Cirrus Logic AN374 User Manual

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.

The CS1630/31 offers tremendous flexibility to achieve constant CCT control to match an incandescent dimming profile. The digital CCT control system provides the ability to program dimming profiles, such as constant CCT dimming and black body line dimming. The CS1630/ 31 optimizes LED color mixing by temperature compensating the LED current with an external negative temperature coefficient (NTC) thermistor. The IC controller is equipped with power line calibration for remote system calibration and end-of-line programming. The CS1630 provides a register lockout feature for security against potential access to proprietary registers.

During the course of two-channel design, several design tradeoffs are made based on cost, size, and performance. This document considers the requirements when designing a system around the CS1630/31. This document helps answer the following system questions:

• How can the light engine be designed or modified to maximize the benefits of the CS1630/31?

• How can clear specifications be derived for the system and the LED driver that can reduce the light bulb

development time with the CS1630/31?

Further Reading

• See data sheet DS954 2-Channel TRIAC Dimmable LED Driver IC to review the features and

specifications of the CS1630/31

• See application note AN368 Design Guide for a CS1630/31 2-Channel TRIAC Dimmable SSL Circuit for questions on how to design the LED driver using the CS1630/31

• See application note AN369 Device Programmer User Guide to review the CS1630/31 application software and graphical user interface (GUI)

Cirrus Logic, Inc.

http://www.cirrus.com

FEB’13

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Contacting Cirrus Logic Support

For all product questions and inquiries contact a Cirrus Logic Sales Representative. To find the one nearest to you go to www.cirrus.com

IMPORTANT NOTICE

Cirrus Logic, Inc. and its subsidiaries ("Cirrus") believe that the information contained in this document is accurate and reliable. However, the information is subject to change without notice and is provided "AS IS" without warranty of any kind (express or implied). Customers are advised to obtain the latest version of relevant information to verify, before placing orders, that information being relied on is current and complete. All products are sold subject to the terms and conditions of sale supplied at the time of order acknowledgment, including those pertaining to warranty, indemnification, and limitation of liability. No responsibility is assumed by Cirrus for the use of this information, including use of this information as the basis for manufacture or sale of any items, or for infringement of patents or other rights of third parties. This document is the property of Cirrus and by furnishing this information, Cirrus grants no license, express or implied under any patents, mask work rights, copyrights, trademarks, trade secrets or other intellectual property rights. Cirrus owns the copyrights associated with the information contained herein and gives consent for copies to be made of the information only for use within your organization with respect to Cirrus integrated circuits or other products of Cirrus. This consent does not extend to other copying such as copying for general distribution, advertising or promotional purposes, or for creating any work for resale.

CERTAIN APPLICATIONS USING SEMICONDUCTOR PRODUCTS MAY INVOLVE POTENTIAL RISKS OF DEATH, PERSONAL INJURY, OR SEVERE PROPERTY OR ENVIRONMENTAL DAMAGE ("CRITICAL APPLICATIONS"). CIRRUS PRODUCTS ARE NOT DESIGNED, AUTHORIZED OR WARRANTED FOR USE IN PRODUCTS SURGICALLY IMPLANTED INTO THE BODY, AUTOMOTIVE SAFETY OR SECURITY DEVICES, LIFE SUPPORT PRODUCTS OR OTHER CRITICAL APPLICATIONS. INCLUSION OF CIRRUS PRODUCTS IN SUCH APPLICATIONS IS UNDERSTOOD TO BE FULLY AT THE CUSTOMER'S RISK AND CIRRUS DISCLAIMS AND MAKES NO WARRANTY, EXPRESS, STATUTORY OR IMPLIED, INCLUDING THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR PARTICULAR PURPOSE, WITH REGARD TO ANY CIRRUS PRODUCT THAT IS USED IN SUCH A MANNER. IF THE CUSTOMER OR CUSTOMER'S CUSTOMER USES OR PERMITS THE USE OF CIRRUS PRODUCTS IN CRITICAL APPLICATIONS, CUSTOMER AGREES, BY SUCH USE, TO FULLY INDEMNIFY CIRRUS, ITS OFFICERS, DIRECTORS, EMPLOYEES, DISTRIBUTORS AND OTHER AGENTS FROM ANY AND ALL LIABILITY, INCLUDING ATTORNEYS' FEES AND COSTS, THAT MAY RESULT FROM OR ARISE IN CONNECTION WITH THESE USES.

