Cirrus Logic AN374 - Manual (Page 7)

AN374
AN374REV2 7
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
10
(dim) or 1/dim. If the power
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
Red
and white current I
White
are calculated using Equations 2 and 3:
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
Red
compensation may be incorrect. For
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
amb
= 25

C and full brightness, the NTC temperature and the junction temperature of the LEDs are at
25

C. Since the curve fitter produces a gain function that is dependent on a combination of the red current
I
Red
I
Red ref  
dim GAIN
DTR
  = [Eq. 2]
I
White
I
White ref  
dim GAIN
DR
  = [Eq. 3]