voltage divider feedback with tny 274
"Under most operating conditions (except when close to noload),
the low impedance of the source follower keeps the
voltage on the EN/UV pin from going much below 1.2 V in the
disabled state. This improves the response time of the
optocoupler that is usually connected to this pin."
i see this information in tny274 datasheet..
is voltage divider feedback possible with tny 274?
Comments
The EN pin is design to be a current input. The disable condition is determined by a current equal to the datasheet parameter IDIS (115 uA from EN pin to SOURCE pin). When using opto coupler feedback this is ideal as the voltage of the EN pin is not critical.
As PI-Crusher mentioned the assumption is that you are interested in a non safety isolated design where the output can be sensed directly without an opto. The issue I can see with just using a resistor divider to sense the output voltage is the tolerance of the EN pin voltage it does not make a good reference (~+/-33%).
A better option is to use a transistor to interface with the EN pin. See:
http://www.powerint.com/sites/default/files/PDFFiles/di122.pdf
Here a zener (VR1) is used as the reference but if the tolerance and temperature variation is acceptable the transistor VBE voltage could be used directly and VR1 removed. The resistor divider would then be R4 and R7, the value of R4 increased.
Cheers
PI-Chekov
Hello savage67,
For TNY274 you must use an optocoupler, unless you do not need an isolated output. On this case I suppose you can just use some dividers. You have to pay attention in this case to your feedback circuit.
Cheers,
PI_Crusher