Use of the formulas, equations, calculations, graphs, and/or other design guide information is at your sole discretion and does not guarantee any specific results or performance. The formulas, equations, graphs, and/or other design guide information are provided as a reference guide only and are intended to assist but not to be solely relied upon for design work, design calculations, or other purposes. Cirrus Logic makes no representations or warranties concerning the formulas, equations, graphs, and/or other design guide information

Cirrus Logic, Cirrus, the Cirrus Logic logo designs, EXL Core, the EXL Core logo design, TruDim, and the TruDim logo design are trademarks of Cirrus Logic, Inc. All other brand and product names in this document may be trademarks or service marks of their respective owners.

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.

Risk of Electric Shock

• The direct connection to the AC power line and the open and unprotected boards present a serious risk of electric shock and can cause serious injury or death. Extreme caution needs to be exercised while handling this board.

• Avoid contact with the exposed conductor or terminals of components on the board. High voltage is present on exposed conductor and it may be present on terminals of any components directly or indirectly connected to the AC line.

• Dangerous voltages and/or currents may be internally generated and accessible at various points across the board.

• Charged capacitors store high voltage, even after the circuit has been disconnected from the AC line.

• Make sure that the power source is off before wiring any connection. Make sure that all connectors are wel

connected before the power source is on.

• Follow all laboratory safety procedures established by your employer and relevant safety regulations and guidelines such as the ones listed under, OSHA General Industry Regulations - Subpart S and NFPA 70E.

Suitable eye protection must be worn when working with or around demonstration boards. Always

comply with your employer’s policies regarding the use of personal protective equipment.

All components and metallic parts may be extremely hot to touch when electrically active.

2 AN374REV2

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TABLE OF CONTENTS

1 OVERVIEW OF THE CS1630 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1 Definition of Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.2 Definition of Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 INTRODUCTION TO THE COLOR SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4 LIGHT ENGINE CHOICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4.1 Considerations for the Color System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Constraint 1: Translating current versus dim requirements into a fourth-order or lower polynomial. . . . . . 7

Constraint 2: The current versus dim plot should have intercept at origin . . . . . . . . . . . . . . . . . . . . . . . . . .7

Constraint 3: Placement of NTC with respect to light engine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

Constraint 4: Red currents increase with an increase in ambient temperature . . . . . . . . . . . . . . . . . . . . . .8

Constraint 5: Maximum allowable color gain is 4.0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

4.2 Constraints Imposed by Second Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Constraint 1: Currents in either channel should not be zero at non-zero dim values . . . . . . . . . . . . . . . . . 8

Constraint 2: No abrupt change in current ratios at a dim point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

Constraint 3: Ratio of peak currents between the two channels must be less than 4 . . . . . . . . . . . . . . . . .9

Constraint 4: Both strings cannot have identical configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

Constraint 5: Maximum differential between string voltages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

Constraint 6: The implications when determining LED string configuration and topology . . . . . . . . . . . . . .9

Constraint 7: Note on Frequency and EMI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16

4.3 Synchronizer Design Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Constraint 1: Non-isolated synchronizer circuit considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.4 Constraints on the Boost Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Constraint 1: Maximum power should be power at full AC sine wave . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Constraint 2: Power should be monotonically increasing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17

5 DESIGN FLOWCHART . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

6 DATA IMPROVEMENTS FOR THE CURVE FITTING PROCESS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

6.1 Typical Light Engine System Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

6.2 Translation into Input Specifications for Calculating Color Gains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

6.3 Methods to Collect Required Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

6.3.1 Experimental Measurement in Lab . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

6.3.2 Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

6.3.3 Experiment and Approximation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

6.4 Improving Data for Feeding the Curve Fitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

6.4.1 Understanding the Data and Data Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

6.4.2 Manipulating Data for Better Curve Fit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

7 PERFORMING CURVE FITTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

7.1 Using the CS1630/ 31 Application Software to Perform Curve Fit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

7.2 Register Bits in OTP Map Affected by the Curve Fitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

7.3 Using Commercially Available Curve Fitting Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

7.4 Process of Converting a Given Gain Coefficients to CS1630/31 OTP Map . . . . . . . . . . . . . . . . . . . . . . . 34

8 REVISION HISTORY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

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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.

