Datasheet

VIPer22ADIP/ VIPer22AS - E Operations
Rev1 9/20
By extracting I
S
:
Using the current sense ratio of the MOSFET G
ID
:
The current limitation is obtained with the FB pin shorted to ground (V
FB
= 0V). This leads to
a negative current sourced by this pin, and expressed by:
By reporting this expression in the previous one, it is possible to obtain the drain current
limitation I
Dlim
:
In a real application, the FB pin is driven with an optocoupler as shown on Figure 4. which
acts as a pull up. So, it is not possible to really short this pin to ground and the above drain
current value is not achievable. Nevertheless, the capacitor C is averaging the voltage on
the FB pin, and when the optocoupler is off (start up or short circuit), it can be assumed that
the corresponding voltage is very close to 0V.
For low drain currents, the formula (1) is valid as long as IFB satisfies I
FB
< I
FBsd
, where
I
FBsd
is an internal threshold of the VIPer22A. If I
FB
exceeds this threshold the device will
stop switching. This is represented on
Figure 11 on page 13, and I
FBsd
value is specified in the
PWM COMPARATOR SECTION. Actually, as soon as the drain current is about 12% of
Idlim, that is to say 85 mA, the device will enter a burst mode operation by missing switching
cycles. This is especially important when the converter is lightly loaded.
Figure 5. I
FB
Transfer function
It is then possible to build the total DC transfer function between I
D
and I
FB
as shown on
Figure 5 on page 9. This figure also takes into account the internal blanking time and its
associated minimum turn on time. This imposes a minimum drain current under which the
device is no more able to control it in a linear way. This drain current depends on the primary
inductance value of the transformer and the input voltage. Two cases may occur, depending
on the value of this current versus the fixed 85mA value, as described above.
I
S
0.23V
R
2
--------------- - I
FB
–=
I
D
G
ID
I
S
⋅ G
ID
0.23V
R
2
---------------- I
FB
–
⎝⎠
⎛⎞
⋅==
I
FB
0.23V
R
1
----------------–=
I
Dlim
G
ID
0.23V
1
R
2
-------
1
R
1
-------+
⎝⎠
⎛⎞
⋅⋅=
I
FBsd
I
Dlim
I
FB
t
ONmin
V
2
⋅
IN
L
------ --------- --------- ---------------- -
t
ONmin
V
1
⋅
IN
L
------ --------- --------- ---------------- -
85mA
I
Dpeak
0
Part masked by the I
FBsd
threshold