Cirrus Logic AN372 - Manual (Page 26)

AN372
26 AN372REV1
For practical winding reasons an integer turns ratio is preferred giving the option of multifilar winding taking
advantage of tight coupling. The maximum FET voltage V
DS
is calculated using Equation 44.
Step 5) Select the Full Brightness Switching Frequency
Minimum and maximum duty cycles, for the tapped buck, are calculated using Equation 9:
expecting a slightly better efficiency
Step 6) Determine the Buck Nominal Timing T1 and T2
At maximum boost voltage V
BST
time T1 is calculated:
and time T2 is calculated:
At minimum boost voltage V
BST
time T1 is calculated:
and time T2 is calculated:
Step 7) Calculate Peak Current on the Buck Primary-side
Calculate average current I
AV
Calculate the peak current I
PK(FB)
at minimum boost voltage V
BST
:
Step 8) Calculate R
Sense
(R21)
The CS1613 current sense threshold is set to 1.4V. The voltage drop across the sense resistor R
Sense
must
equal 1.4V when a maximum current of 0.15A flows through resistor R
Sense
. Use Equation 14 to solve for
R
Sense
(R23).
V
DS
V
BST max  
N V
OUT max  
   + 445V 4 25.2  V   + 546V = = = [Eq. 44]

C min/max  
V
OUT min  
N 1 +   
N V
OUT min  
   V
BST max  
+
----------------------------------------------------------------------------
23.8V 5 
4  23.8V  445.5V + 
--------------------------------------------------------- = 22% = = [Eq. 45]

C max/min  
V
OUT min  
N 1 +   
N V
OUT min  
   V
BST max  
+
---------------------------------------------------------------------------- =
25.2V 5 
4  25.2V  365V + 
---------------------------------------------------- 27% = = [Eq. 46]
P
IN
P
OUT

----------------- =
9.6W
0.95
-------------- 10.1W = = [Eq. 47]
T1 
C min/max  
TT  0.22 8 s  1.76 s = = = [Eq. 48]
T2 TT T1 – 8 s 1.76s – 6.24 s = = = [Eq. 49]
T1 
C max/min  
TT  0.27 8 s  2.16 s = = = [Eq. 50]
T2 TT T1 – 8 s 2.16s – 5.84 s = = = [Eq. 51]
I
AV
P
IN
V
BST min  
--------------------------
10.1W
405V 40.5V –  
----------------------------------------- = = 28mA = [Eq. 52]
I
PK FB  
2 I
AV


C
------------------ =
2 28mA 
0.27
-------------------------- 207mA = = [Eq. 53]
R
Sense
1.4V
I
PK FB  
------------------
1.4V
0.207A
------------------- 6.76  = = = [Eq. 54]