An368 – Cirrus Logic AN368 User 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
3543H
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]