Hello guys im working on small project, its LED billboard with ten letters, it has 98 series with 6 LED in each. I want to be able to power up and down every letter with ATmega16 microcontroller. I have selected IRL510 as N-ch mosfet to act as a switch for currents 0.5A +-0.2A. My question is can i wire mosfet directly to ATmega16 and can somebody draw me a circut i have uploaded my vison of circut. I have only 10 mosfets, but atmega16 has 32 O/I, would it help to wire 2 pins of microcontroller to the gate of mosfet. Please somebody explain or give me some suggestion of how to solve the switching off and on LED with atmega16 and IRL510. Thank you. http://www.pixhost.org/show/912/23146358_001.jpg

 

Sure you can drive a logic level mosfet directly from micro. All you need is to add a "pull-down" resistor.

 

You might wish to consider using IRLD014PBF or IRLD024PBF instead; they are 1.7A and 2.5A logic-level MOSFETs respectively which have a lower ON-resistance than the IRL510, and they come in a 4-pin DIP package which is easier to breadboard with; the TO220 package of the IRL510 will tend to overstretch the sockets on a breadboard, making connections unreliable.

 

Sure you can drive a logic level mosfet directly from micro. All you need is to add a "pull-down" resistor.

You mean that i connect output of micro directly to mosfet gate and between micro and mosfet gate i put resistor to ground.
How much 1k,5k,10k?

 

You might wish to consider using IRLD014PBF or IRLD024PBF instead; they are 1.7A and 2.5A logic-level MOSFETs respectively which have a lower ON-resistance than the IRL510, and they come in a 4-pin DIP package which is easier to breadboard with; the TO220 package of the IRL510 will tend to overstretch the sockets on a breadboard, making connections unreliable.

I have chosen IRL510 because of its input capacitance i have been told that is the main factor besides Rds for driving mosfet from micro.
IRL510 has 250pf input capacitance, IRLD014PBF has 400pf and IRLD024PBF has around 700 if i recall well. :/

 

How fast do you need to go?
If your ATmega can supply 1 ma into 700 pf, you can get the mosfets on in about 3.5 microseconds.

 

50-500 miliseconds

How fast do you need to go?
If your ATmega can supply 1 ma into 700 pf, you can get the mosfets on in about 3.5 microseconds.

 

Just to add some words i found about "pull-down" resistor. For my case i think i will go with 10k?!?


Think about what the purpose of a pulldown is and when it matters. During normal operation the gate is generally actively driven both ways. A pulldown resistor does nothing useful then, and a best doesn't get in the way.

Usually the purpose of a pulldown is to keep the FET off during startup while the active gate drive circuit is high impedance. This could happen, for example, if the gate is being directly driven from a microcontroller pin. It may be 10s of ms before the micro's clock starts running and it gets around to executing the instructions that put the pin in a known output state. That could be bad if the FET should only be on for a few µs at a time to prevent some inductor from saturating, for example. In cases like that, not only could the FET waking up on cause excessive current, but that excessive current could actually prevent the supply from coming up all the way, essentially latching the circuit in crowbar mode indefinitely.

So what are the criteria for deciding the value of the pulldown? On one end, the resistance needs to be low enough so that the gate is discharged in time, and can be held in the low state despite capcitive coupling from startup transients. The gate of a FET has very high resistance and mostly looks capacitive. Even a large resistor can eventually discharge the gate capacitance. The limiting factor there is how fast the device might be turned off and then back on again. Usually this isn't the issue though. Keeping the gate low despite startup transients is much harder to judge since it's almost impossible to know where these transients may be coming from and how strongly they will couple onto the gate node. This is why you see such a range. Nobody really knows what's needed, so they experiment and derate, or more likely, pick a nice number. Different people's idea of nice varies.

On the other end, you don't want the pulldown to draw significant current that would otherwise go to driving the gate high quickly or at all. If you're using a FET driver that can source 1 A during switching, the extra 10 mA from 1 kΩ pulldown is pretty much irrelevant. On the other hand, if the gate is being driven straight from a micro pin, then the extra 5 mA of a 1 kΩ pulldown could be a significant inconvenience. In that case, 10 kΩ would be better. It's rarely necessary to go higher than that, but in some low power circuits where the FET is on for long periods of time you might want 100 kΩ.

So like I said, 1 kΩ, 10 kΩ, or 100 kΩ ought to work.

 

Can somebody verify this circuit, will it work? i connected two pins on each mosfet gate that will be at the same time logic 1 or logic 0. The mosfets on schematic are IRL510 not IRFZ44N. Thank you. http://www.pixhost.org/show/919/23153865_001.jpg

 

It looks to me like this will work.

 

This is not a good idea to connect micro pins together. This is a disaster waiting to happen.

PS. recently I drive a mosfe's (3xIRL630) from ATtiny13 using only single pin without any problem.

 

My bad. I know nothing about microprocessors and would have assumed the pins switch simultaneously. Perhaps a resistor in series with each drive pin? Or would that simply defeat the intent of using two pins?

And why use two pins when the current from one pin will easily drive the mosfets way faster than 50 milliseconds?

 

My bad. I know nothing about microprocessors and would have assumed the pins switch simultaneously.

In theory they should switch simultaneously. But what about in "start up" or when reset?