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What specifically causes the Tinyswitch 4 to go into autorestart?

Posted by: willieb3 on Fri, 04/30/2021

I am trying to do some troubleshooting/debugging on a power supply built using a Tinyswitch 4. I have removed the tinyswitch from the board and isolated it in it's own circuit, and basically apply 30VDC at the drain pin. The BP/M min has a 0.1uF cap to source, and for the EN/UV pin I have placed a potentiometer to adjust the current leaving between 25 uA to 120 uA. My hopes were that I could use the potentiometer to adjust the current, and there by change the duty cycle.

The problem is, the TNY is going into restart mode! From the TNY datasheet:

"An internal counter clocked by the oscillator is reset every time the ENABLE/UNDERVOLTAGE pin is pulled low. If the ENABLE/UNDERVOLTAGE pin is not pulled low for 64 ms, the power MOSFET switching is normally disabled for 2.5 seconds (except in the case of line undervoltage condition, in which case it is disabled until the condition is removed)."

I don't understand this. The internal counter will reset when the EN/UV is pulled low (from what I understand below 25uA), does this mean it will enter Auto-restart mode? It also says if it is not pulled low for 64ms, then it will enter Auto-restart. So in which case will it not enter auto-restart? Does the EN/UV pin need to have a fluctuating current? How would a fluctuating current be maintained in a regular power supply?

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