Cirrus Logic AN368 - Manual (Page 56)

AN368
56 AN368REV2
h. Determine the RMS Current in the Winding
The RMS current in the primary winding, I
PRI(RMS)
, is calculated using Equation 29:
The RMS current in the secondary winding, I
SEC(RMS)
, is calculated using Equation 30:
i. Calculate R
Sense
(R21)
Calculate sense resistor R
Sense
(R21) for flyback using Equation 31:
where,
f
scale
= Scaling factor
R21 = R
Sense
in 
Calculate the power P
Sense
dissipated by the sense resistor R
Sense
(R21) using Equation 32:
j. Calculate Flyback Zero-current Detection
Bit VALLEYSW in register Config2 at Address 34 is set to ‘1’, enabling quasi-resonant switching (valley
switching) on the second stage. Bit POL_ZCD in register Config4 at Address 36 is set to ‘0’ configuring the
comparator output to active-low. The transformer TX1 auxiliary winding turns ratio
is calculated using Equation
33:
I
PRI RMS  
I
PK1 FB  
2
D
MODE1
3
--------------------
 
 
 I
PK2 FB  
2
D
MODE2
3
--------------------
 
 
 + =
[Eq. 108]
299mA   2
0.37
3
-----------
 
 
 237mA   2
0.225
3
---------------
 
 
 + =
123.5mA =
I
SEC RMS  
N
2
I
PK1 FB  
2 
1 D
MODE1
–
3
-----------------------------
 
 
 N
2
I
PK2 FB  
2   
1 D
MODE2
–
3
-----------------------------
 
 
 + =
[Eq. 109]
5.57
2
299mA   2 
1 0.37 –
3
---------------------
 
 
 5.57
2
237mA   2 
1 0.225 –
3
------------------------
 
 
 + =
1.016A =
R
Sense
1.4V
f
scale
I 
PK1 FB  
-------------------------------------
1.4V
1.1 0.299A 
-------------------------------- 4.26  = = = [Eq. 110]
P
Sense
I
PRI RMS  
2
R
Sense
 123.5  mA  2
4.26   65mW = = = [Eq. 111]
[Eq. 112]
N
P
N
FBAUX
-------------------
2 N V
OVP
R23   
1.25V R22 R23 +   
------------------------------------------------------ =
2 5.57 15V 1.1k    
1.25V 14k  1.1k +   
--------------------------------------------------------------- =
9.7 =