2.1 Definition of Acronyms

Acronym Description

PFC Power Factor Correction

ZCD Zero-current Detection

BOP Boost Overvoltage Protection

COP Clamp Overpower Protection

OVP Second-stage Output Open Circuit Protection and Overvoltage Protection

OCP Second-stage Overcurrent Protection

OLP Second-stage Open Loop Protection

SCP Short Circuit Protection

iOTP Internal Overtemperature Protection

eOTP External Overtemperature Protection

PLC Power Line Calibration

OTP One-time Programmable

LED Light Emitting Diode

TX Transformer

TRIAC

NTC

SSL

CSV Comma-separated Values File

CCT Correlated Color Temperature

DAC Digital-to-Analog Converter

TRIode for Alternating Current, which is an electronic component that can conduct current in either direction when it is triggered. It is formally called a bidirectional triode thyristor.

Negative Temperature Coefficient thermistor

Solid-state lighting. Refers to a type of lighting that uses semiconductor LEDs as a source of illumination rather than electrical filaments, plasma, or gas.

4 AN374REV2

2.2 Definition of Symbols

CH1

----------------





CH2

----------------





Symbol Description

& F

sw1

sw2

TT Second-stage switching period

& TT

CH1

& T1

& T2

& T3

CH2

Second-stage switching frequency

Switching frequency for channel 1 and channel 2

Switching period for channel 1 and channel 2

Second-stage primary FET ‘ON’ time for channel 1 and channel 2

Second-stage secondary rectifier diode conduction time for channel 1 and channel 2

Time the second-stage FET and rectified diode are ‘OFF’ for channel 1 and channel 2

AN374

MODE1

PK1(FB)

MODE1

GAIN

& D

Reflected

CLAMP

PK(FB)

& I

& V

Sense

& T

NTC

PK(BST)

BST

CH1

CH2

& I

CH1

OUT

Red

White

dim

& GAIN

MODE2

PK2(FB)

MODE2

NTC

CH2

MODE2

DTR

Duty ratio for Mode 1 and Mode 2

Input line voltage

Voltage across secondary winding reflected onto primary

Primary clamping voltage above boost output voltage (V

BST

)

Maximum second-stage peak current in primary-side FET

Maximum second-stage peak current in primary-side FET for Mode 1 and Mode 2

Output current for Mode 1 and Mode 2

Output voltage for Mode 1 and Mode 2

Second-stage primary current sense resistor

Negative temperature coefficient resistance and corresponding temperature

Maximum boost inductor current

Second-stage primary inductance

Boost inductance

Boost output voltage

Flyback transformer turns ratio N

P/NS

Channel 1 secondary output VDC (channel 1 LED string supply voltage)

Channel 2 secondary output VDC (channel 2 LED string supply voltage)

Channel 1 and channel 2 LED string current

Load power = Power to the LED string

Output current that flows through the amber/red color LED string

Output current that flows through the white/blue color LED string

The CS1630/31 color control system has the ability to maintain a constant CCT or change CCT as the light dims. OTP configurations allow the selection of the dimming profile. A specific CCT profile can be programmed to the digital mapping device. The mapping is two-dimensional: one current versus temperature profile is generated for each dim level. The CS1630/31 provides two-dimensional mapping for the color LED’s current only, and one-dimensional mapping (current versus dim level) for the other string.

