Cirrus Logic AN368 - Manual (Page 54)

AN368
54 AN368REV2
e. Determine the Flyback Nominal Timing T1 and T2
Calculate Mode 1 duty ratio D
MODE1
, and Mode 2 duty ratio D
MODE2
, using Equations 11 and 12, respectively:
Calculate period TT
CH1
using Equation 13:
Calculate switching frequency F
sw2
using Equation 14:
Calculate period TT
CH2
using Equation 15:
Calculate total switching periods for channel 1, TT
CH1
, and channel 2, TT
CH2
, using Equations 16 and 17,
respectively:
Calculate total period TT using Equation 18:
Calculate switching frequency F
sw
using Equation 19:
D
MODE1
N V
MODE1

V
BST
N V
MODE1
   +  
----------------------------------------------------------
5.57 20.95V 
200V 5.57 20.95V    +  
------------------------------------------------------------------ 0.37 = = = [Eq. 90]
D
MODE2
N V
MODE2

V
BST
N V
MODE2
   +  
----------------------------------------------------------
5.57 10.4V 
200V 5.57 10.4V    +  
-------------------------------------------------------------- 0.225 = = = [Eq. 91]
T1
CH1
T2
CH1
+
1
F
sw1
-----------
1
70kHz
------------------ 14.29 s = = = [Eq. 92]
F
sw2
F
sw1
V
BST
N V 
MODE1
  +  
V
BST
N V 
MODE2
  +  
----------------------------------------------------------
I
MODE1
V
MODE2

I
MODE2
V
MODE1

------------------------------------------   = [Eq. 93]
70kHz =
200V 5.57 20.95V    +  
200V 5.57 10.4V    +  
------------------------------------------------------------------
213mA 10.4V 
275mA 20.95V 
-------------------------------------------  
53.3kHz =
T1
CH2
T2
CH2
+
1
F
sw2
-----------
1
53.3 kHz
---------------------- 18.76 s = = = [Eq. 94]
TT
CH1
T1
CH1
T2
CH1
T3
CH1
+ + = [Eq. 95]
15.29 = s
14.29 = s 1s +
TT
CH2
T1
CH2
T2
CH2
T3
CH2
+ + = [Eq. 96]
19.76 = s
18.76 = s 1s +
TT TT
CH1
TT
CH2
+ 15.29 s 19.76s + 35.05 s = = = [Eq. 97]
F
sw
1
TT
-------
1
35.05 s
---------------------- 28.53kHz = = = [Eq. 98]