VIPER22A-E STMicroelectronics, VIPER22A-E Datasheet - Page 9

• Manufacturer Part Number: VIPER22A-E
• Description: Fixed frequency off line converter
• Manufacturer: STMicroelectronics

Specifications of VIPER22A-E

• Constant Current Output: 350 mA, 700 mA, 1.05 A (selectable)
• Input Voltage Range: 90 to 264 VAC 50/60 Hz

VIPer22A-E, VIPer22ADIP-E, VIPer22AS-E
The Power MOSFET delivers a sense current I
R2 receives this current and the current coming from the FB pin. The voltage across R2 is
then compared to a fixed reference voltage of about 0.23 V. The MOSFET is switched off
when the following equation is reached:
By extracting I
Using the current sense ratio of the MOSFET G
The current limitation is obtained with the FB pin shorted to ground (V
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
In a real application, the FB pin is driven with an optocoupler as shown on
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 0 V.
For low drain currents, the formula (1) is valid as long as IFB satisfies I
I
stop switching. This is represented on
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.
FBsd
is an internal threshold of the VIPer22A. If I
Dlim
:
S
:
I
Doc ID 12050 Rev 2
D
I
Dlim
=
R
G
2
ID
⋅
=
(
Figure 12 on page 14
I
I
⋅
S
S
G
I
I
FB
S
+
ID
=
I
=
FB
⋅
0.23V
--------------- - I
=
0.23V
R
G
)
–
2
ID
s
0.23V
--------------- -
=
ID
which is proportional to the main current Id.
R
⋅
FB
0.23V
–
⎛
⎝
:
1
⋅
0.23V
--------------- - I
⎛
⎝
FB
exceeds this threshold the device will
------ -
R
R
1
2
2
+
–
------ -
R
1
, and I
1
FB
⎞
⎠
⎞
⎠
FBsd
value is specified in the
FB
FB
= 0 V). This leads
< I
Figure 5.
FBsd
Operations
, where
which
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