Cirrus Logic AN364 - Manual (Page 21)

AN364
AN364REV3 21
Step 5) Determine the Flyback Nominal Timing T1 and T2
Use Equation 7 to solve for T1:
Use Equation 8 to solve for T2:
Step 6) Calculate Peak Current on the Flyback Primary-side
To deliver constant power, I
PK(FB)
is at its maximum when V
BST
is at its minimum. Use Equation 9 solve for
I
PK(FB)
:
Although I
PK(FB)
is calculated to equal 0.131A, the flyback transformer is specified at I
PK(FB)
= 0.15A to
guarantee margin against saturation.
Step 7) Calculate R
Sense
(R23)
The CS1611 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 10 to solve for
R
Sense
(R23).
Choosing a 10Ω standard value will assure margin against resistor tolerance. To prevent false triggering by
the comparator, pin FBSENSE has an internal blanking time of 550ns. To reduce switching spikes, it is
recommended to add an additional RC filter circuit using a 1k
 resistor followed by a 100pF capacitor.
Step 8) Calculate the Flyback Primary-side Inductance
Use Equation 11
and the minimum I
PK(FB)
from Equation 33 to calculate the primary inductance:
Step 9) Calculate R
FBGAIN
(R17)
Using Equation 12, solve for R
FBGAIN
(R17):
Step 10) Determine the RMS Current in the Winding
Using Equation 13
, calculate transformer T1 primary RMS current:
T1
1
F
sw
--------- T3 –
V
Reflected
V
Reflected
V
BST
+  
----------------------------------------------------  =
1
85kHz
------------------ 1 s –
220V
220V 405V +
-----------------------------------  4.14 s = = [Eq. 31]
T2
1
F
sw
--------- T3 –
V
BST
V
Reflected
V
BST
+  
----------------------------------------------------  =
1
85kHz
------------------ 1 s –
405V
220V 405V +
-----------------------------------  7.62 s = = [Eq. 32]
I
PK FB  
2 P
OUT max  
TT
fb
 V
BST
 T1
fb
  
----------------------------------------------   = 2 6.6W
12.5 s
0.85 365V  4.05 s 
---------------------------------------------------------   0.131A = = [Eq. 33]
R
Sense
1.4V
I
PK FB  
------------------
1.4V
0.131A
------------------- 10.7  = = = [Eq. 34]
L
P
V
BST
T1   
I
PK FB  
--------------------------------
365V 4.05 s   
0.131A
-------------------------------------------- 11.3mH = = = [Eq. 35]
R
FBGAIN
4M 
TT
fb
T2
fb
------------ 128 
 
  64
–
---------------------------------------------- =
4M 
12.5 s
7.65 s
------------------ 128 
 
  64
–
-------------------------------------------------------- = 26.5k  =
[Eq. 36]
I
RMS
I
PK FB  
T
current
3 TT 
--------------------  = 0.131A
4.05 s
3 12.5 s 
----------------------------  43 mA = = [Eq. 37]