On\Off as a fuse on a switching regulator (LM2596)

[Alex30]

So I just wanted to run this by the forum before I etch a board to test this.

I am building a power supply and already have a current limiting circuit that works. This current limiting works in the usual manner of using a comparator to turn on a transistor and drive the voltage adjust low on the adjustable linear regulator.

I wanted to add a secondary mode that you can switch on that would turn the power supply off when it is over the set current (like a re settable fuse). My idea is to use the comparator's output to latch an SCR open that would then apply a positive voltage to the ON\OFF pin of the switching regulator, that way when the comparator's voltage returns to low due to there being no current, the SCR will still remain open and keep driving the ON\OFF pin high. That is until the supply is manually switched off at the power switch which allows the SCR to reset.

My main concern is that I'm not sure of the On\Off state time on the regulator as it is not shown

I am wondering if anyone has any idea of the normal transient response of an On\Off pin for a switching regulator and if they have used this pin in this way before.

Also I am still working on a schematic so I do not yet have one, hopefully my description is adequate enough to not need me to finish my schematic to ask this question.

[Paul Moir]

What you're describing is *almost* a crowbar protection circuit.  The only difference is rather than shutting down the regulator, the crowbar SCR shorts the output.  That way no matter how messed up the regulator is, nothing unsafe is outputted.  No worries about settling times etc.  Would this be a better way to go for you?
 
("Crowbar" circuit meaning:  throw a figurative crowbar across the power supply output to short it out.)

[Alex30]

Originally I had planned to just do that but I thought it might be a nicer solution to use the On\Off pin if possible\practical. It's not so much a matter of fusing the rest of the power supply, it's more of a feature I wanted for things I'm going to use the supply for.

[Seekonk]

I am not clear what you are trying to do.  Pin #5 is very fast to turn the regulator off and on.  I have used it to make a 2596 a power point controller for solar. A comparator could be used to make it latch off or constant current. 

[Alex30]

I am not clear what you are trying to do.  Pin #5 is very fast to turn the regulator off and on.  I have used it to make a 2596 a power point controller for solar. A comparator could be used to make it latch off or constant current.

That sounds good. I basically have a shunt resistor and an opamp that is sensing current. When the voltage output of this opamp is over that I have set using a variable resistor it drives a comparator. Only thing is that if I drive the On\Off pin directly from this comparator it would most likely just ring the regulator on and off (as the supply is turned on and off). This is why I am planning to use an SCR (thyristor) where the voltage from the comparator will upon the latch on the SCR and keep it turned on and driving the On\Off pin high until I power down the entire supply

[macboy]

You can add a large amount of hysteresis to your comparator, so that after tripping, even zero current will not turn off the output. Then, add a pushbutton to yank the appropriate input of the comparator to the appropriate level to 'reset' it (overriding the effect of the hysteresis).

Even if you are not up to calculating the input and positive feedback resistors, a few minutes of trial and error with a simulator should get you there. Once you have the resistor values, it is a very simple modification to your existing circuit, just adding or changing two or three resistors.

[void_error]

Another way to do it is to add a monostable after the comparator so whenever the current limit is reached the regulator is turned off for a certain period of time then back on and will remain in that state untill the next time the limit is reached. This is called hiccup mode overcurrent protection.

[rob77]

yet another aproach is to drive a set input of a R-S flip flop with the comparator - that way you will have a latching over current protection with a possibility of reset. you can make the R-S flip-flop from a comparator, a comparator have a open collector output => it's compatible with virtually anything.

[Alex30]

Thanks for the suggestions. A monostable sounds like a really nice idea, might go with that. Thanks!

[dannyf]

My idea is to use the comparator's output to latch an SCR open that would then apply a positive voltage to the ON\OFF pin of the switching regulator, that way when the comparator's voltage returns to low due to there being no current, the SCR will still remain open and keep driving the ON\OFF pin high.

Fundamentally sound and certainly doable - essentially a craw bar.

You may get by without using a comparator as the SCR's threshold level is essentially a comparator.