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Powering soft-start/soft-stop SMPS

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king.oslo

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Hello there,

I've chosen a MAX8792 buck for my SMPS. Here is the datasheet: https://www.electro-tech-online.com/custompdfs/2012/01/MAX8792.pdf When pin EN is pulled high and VDD is 5V, soft start is triggered. When EN is pulled low, the buck ramps down Vout, and goes to sleep.

Ideally, I'd like to use a switch to cut the AC power on start up/shutdown. How is this whole operation normally executed? I made this suggestion. See the attached schematic.

What are your views?

Kind regards,
Marius
 
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Normally you just switch the AC power and the converter starts up and shuts down from the application and loss of its DC input power.

Do you have a particular reason you want to switch both at the same time?
 
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I think I would use 39k to +18.9 and 10k to ground - no diode. I'm thinking your schematic is wrong in the polarity of the bulk supply??
The 75000 Ufd caps on the bulk supply will make the lights dim from inrush as well.:D What are the current and voltage specs you are after.
 
On another look it is just that C1 is backwards. The + should go to the positive suppy voltage.
But seeing your wide voltage range makes me think a simple divider may not work well.
If you set the enable voltage to 2.2 volts (maximum turn on spec) at say 15 volts (higher than 14.4 by a small margin) then you got a part that turned on at 1.2 volts (minimum turn on spec.) it would want to turn on at 8 volts or so. (below the 14.4 spec. you have). A better solution might be to use a comparitor set to 15 volts to just switch the enable between 0 and 5 volts.
 
Datasheet says it has soft start/stop feature to avoid inrush problem. This is the only reason.M
I can see a reason for a soft start but not a soft stop (whatever that would be). If you want a soft start just use a RC filter delay to generate the soft-start signal from the transformer output DC power. Then when you apply AC power to the circuit, the converter will soft-start shortly after, as determined the RC delay time.
 
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