Cirrus Logic AN375 - Manual (Page 20)

AN375
20 AN375REV4
Using turns ratio N derived in Equation 36 on page 19, calculate minimum reflected voltage V
Reflected(min)
and
maximum reflected voltage V
Reflected(max)
. The design chooses a zero tolerance for output voltage V
OUT
on
the assumption that the LED string has a degree of compensation. See Equation 37 and 38:
Step 4) Predicting T3
Setting period T3
PFC(90°)
equal to 1.5 s is a good approximation as a starting point until the resonant
frequency can be measured.
Step 5) Calculate Duty Cycle in Dimmer Mode
Use Equation 19 on page 13 to calculate nominal ratio Z at 90°. See Equation 39:
Since ratio Z is equal to 0.725 and less than 1.31 where the frequency is maximum at a 90° phase angle and
minimum at the conduction threshold of 30V, a switching Frequency Profile A is selected.
A maximum switching frequency F
SW(max)
of 56.7kHz implies a switching period TT equal to 17.6µs. See
workbook SS375 CS1615/16 Calculator to Support Application Note AN375 for more details regarding the
calculation of the switching frequency in Dimmer Mode. In Dimmer Mode, period T3 is a maximum of 2µs. Use
Equation 13 on page 11 to calculate the minimum duty cycle

min
. See Equation 40:
Step 6) Calculate Peak Current
Use Equation 14 on page 12 to solve for peak current I
PK
. See Equation 41:
Step 7) Calculate Primary Winding Inductance
Using Equation 15 on page 13, calculate primary inductance L
P
to allow for sufficient current build up at the
90° point when AC line voltage V
IN
is at a peak. See Equation 42:
In the example design the primary inductance L
P
is rounded up to 2.6mH.
V
Reflected min  
N V
OUT
1.0    V
F
+    5.75 27.9V 1.0    0.9V +    165.6V = = = [Eq. 37]
V
Reflected max  
N V
OUT
1.0    V
F
+    5.75 27.9V 1.0    0.9V +    165.6V = = = [Eq. 38]
Z
V
IN
V
Reflected
--------------------------
120V
165.6V
------------------- 0.725 = = = [Eq. 39]

min
1
T3
TT
------- –
 
 
1
V
INPK max  
V
Reflected min  
--------------------------------------- +
 
 
--------------------------------------------------------
1
2 s
17.6 s
------------------ –
 
 
1
1.1 2  120V 
165.6V
------------------------------------------ +
 
 
------------------------------------------------------------ 0.417 = = = [Eq. 40]
I
PK
2 I
IN CC  


min
---------------------------
2 107mA 
0.417
----------------------------- 513.6mA = = = [Eq. 41]
L
P
TT
PFC 90   
T3
PFC 90   
–   2
V
INPK
V
Reflected

V
INPK
V
Reflected
+
-------------------------------------------------
 
 
2

4 TT
PFC 90   
P
IN
 
---------------------------------------------------------------------------------------------------------------------------------------- =
[Eq. 42]
15.2 s 1.5s –   2
2 120V  165.6V 
2 120V    165.6V +
----------------------------------------------------------
 
 
2

4 15.2 s 8.5W  
------------------------------------------------------------------------------------------------------------------------- =
2.54mH =