Cirrus Logic AN364 - Manual (Page 8)

AN364
8 AN364REV3
For optimum efficiency, the increase in transformer losses (created by an uneven duty cycle) must balance the
reduction of the losses caused by discharging the leakage inductance (obtained by increasing the overshoot
voltage). Equation 3 is used to balance all voltages contributing to the FET voltage drain and source.
where
V
Overshoot
= V
CLAMP
- V
Reflected
Step 3) Determine the Flyback Transformer Turns Ratio
Select a turns ratio based on the output voltage, V
OUT
, and V
Reflected
using Equation 4.
where
V
OUT(max)
= the maximum LED string forward voltage V
OUT
at full current plus the rectifying diode voltage V
F
.
Step 4) Select the Full Brightness Switching Frequency
The CS1611 maximum switching frequency is 200kHz. Test results indicate that optimal performance is
obtained in the range of 75kHz to 120kHz. Higher frequencies allow the use of smaller magnetics, but
switching losses increase. Transformer size versus switching frequency is limited by designs that require
isolation. Selecting too low a full brightness switching frequency risks impairing dimmer compatibility while also
allowing the minimum frequency to drop into the audible range.
From the full brightness frequency, determine the value of (T1 +T2) using Equation 5.
where
T3 is 1/2 the transformer resonant period
The transformer primary inductance resonates with the total parasitic capacitance of the drain node. For initial
calculations, T3 is estimated as 1
s and must be measured for final accuracy.
Step 5) Determine the Flyback Nominal Timing T1 and T2
From the reflected voltage and the turns ratio, determine T1 and T2. Equation 6 ensures zero DC voltage
across the transformer T1 magnetizing inductance.
V
Breakdown
V
BST
V
Reflected
V  +
CLAMP
V
Reflected
 – V
M in arg
+ + = [Eq. 3]
N
V
Reflected
V
OUT max  
---------------------------- = [Eq. 4]
T1 T2 +
1
F
sw
---------
 
  T3
– = [Eq. 5]
T1
T2
-------
V
Reflected
V
BST
-------------------------- = [Eq. 6]