The dim-regulated gain and dim-regulated plus temperature-regulated gain

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dim

NTC

(From Boost)

÷ 4096

÷ 256

Normalize

Saturation

Logic

GAIN

= Q3 × D3 + Q2 × D2 + Q1 × D + Q0

GAIN

DTR

= P30 × T3 + P20 × T2 + P10 × T + P03 × D3 + P02 × D2 + P01 × D +

P21 × T

× D + P12 × T × D2 + P11 × T × D + P00

White(ref)

Red(ref)

White

Red

dim

Temperature

(ADC Fast Filter)

Figure 1. Color Control System

GAIN

DTR

P30 T3 P20 T2 P10++T P03 D3 P02++D2 P01 D P21 T2D++= P12 T D2 P11 T D P00+++

GAIN

Q3= D3 Q2 D2 Q1 D Q0+++

[Eq. 1]

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. In many such designs, one of the LED strings is composed of red or amber LEDs, and the other string is composed of cool-white or blue-white LEDs. While the lumen output of white LEDs does not vary significantly across temperature, the lumen output of red LEDs can vary as much as 40% across temperature. To achieve a consistent light output across temperature, the current in the red LED string needs to be compensated with respect to temperature. Depending on the design, either channel can be temperature compensated.

Cirrus Logic, Inc. and its affiliates and subsidiaries generally make no representations or warranties that the combination of Cirrus Logic’s products with light-emitting diodes (“LEDs”), converter materials, and/or other components will not infringe any third-party patents, including any patents related to color mixing in LED lighting applications, such as, for example, U.S. Patent No. 7,213,940 and related patents of Cree, Inc. For more information, please see Cirrus Logic’s Terms and Conditions of Sale, or contact a Cirrus Logic sales representative.

Figure 1 illustrates the block diagram of the color control block inside of the CS1630. The color temperature of the light engine can be modified by changing the gains of each channel based on the current dim level. On the temperature-controlled channel, the currents can be varied according to the temperature sensed by an external NTC.

The required gain value for a particular combination of dim and temperature is obtained using polynomial curves, the coefficients of which are programmed into the CS1630/31 OTP memory. A different polynomial is used for each channel. One of these is a polynomial in two variables, dim and temperature, while the other is a polynomial in dim only. If D and T are assumed to be the normalized dim and temperature values, respectively, between 0 and 1.0, then GAIN

refers to the dim-regulated gain and temperature-regulated gain, and GAINDR refers to the dim-reg-

DTR

ulated gain.

As shown in Equation 1, the gain equation for the white is a third-order polynomial in dim, and the gain equation for the red is a third-order polynomial in both dim and temperature. The color gain is third order to provide good tradeoffs between computational overhead and being able to operate over the largest variety of LEDs across a wide range of temperature. A lower-order polynomial fit, such as a quadratic or a linear fit, would not allow a large range of gain values across the entire operating range. This would limit the sample space of available LEDs, since the gain is an indirect reflection of its variation across temperature and required dim. As a result, a third-order fit allows the system engineer to achieve a large variation in CCT and lumens across dim.

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RedIRed ref

dim GAIN

DTR

=

[Eq. 2]

WhiteIWhite ref

dim GAIN

=

[Eq. 3]

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.

4.1 Considerations for the Color System

Constraint 1: Translating current versus dim requirements into a fourth-order or lower polynomial

Numerous smooth curves that do not have asymptotes at the origin can be modeled as a polynomial function. The few exceptions are notable functions where the output current equals log converter design requires a light engine that has a color curve that trends to any of these functions, then some design tradeoffs must be considered. Such output current profiles are not widely prevalent in the consumer or commercial lighting applications in which two-channel LEDs are typically used.

Constraint 2: The current versus dim plot should have intercept at origin

The final output currents for red current I

and white current I

Red

are calculated using Equations 2 and 3:

White

(dim) or 1/dim. If the power

These equations show that the current versus the dim will tend to the origin even for non-zero coefficients P00 and Q0.

The dim axis is an imaginary axis with no physical significance to an LED designer. This constraint causes problems that can be substantially mitigated by remapping the dim axis, as demonstrated in section Manipulating Data for Better Curve Fit on page 27.

Constraint 3: Placement of NTC with respect to light engine

The NTC can be placed close to the LEDs when the LEDs represent the junction temperature of the LEDs or it can be placed farther away from the LEDs, where it represents the heat sink temperature or even the ambient temperature inside the bulb. Each placement comes with its own advantages, and Steps 1 and 2 below describe the implications of placing the LEDs in either location.

