Cirrus Logic AN368 - Manual (Page 50)

AN368
50 AN368REV2
The EMI filter and the reactances associated with the dimmer constitute a complex reactive network that has
minimal damping. This reactive network will ring as it is excited by the dimmer turn on and the boost stage
conduction. If the current in the dimmer's TRIAC reverses, the TRIAC opens, which disturbs the dimmer timing
and causes flicker. For this reason stringent limitations are imposed on the values assigned to the EMI
components.
a. I
Sense
DAC Dithering
Dither is used to spread the EMI energy from the flyback over a frequency range to avoid high energy peaks
at the switching frequency. The CS1630 controller provides the capability to apply dithering to the reference
level used by the I
Sense
comparator to compare against the voltage on the FBSENSE pin. The maximum dither
level is approximately 12.3%. Dithering on the I
Sense
reference is enabled by asserting the DITHER bit in the
Config17 register at Address 49. To configure dithering at full dim levels (No-dimmer Mode only), the
DITNODIM bit in the Config61 register at Address 93 must be set to ‘1’.
The amount of dithering is configured using the DITLEVEL[1:0] bits in register Config61 at Address 93.
The amount of dithering can be varied between the two channels. The dither magnitude set by the
DITLEVEL[1:0] bits can be reduced and applied to one of the I
Sense
references for a channel. The channel that
receives the reduced dithering is selected by the DITCHAN bit in register Config61 at Address 93. A value of
‘0’ selects channel 1, and a value ‘1’ selects channel 2. The dither attenuation is configured by the DITATT[1:0]
bits in register Config12 at Address 44.
The amount of dithering can vary between the two channels. Ideally the dither should be in correlation with the
ratio of the peak currents. For example, if the peak current in Mode 2 is half of the peak current in Mode 1 set
bits DITATT[1:0] to ‘01’ to apply a 50% dither attenuation.
DITLEVEL[1:0] Dither Magnitude
0 1.3% of DAC full scale
1 2.9% of DAC full scale
2 6.0% of DAC full scale
3 12.3% of DAC full scale
Table 9. I
Sense
Reference Dither Magnitude
DITATT[1:0] Dither Attenuation
0 No attenuation
1 50% attenuation
2 25% attenuation
3 12.5% attenuation
Table 10. I
Sense
Dither Attenuation