 40°C Tamb = 55°C 0.02 Dim 1.0 Figure 15.
AN374 6.3.3 Experiment and Approximation In many cases, collecting data at room ambient is not difficult. However, getting data across various ambient temperatures, particularly at low temperatures, is not easy. In an event where getting data is not feasible at ambient temperatures outside of room temperature, existing 25C data can be extended to other temperatures based on data sheet numbers of the LED and measured temperature rises.
AN374 6.4 Improving Data for Feeding the Curve Fitter This section demonstrates the use of a spreadsheet to manipulate the data collected in the previous section to get a more accurate curve fit. The data is presented in a format compatible with the curve fitter software. 6.4.1 Understanding the Data and Data Format The final data can be collected in a spreadsheet. An example of the spreadsheet is shown in Figure 16. Steps are provided to the left to describe the process.
AN374 If resistance is measured, then 4M NTC = -------------------------------------R NTC + R Series [Eq. 23] The data below shows the white/blue currents across the dim. The format in Figure 18 is useful because it is required when the data is imported into the curve fitter. enter љ BwDim enter љ I_bw (A) 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Enter dim values in ascending order 0.068 0.131 0.190 0.246 0.297 0.344 0.386 0.425 0.460 0.490 At 50% dim, IWhite = 0.297 A Figure 18.
AN374 6.4.2 Manipulating Data for Better Curve Fit As mentioned previously, the curve fitter looks for a polynomial fit with a maximum of fourth order. There are a few quick techniques that can be performed to make a better fit. This section shows some of the common techniques used to improve the quality of data and therefore the quality of the fit. In this section, it is assumed that the data, if attempted to be fitted using a curve fitter, provides a large error, so improvements are necessary.
AN374 When re-mapping the dim axis, remember the following points: • • The dim axis needs to be remapped to the white current and the red current axis simultaneously. Otherwise, the data will be altered. In the plot in Figure 20 it can be seen that the data point is to the left of the trendline for the red current and the white current. Moving the dim axis helps mitigate the problem under these circumstances. 0.6 0.5 Dim1 IWhite (mA) IRed (mA) 0.10 0.050 0.030 0.25 0.125 0.075 0.40 0.250 0.
AN374 previous sections. If some of the points in the middle region of the dimming curve are removed, then the fit can be significantly improved in the other regions of interests. 400 350 Output Current (mA) 300 250 200 150 y = 887.97x4 - 1549x3 + 733.71x2 + 301.04x R² = 0.9969 100 50 0 0.00 0.20 0.40 0.60 0.80 1.00 1.20 Dim Figure 22. Best Fit Trendline for a Light Engine By removing the points of interest, the dim map is reconstructed as illustrated in Figure 23.
AN374 7 Performing Curve Fitting 7.1 Using the CS1630/31 Application Software to Perform Curve Fit The CS1630/31 application software and graphical user interface (GUI) are designed to perform all necessary operations to program the one-time programmable (OTP) registers. In this section, an overview of the utility is provided to help understand how to perform the curve fit. The curve fitting utility is incorporated in the application software and can be downloaded from the Cirrus web site.
AN374 Once the data has been entered, the curve fitter can be understood by performing the following steps: Step 1) White Gain setting The entry procedure that is required to preform a curve fit for the White Gain polynomial is detailed in Figure 27. Step 1: Enter the white LED currents and dim. Step 2: Enter the Weight Factor. Error graph give ratio of fit to the data. Step 3: Click the Calculate Coefficients button. Step 4: Adjust results. Step 5: Click the UPDATE REGISTERS button. Figure 27.
AN374 Step 2) Red Gain setting The entry procedure that is required to preform a curve fit for the Color Gain polynomial is detailed in Figure 28. Select graph to view to check quality of fit. Select 3D view perspective. Step 1: Enter the color LED currents for different dims. Step 2: Select “Use Mask”. Step 3: Enter the Weight Factor. Step 4: Click the Calculate Coefficients button. Step 5: Adjust Results. Step 6: Click the UPDATE REGISTERS button.
AN374 Step 3) NTC setting In Figure 29, the value can be entered as an NTC parameter, which corresponds to temperature or in resistances themselves. In addition, the resistance in series to the NTC should be specified. Check application note AN368 for schematic and typical connection to the eOTP pin. Figure 29. SYSTEM DESIGN - NTC Data Tab Once the gains are computed, note that the UPDATE REGISTERS button does write the registers to the IC.
AN374 • Registers CH1CUR and CH2CUR: Channel 1 Reference targets should be the current at 100% brightness on the white string. Channel 2 reference targets should be the currents at 100% brightness at 25C ambient temperature. The currents can be translated into their corresponding digital representation as shown in application note AN368. 7.
AN374 8 Revision History Revision Date REV1 NOV 2012 Initial release. REV2 FEB 2013 Corrected typographical errors.