Step 1) Advantages of placing the NTC close to the LEDs

1. The problem of thermal mass is greatly reduced, which allows using simpler temperature protection and thermal fold back.

2. Since the NTC is a good representation of the junction temperature, it is simpler to make light-system models that can be used for designing the light engine.

3. Temperature protection systems such as thermal fold back and thermal shutdown can be more accurately designed if needed, improving reliability in such applications.

Step 2) Disadvantages of placing the NTC close to the LEDs

1. The

 of the NTC varies across temperature, and a narrow temperature range produces a more accurate

linear approximation. By having a large temperature rise on the NTC, the NTC is no longer accurate, since the CS1630/31 can only accept a single

 value and a single expression for the temperature.

2. If the thermal impedance between the NTC and the LED junction is large, the NTC will not accurately represent the LED junction temperature, and the red current I

compensation may be incorrect. For

Red

example, when the NTC is placed close to the LED strings, then the measured temperature is a close representation of the LED junction temperature. In a tight form factor and at full brightness, the temperature rise from ambient to thermal equilibrium is significant. Typically, LED strings can take up to a few minutes before they reach thermal equilibrium. When the system is started at ambient temperature T

= 25C and full brightness, the NTC temperature and the junction temperature of the LEDs are at

amb

C. Since the curve fitter produces a gain function that is dependent on a combination of the red current

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PK1 FBVBST

CH1

----------------





 200V

5.3s

3543H

---------------------





 299m A== =

[Eq. 4]

MODE1 avgIPK1 FB

CH1

2TT

----------------





 299m

A 5.57

9.0s

2 35.05s

------------------------------





 214m A== =

[Eq. 5]

, NTC, and dim variables, the curve fitter will have to compute gains across a larger number of points,

Red

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 I value.

in the red channel should increase monotonically with ambient temperature at a given dim

Red

Ambient Temperature

C300mA

C330mA

C360mA

Red Current Flowing in

Current Compensated Channel

Table 1. Maximum Measurable Switching Period

For example, if the current at ambient temperature T T

= 25C, then the currents are non-monotonic with ambient temperature.

amb

= 50C is less than the current at ambient temperature

amb

Constraint 5: Maximum allowable color gain is 4.0

The recommended maximum gain value that the color system generates is 4.0. For example, the red current at 25

C cannot be four times larger or smaller than any other current at the same dim value.

4.2 Constraints Imposed by Second Stage

Constraints in this section are imposed by the second stage. Calculations of these parameters are detailed in application note AN368 Design Guide for a CS1630/31 2-Channel TRIAC Dimmable SSL Circuit. All variables referenced in this document are computed in application note AN368.

Constraint 1: Currents in either channel should not be zero at non-zero dim values

Calculate peak current I

PK1(FB)

during Mode 1 using Equation 4:

Calculate the average current I

MODE1(avg)

during Mode 1 using Equation 5:

Equations 4 and 5 show that both channel currents share the same peak currents. It is difficult to have zero current in one channel and have a non-zero current in the other channel. As a result, currents must be greater than zero at all non-zero dim values.

Constraint 2: No abrupt change in current ratios at a dim point

It is recommended to avoid any abrupt change in the ratio of two-channel currents because it may cause color flicker. The ratio of the channel current and the actual value of the string current should be a smooth function which does not have any abrupt changes that become visible.

8 AN374REV2

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PK1 FB

PK2 FB

-------------------

MODE1

MODE2

--------------------=

[Eq. 6]

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 I 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 2.5 is considered a difficult requirement for the driver to guarantee regulation across the dimming range. If the resonant reset time T3 is short and can be neglected, then the ratio of the peak currents can be given as follows:

Refer to application note AN368 for detailed calculation.

Constraint 4: Both strings cannot have identical configuration

Either the output voltage differential should be maintained, or the output current differential should be maintained. If the strings are placed in series, then the currents should never be the same at any point of the dimming curve. If the strings are placed in parallel, then the voltage cannot be the same. The currents can cross in a parallel load configuration. In a commercial lighting space that requires mixing white LEDs of different wavelengths to produce a higher CCT white light (>3000K), it is recommended to have a different number of white LEDs in the two LED strings when using the CS1630/31.

