VIPER12ASTR-E STMicroelectronics, VIPER12ASTR-E Datasheet - Page 10

• Manufacturer Part Number: VIPER12ASTR-E
• Description: IC OFFLINE SWIT PWM SMPS 8SOIC
• Manufacturer: STMicroelectronics
• Series: VIPER™r

Specifications of VIPER12ASTR-E

• Output Isolation: Isolated
• Frequency Range: 54 ~ 66kHz
• Voltage - Input: 8 ~ 50 V
• Voltage - Output: 730V
• Power (watts): 5W
• Operating Temperature: 25°C ~ 125°C
• Package / Case: 8-SOIC (0.154", 3.90mm Width)
• Operating Temperature (max): 150C
• Operating Temperature (min): -40C
• Pin Count: 8
• Mounting: Surface Mount
• Package Type: SO
• Case Length: 5(Max)mm
• Screening Level: Automotive
• Mounting Style: SMD/SMT
• Operating Supply Voltage: 0 V to + 50 V
• For Use With: 497-6413 - BOARD EVAL VIPER12A LP AC/DC497-5084 - EVAL BOARD TRAVEL ADAPTOR 3.6W497-5082 - EVAL BOARD FLYBACK PWR SUPPLY 6W497-4932 - BOARD POWER SUPPLY 1.8W VIPER12A497-4931 - BOARD 6W DUAL OUTPUT VIPER12A497-4930 - BOARD 6W SINGLE OUTPUT VIPER12A
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 497-6792-2; VIPER12ASTR-E

Feedback pin principle of operation
10/21
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
the PWM COMPARATOR SECTION. Actually, as soon as the drain current is about 12 % of
Idlim, that is to say 50 mA, the device will enter a burst mode operation by missing switching
cycles. This is especially important when the converter is lightly loaded.
Figure 6.
It is then possible to build the total DC transfer function between I
Figure 6 on page
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 50 mA value, as described above.
FBsd
is an internal threshold of the VIPER12A. If I
Dlim
I
FB
:
transfer function
10. This figure also takes into account the internal blanking time and its
Doc ID 11977 Rev 2
I
Dlim
=
Figure 12 on page
G
I
FB
ID
⋅
=
0.23V
–
0.23V
--------------- -
R
1
⋅
FB
⎛
⎝
------ -
R
1
exceeds this threshold the device will
2
+
------ -
R
1
14, and I
1
⎞
⎠
D
FBsd
and I
FB
FB
value is specified in
FB
= 0 V). This leads
< I
Figure 5
as shown on
FBsd
VIPER12A-E
, where
which