Cirrus Logic CS1631 - Manual (Page 13)

CS1630/31
DS954F3 13
waveform. The information from the probe event is used to
maintain proper operation with the dimmer circuitry.
5.3.5 Trailing-edge Mode
In Trailing-edge Mode, the CS1630/31 determines its operation
based on the falling edge of the input voltage waveform (see
Figure 10). To provide proper dimmer operation, the
CS1630/31 executes the boost algorithm on the falling edge of
the input line voltage that maintains a charge in the dimmer
capacitor. To ensure maximum compatibility with dimmer
components, the device boosts during this falling edge event
using a peak current that must meet a minimum value. In
Trailing-edge Mode, only the CRM boost algorithm is used.
5.4 Correlated Color Temperature Control
The CS1630/31 color control system can adjust and maintain
the correlated color temperature (CCT) for the LED color-
mixing application by connecting an external negative
temperature coefficient (NTC) thermistor to the eOTP pin. The
LED temperature variation can be accurately detected by the
internal eOTP feedback loops (see "External Overtemperature
Protection" on page 19).
Red and amber LEDs are necessary components in
color-mixing applications when providing warm white or other
CCTs. When mixing colors, red and amber LEDs are the most
temperature sensitive, so they cause a large variation in
temperature. The CS1630/31 is capable of providing LED
CCT and luminosity with temperature compensation using the
NTC thermistor to resolve the significant change in the
luminous output due to temperature variations.
Since LED lumens are mainly a function of temperature and
forward current, color temperature and luminosity can be
maintained by independently adjusting each string's output
current as the ambient temperature changes. This can be
done by mapping the NTC reading to a required value of the
current in each string using a digital mapping block.
In the CS1630/31, only one of the LED string currents is
compensated for due to temperature variations. The current in
the other string is kept constant over temperature, which may
result in the luminosity decreasing slightly as temperature
increases. In order for the ADC to resolve the entire range of
possible temperature variation in the LEDs, it is recommended
to select series resistor R
S
and NTC resistor R
NTC
with the
appropriate Beta value, which retains the total resistance
(R
S
+R
NTC
) at all possible operating temperatures within the
tracking range of the ADC. The final temperature-to-digital
code mapping depends on these variables.
The CS1630/31 color control system also 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. In this case, 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. A
simplified block diagram of the color control system is shown
in Figure 11.
Figure 9. Leading-edge Mode Phase-cut Waveform
Figure 10. Trailing-edge Mode Phase-cut Waveform
Gain
White
dim
I White
I ref
White
I ref
Color
X
X
ADC
NTC
I
Color
Gain
Color
X
X
Figure 11. Block Diagram of Color Control System