LNK304 - Output voltage regulation is not working
hi,
I have designed a power supply using LNK304DG for 3.3V/100mA (constant voltage)if I connect a load, the voltage drops to about 2.8V or lower. What is wrong with the circuit?We used the application note AN-37 to design the circuit: A High-Side Buck-Boost – Direct Feedback SMPS Some inputs:
VACmin = 195 Volt / VACmax = 275 Volt / L = 680 uH / R_FB = 2.06 kO / R_BIAS = 2.2 kO / CO is a RR71C680MDN1 with 7 mOhms ESR
VO_real is about 3.26V without additionnal load (only RPL 1.1kohms) (~3mA), VO_real is about 2.94V with an additionnal load of 390ohms (~11mA), VO_real is about 2.70V with an additionnal load of 68ohms (~50mA), VO_real is about 2.62V with an additionnal load of 36ohms (~95mA)
Thanks for any help
Comments
Well, my problem is that my load will range from 10mA to 100mA. The application note doesn't account for load when choosing Vo.
If I adjust R_fb for 3.3V at 100mA (Something I can't do, without trial and error, since there is nothing in the application note) I will fry my uC when disconnecting the load.
Is there a better way to fix the PSU ?
is it possible to build a constant voltage alim with LNK304 ?
Hi, we tried a lot of things:
We adjusted to feedback resistor to 4.4k to produce a 5.1V output, then added a 3.3V LDO to regulate the output. Without a load we have approximately 3.3V.
We added a 33Ohms load to test the output at 100mA and the output voltage dropped to 2.2V.
It seems we can't draw more than 70/80mA from the PSU.
Things we tried:
- Doubling CIN. No effects.
- Doubling LIN no effects
- Doubling Lout no effects
Did we missed something ?
LinkSwitch-TN is not intended to provide the best output voltage regulation. In order to provide 3.3V at 100mA, you need to load the output of your design to 100mA and observe the output voltage. Adjust the value of Rfb until your full-load output voltage is within regulation.
If you're finding that LNK304 is not able to meet the power demands of this design, you might try moving to a larger device such as LNK305 or LNK306.
-The Traveler
If the feedback components have been chosen such that the output voltage is 3.3V at no-load this would explain the voltage drop at full-load. Try adjusting your feedback resister values for the full-load output voltage.
-The Traveler