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As discussed in section 2.2.7, the lowest switching
section, how the
is actually set by the IC is explained.
f
min
Based on the definition of oscillator as in the datasheet and the external circuit around pin FREQ in
Figure 1, the minimum switching frequency will be achieved when pin SS is 2V (usually after softstart),
opto-coupler transistor is open and only
corresponding
R
FREQ
for
.
R
min
F
The maximum operation frequency can possibly be seen when maximum input voltage, say 425V, is
applied, and the converter run in no load condition (
condition can be given as:
V
_
=
in
nom
M
M
min
nom
V
_
max
in
From the gain equation, we get:
−
2
(
) 1
F
m
=
(
,
)
G
F
Q
2
−
(
) 1
F
m
The corresponding normalized frequency
1
=
F
−
+
1
m
mM
min
=
*
85
Therefore
f
F
max
For 180 kHz switching frequency, the corresponding equivalent resistance
according to Figure 3. Under no load normal operation, pin SS is already 2V after soft start, and collector
of opto-coupler transistor is pulled to ground, therefore
=
//
R
R
R
eq
reg
FMIN
The
is calculated to be 8.8kΩ. A standard value resistor of 8.2kΩ is selected for the actual design.
R
reg
2.3.3
Frequency setting for OCP:
Assuming the maximum rms current during over-current should be limited by the IC to 1.2 times the
maximum normal operation, i.e.
=
1
2 .
I
I
_
_
_
max
ocp
rms
in
rms
The corresponding impedance of the resonant network during over-current can be estimated as:
400
V
_
=
=
in
rms
Z
π
ocp
*
I
ocp
During over-current, the load impedence is considered to be shorted, and thereofore the impedance of
the resonant network can be calculated as:
π
=
*
2
*
Z
j
f
L
ocp
ocp
r
Application Note
R
found from Figure 3 is 50kΩ. A standard value resistor of 51kΩ is selected
400
=
=
1 *
. 0
94
425
=
=
( ,
) 0
M
Q
min
F
max
=
. 2
13
=
180
.
kHz
kHz
=
=
1
2 .
*
. 2
06
. 2
47
A
*
2
=
Ω
73
. 2
47
1
+
=
2
π
*
2
*
j
f
C
ocp
r
will be seen in full load operation at
f
min
is connected to pin FREQ. For
min
F
=
0
), if burst mode is disabled. The gain in this
Q
can be found by:
1
π
−
*
f
L
π
ocp
r
2
*
f
C
ocp
11
V
=
f
min
at FREQ pin is 7.5kΩ
R
eq
r
. In this
_
min
in
30
, the
kHz
[14]
[15]
[16]
[17]
2011-07-06
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