An368 – Cirrus Logic AN368 User Manual
Page 53
AN368
AN368REV2
53
It is recommended to choose V
Reflected
to equal 37% of V
Zener
. Voltage V
Reflected
is set to 116.6V, leaving
183V overshoot to dissipate as leakage inductance energy. The actual overshoot range is from 65V to 95V,
depending on the clamp zener tolerance. See Equation 82:
c. Determine the Flyback Transformer Turns Ratio
Calculate a turns ratio N using Equation 4:
d. Select the Full Brightness Switching Frequency
The maximum switching frequency F
sw(max)
for the second stage is configured at ~35.7kHz. Register TTFREQ
at Address 46 is set to ‘10001100’, and the minimum switching period is calculated using Equation 5:
The minimum switching frequency F
sw(min)
for the second stage is configured at ~5kHz. Register TTMAX at
Address 38 is set to ‘00011111’, and the maximum switching period is calculated using Equation 6:
The maximum second-stage switching period that is measured by the controller algorithm is configured to
204.75
s. Bits TT_MAX[1:0] in register Config3 at Address 35 are set to ‘11’.
Calculate output voltage V
MODE1
and output current I
MODE1
using Equations 7 and 8, respectively:
The output voltage, V
MODE2
, and output current, I
MODE2
, are calculated using Equations 9 and 10,
respectively:
V
Breakdown
V
BST max
V
Zener
V
M
in
arg
+
+
220V 315V 65V
+
+
600V
=
=
=
[Eq. 82]
N
V
Reflected
V
CH1 max
V
CH2 max
+
--------------------------------------------------------------
116.6V
9.7V 10.3V 0.7V 0.25V
+
+
+
---------------------------------------------------------------------------------
5.57
=
=
=
[Eq. 83]
1
F
sw max
---------------------
TTFREQ[7:0] 4
50ns
=
[Eq. 84]
140 4
=
50ns
28
s
=
1
F
sw min
--------------------
TTMAX[7:0] 128 127
+
50ns
=
[Eq. 85]
31
128 127
+
=
50ns
204.75
s
=
V
MODE1
V
CH1
V
CH2
V
D15
V
D5
+
+
+
9.7V 10.3V 0.7V 0.25V
+
+
20.95V
=
+
=
=
[Eq. 86]
I
MODE1
I
CH2
213mA
=
=
[Eq. 87]
V
MODE2
V
CH1
V
D15
+
9.7V 0.7V
10.4V
=
+
=
=
[Eq. 88]
I
MODE2
I
CH1
I
CH2
–
488mA 213mA
–
275mA
=
=
=
[Eq. 89]