Application Note
11
2011-07-06
As discussed in section 2.2.7, the lowest switching
min
f
will be seen in full load operation at
min
_
in
V
. In this
section, how the
min
f
is actually set by the IC is explained.
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
min
F
R
is connected to pin FREQ. For
kHz
f
30
min
=
, the
corresponding
FREQ
R
found from Figure 3 is 50k
Ω
. A standard value resistor of 51k
Ω
is selected
for
min
F
R
.
The maximum operation frequency can possibly be seen when maximum input voltage, say 425V, is
applied, and the converter run in no load condition (
0
=
Q
), if burst mode is disabled. The gain in this
condition can be given as:
94
.
0
1
*
425
400
max
_
_
min
=
=
=
nom
in
nom
in
M
V
V
M
[14]
From the gain equation, we get:
)
0
(
,
)
1
(
)
1
(
)
,
(
min
2
2
=
=
−
−
=
Q
M
m
F
m
F
Q
F
G
[15]
The corresponding normalized frequency
max
F
can be found by:
13
.
2
1
1
min
=
+
−
=
mM
m
F
Therefore
kHz
kHz
F
f
180
85
*
max
=
=
.
For 180 kHz switching frequency, the corresponding equivalent resistance
eq
R
at FREQ pin is 7.5k
Ω
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
reg
eq
R
R
R
FMIN
//
=
The
reg
R
is calculated to be 8.8k
Ω
. A standard value resistor of 8.2k
Ω
is selected for the actual design.
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.
The corresponding impedance of the resonant network during over-current can be estimated as:
[16]
During over-current, the load impedence is considered to be shorted, and thereofore the impedance of
the resonant network can be calculated as:
[17]
A
I
I
rms
in
rms
ocp
47
.
2
06
.
2
*
2
.
1
2
.
1
max
_
_
_
=
=
=
Ω
=
=
=
73
47
.
2
*
2
*
400
_
π
ocp
rms
in
ocp
I
V
Z
r
ocp
r
ocp
r
ocp
r
ocp
ocp
C
f
L
f
C
f
j
L
f
j
Z
*
2
1
*
2
*
2
*
1
*
2
*
π
π
π
π
−
=
+
=