Cirrus Logic AN364 - Manual (Page 12)

AN364
12 AN364REV3
Notes on Circuit Fine Tuning
• Going beyond the R
FBGAIN
limitation will not have any further effect on the design.
• R
Sense
and R
FBGAIN
are frequently adjusted simultaneously to reach the desired operating point.
• The optimized final design will have a slightly different switching frequency variation than the first design
iteration.
• When the load is increased or decreased by 10%, then R
Sense
needs to be decreased or increased by less
than 10%, respectively. Adjusting R
FBGAIN
is required when the load changes so that the flyback FET turn
‘ON’ hits the resonant voltage valley.
• Figure 6 illustrates a typical frequency profile with dimming. This is only one typical profile. The breakpoints
can move depending on various tolerances and specific design choices.
At this point, full power and valley switching are close to the target frequency. Further adjustment of the
frequency is done by changing flyback transformer primary inductance L
P
. The core gap can be changed for
small inductance variations.
3.4 Boost Stage Design
The design process for the boost stage is outlined below:
1. Determine I
PK(BST)
and a tentative I
PK(BST)
resistor value R
IPK
(R13)
2. Determine Boost Inductor Specifications
3. Calculate Boost Input and Output Capacitors
The boost stage is designed in No-dimmer Mode, which has a considerable degree of freedom in its design
parameters. For the boost stage to operate in dimmer mode and with the largest variety of dimmers, the design
is constrained within a more limited set of parameters. Even in No-dimmer Mode, the several operating states
of the boost stage leave an exclusively formula-based design impractical.
5% Light
100% Light
(Full Bright)
F
sw(fb)
>F
MIN
S
w
itc
h
in
g
F
re
q
ue
nc
y,
F
sw
Further peak current reduction
and frequency increase
still reducing average current
Increasing T2 at reduced peak current
CRM with valley switching region. Reducing peak
current forces a frequency increase.
Increasing T2 at minimum peak current
Dimming
Figure 6. Flyback Switching Frequency Profile vs. Dimming