Constraint 5: Maximum differential between string voltages

The overvoltage protection (OVP) threshold is set to 1.25V and the FBSENSE comparator threshold is set at a 200mV. As the dimmer conduction angle is reduced, the flyback operates continuously in DCM, while maintaining valley switching for low losses. At very low conduction angles, the flyback may switch on the third valley. Therefore, a minimum signal of 400mV on the FBAUX pin is recommended, which means that the string voltage of one channel cannot be greater than three times the string voltage of the other channel in a parallel load configuration. The FBAUX pin is used exclusively for overvoltage protection. In the event that a higher string voltage differential is desired, an external circuit must be designed such that different divider ratios are chosen for both modes of operation.

The OVP point is fixed for the higher of the two strings. For example, if one string has an output voltage of 5V and the other string has an output voltage of 15V, the OVP point is still considered to be at greater than 15V. Therefore, both output capacitors must be rated for the OVP voltage, which is in this case greater than 15V.

at full brightness threshold that is usually set at approximately 25%, so the theoretical maximum

PK(FB)

Constraint 6: The implications when determining LED string configuration and topology

Identify the light engine and select an appropriate second stage topology. There are two aspects that need to be considered: first, whether to use the series or parallel LED configuration and second, which power train topology to use.

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Figure 2a. Flyback Series Output Model Figure 2b. Flyback Parallel Output Model

R22

R21

R23

CS1630 /31

FBAUX

GND

FBSEN SE

TX1

BST

C10

C15

GND

_ Q

VCC

D15

R12 D10

C16

Channel 1 LED (White)

Channel 2 LED (Red)

GND

IGND

MODEx

PRI

MODEx

R22

R21

R23

CS1630 /31

FBAUX

GND

FBSEN SE

TX1

BST

C10

C15

GND

_ Q

VCC

D15

R12 D10

C16

Channel 1 LED (Whi te )

Channel 2 LED (Red)

GND

IGND

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. When considering a series design, it is recommended that the current flowing through one of the LED channels be 80% or lower than that of the other LED channel at all times. The LED string that has current flowing continuously is referred to as channel 1 LED (I as channel 2 LED (I

CH2

); I

CH2

 0.8I

CH1

), while the string with the bypass FET is referred to

CH1

Figure 2b illustrates a parallel light configuration. The two LED strings are networked in parallel so that current flows to either the channel 1 LED string or the channel 2 LED string at any given T2 time. The CS1630/31 controller uses the LED forward voltage to detect which LED string is being driven. One LED string must always have a larger forward voltage compared to the other LED string. The LED string with the higher voltage is referred to as channel 1 LED with forward voltage V to as channel 2 LED with a forward voltage of V

CH2

and the LED string with the lower voltage is referred

CH1

A good rule of thumb is that channel 2 LED must always have a forward voltage of 85% or lower than the channel 1 LED to consider a parallel design. Table 2 defines the selection process based on the requirements of the series and parallel output configuration.

< V

CH1

CH2

and I

CH2

< I

CH1

Cross Parallel Configuration Not Functional

CH2

Series or Parallel Configuration Series Configuration

CH1

Table 2. Series vs. Parallel

If there are problems converging on a target design using an existing light engine, the current and voltage profile can be modified by adding, removing, or moving LEDs between the two LED channels. Figure 3 illustrates the parallel and series scenarios that can be configured using the CS163X Customer Application tool.

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CH1

and V

CH2

Cross

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Step 2a:

Select Series

vs. Parallel

Step 2b:

Select Flyback,

Buck, or

Tapped Buck

Figure 3. Second Stage String Configuration

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 2) Select power train topology

The CS1630/31 supports three possible power train configurations: tapped buck, buck, and flyback (see Figure 4a, 4b, and 4c). The two most important factors for selecting a power train configuration is whether the output requires Underwriters Laboratory (UL) approved isolation and the input to output voltage ratio. The flyback power train can be either isolated or non-isolated. Buck and tapped-buck designs are expected to always be non-isolated. If isolation is not required, one of the three possible solutions must be selected. If isolation is required, the design will be a flyback.

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