Hi All
I know this is very basic stuff but any help would be appreciated. I need to spec a voltage regulator chip and a mosfet. I'm running off a 18650 Lithium battery and can get surges of 20A (at least that is what I'm told). The voltage regulator will have input of 3.7V and output of 3V. The output will be used to bias the mosfet and run a 555 chip for a pwm.
So when I look at voltage regulators which value do I check to make sure I won't fry the chip? Same with the mosfet, it will be handling 3.7 V at about 6A but might see a 20A surge. Which datasheet value do I need to check to make sure I'm ok.
thanks much

 

What is the maximum duration of the surge? If it's very short then maybe a capacitor can handle the surge to relieve the regulator. A MOSFET to handle 20A isn't a problem.

 

Surge would be very short as in something burned out. I know that the parts are easily available but I want to make sure I am picking out the correct part. I'm confused by some of the specs and not sure which one to look at.
This part looks fine, overkill even, 60V 16A continuous. But how do I find out about the surge, will this take 20A for a flash?
http://www.digikey.com/product-detail/en/stmicroelectronics/STB16NF06LT4/497-4322-1-ND/725594
For the voltage regulator if it is 5V 1A is that the input amperage or the output amperage? If my input amperage could surge do I need a 5V 20A voltage regulator?
thanks very much

 

Maybe one like this. A little cheaper I think.
http://www.infineon.com/dgdl/IPP055N03L_rev1.04.pdf?fileId=db3a30432313ff5e01239ec9d89070ad
If all you are running from the 3 volts is a 555 and gate drive for the FET you don't need a regulator. Just run them off the battery.

 

Thanks much. So the 555 can be run directly from the 3.7V 18650 battery? As the battery drains won't that effect the circuit?
On the datasheet is the Continuous drain current (Id) the spec that I'm looking for? The part Ronv mentions has a value of 50A and a Vds of 30V.
If I do use a voltage regulator is the amp rating in the datasheet input limit or max output current?

 

HI,

YOU CAN SEE THIS DATA ON W.W.W:ALLDATASHEET.NET. TRY AND SEE.

 

Thanks much. So the 555 can be run directly from the 3.7V 18650 battery? As the battery drains won't that effect the circuit?
On the datasheet is the Continuous drain current (Id) the spec that I'm looking for? The part Ronv mentions has a value of 50A and a Vds of 30V.
If I do use a voltage regulator is the amp rating in the datasheet input limit or max output current?

The '555 timings are pretty much unaffected by supply voltage.
The regulator rating you need to consider is the maximum output current.

 

what value from the datasheet tells me what the maximum input current that the voltage regulator can take? For this application an output current of 500 mA is probably sufficient but the surge might be 20A. I see current output and current limit but not sure if either of these is what I need to consider. thanks for the help

 

As your surge is very short I would suggest doing some calculations on the surge current, the surge time and select a capacitor that can supply the surge current for long enough to keep the supply voltage high enough.
it = CV
C = it/V
Let's make the surge 1uS long and 20A. Let's say the acceptable voltage drop is 0.1V:
C = 20A * 1uS / 0.1V = 220uF
So a 220uF capacitor will be sufficient to absorb that 20A pulse, but it will need to be very low ESR.

Otherwise you need a regulator which can supply 20A which is definitely overkill.

 

You seem to be confused. If the maximum output current of the regulator is 500mA the maximum input current is 500mA + a few more milliamps used by the regulator itself. It cannot have an input of 20A and and output of 500mA. Where would all that current go if it did?

Does you load actually draw 20A at times, or are you basing that on the maximum surge that the battery can produce? If the latter, that is nothing to worry about, it will not "produce" 20A unless something is trying to draw 20A. I.e. probably a short circuit. And most regulators have the max output current limited, so that would never happen even if you did short circuit the output.

Bob

 

Yes I am confused. The surge would be from a short. Normally the load, which is not on the regulator, would be about 10A max.
the regulator would just be diverting power to the 555 and other circuit components. The only time the circuit board sees high amps is when the mosfet opens and that is just on the drain/source. So I just need to be sure that the regulator isn't going to fry, I'm guessing 1.5A regulator would be ok, is that right?

 

So this surge is nothing to do with the regulator?
So why would the surge affect the selection of the regulator output current?
You're confused? I certainly am.

 

I'll post a schematic soon

 

Here is what I was thinking. Voltage reg supplies 555. 555 triggers mosfet to complete circuit to load. If the load shorts and there is a surge will the voltage reg even see it? If the voltage reg does see the surge then I have to get one rated at the surge amps or use a capacitor?
thanks for the input.

 

If the load shorted all the regulator would see is its input would fall.
Also you don't need the regulator at all. The '555 can be powered direct from the battery.

 

thanks to all