Yet another startup problem with TNY280
Posted by: perihelius
on
I have a startup problem with a 12V .5A buck supply using the TNY280. The supply starts up with a resistive load >57 ohms and operates normally when I decrease the load down to 24 ohms. If the load is less than 57 ohms the supply gets stuck in a perpetual restart cycle.
During startup it seems to skip 4 cycles so I assume it does not deliver enough energy to bring the output higher than ~5V. After ~220ms with the EN/UV never pulled low it restarts. The output is flat at 5V for 210+ ms.
-Why does it skip cycles on startup?
-Why it restarts after 220 ms and not 64ms?
-What am I missing?
Thanks
Hello,
Could you please clarify what you mean by "it skips cycles on startup"? During startup and overload conditions (when conduction time is less than 400 ns) the device will reduce the switching frequency. Perhaps this is what you're observing? Attach a scope plot if possible to better describe what you're seeing.
Once the power supply enters auto-restart it should restart about ever 2.5 seconds, not 220 ms and not 64 ms.
If the EN/UV pin is not pulled low (meaning that optocoupler has not drawn any current from this pin) for 64 ms then the MOSFET is disabled for 2.5 seconds. After this time has passed the supply with try to start again. This assumes that there is no line undervoltage condition present.
Lastly and maybe most importantly, what value bypass capacitor (the capacitor connected to the BP/M pin) are you using? It looks like it is 100 uF. The bypass capacitor should not be this large. It should either be 0.1 uF for standard current limit, 1 uF for reduced current limit or 10 uF for increased current limit.
Also to help simplify this problem removing the UV resistor connected to the EN/UV pin can be helpful.
I hope this helps!
-Wesley