Cirrus Logic AN368 - Manual (Page 57)

AN368
AN368REV2 57
The FBAUX pin current must be limited to less than 1mA. Resistor R22 plus R23 should be chosen such that
current V
AUX
/(R22 + R23) should be less than 1mA during the time when FET Q5 is ‘ON’ or ‘OFF’. A series
resistor of at least 22k  must be used to limit the current.
k. Determine Output Capacitors
Channel 1 output capacitor C8 ripple current I
Ripple(RMS)
is the vectorial difference between transformer TX1
secondary current and the DC load current. See Equation 113.
Capacitor C8 is selected to be a 16volt, 100 F electrolytic capacitor. Channel 2 output capacitor C15 is
calculated using Equation 114:
Capacitor C15 was selected to be a 68 F with a16V voltage rating.
l. Flyback Transformer Design
The flyback transformer is specified with a EE13/6/6 core and the effective cross-sectional area A
e
of the core
is 20.1mm 2 . The peak flux density B
PK
is equal to 213mT. Calculate the flyback transformer TX1 gap length I
g
using Equation 34:
The primary turns N
P
is calculated using Equation 35:
The secondary turns N
S
is calculated using Equation 36:
I
Ripple RMS  
1.016A   2
0.488A   2
– 0.89A = =
[Eq. 113]
C15
0.213A 100 F 
0.488A
------------------------------------------- = 43.65 F = [Eq. 114]
l
g
0.5 
o
L
P
I
PK1 FB  
2
B
PK
2
A
e
  
--------------------------    =
0.5 = 1.26 H m    3543H
299mA   2
213mT   2
20.1mm
2 
--------------------------------------------------------   
0.219mm =
[Eq. 115]
N
P
L
P
I
PK1 FB  
B
PK
A
e

---------------------  3543 H
299mA
213mT 20.1mm
2 
------------------------------------------------ 247 =  = = [Eq. 116]
N
S
N
P
N
-------
247
5.57
----------- 44 = = = [Eq. 117]