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Topic: Trying to find a mosfet for driving LED array (Read 879 times) previous topic - next topic

jshields

Hello everybody,

I am trying to prototype an LED light that needs 350ma at 3V.  I am using a Microcontroller (probably an Attiny85) for other operations but need a mosfet to be able to control the LEDs.  I will be using a linear driver (probably an AMC7135) to control the current to the LEDs.  My little prototype needs to run on 3AA  alkaline batteries so I need a mosfet that can operate with around 3 to 4.5v.  I am also concerned with efficiency and voltage drop if you know anything about that.

There are so many options out there but the one I found is this:

 http://www.digchip.com/datasheets/parts/datasheet/232/IRL3803.php

Does this look like an appropriate mosfet?  Do you forsee any problems with the circuit?

Thanks any help you can provide :)

jshields

Please note,  I am using the mosfet as a switch not a driver.  Sorry for any confusion.

DrAzzy

#2
Mar 18, 2016, 07:14 pm Last Edit: Mar 18, 2016, 07:17 pm by DrAzzy
In SMD, there are hundreds or thousands of options.

Digikey search form

However - if you need through-hole, you're in a much tighter situation, because you need to switch it with like 4v or less as the batteries are getting low. There's the IRF3708PBF - it's only spec'ed as 4.5v logic level, but if you look at the graphs, it's got decent response with around 3v on the gate. They're kinda pricy though, and bigger than you need. There's also a TO-92 MOSFET that works with 1.5v on the gate..... but 150 or 100mA max current, so that's no good.


As it happens, I sell nice low-voltage logic level SOT-23 MOSFETs pre-installed on breakout boards for those who don't like SMD soldering, as well as the blank breakout boards. Those would work fine for switching a 1W led.

I like AMC7135's too - they're a good part, incredibly low dropout, and you can switch the low side with a fet and it works fine. The 7140 is also a really cool part - but more than you need for this.
ATTinyCore and megaTinyCore for all ATtiny, DxCore for DA/DB-series! github.com/SpenceKonde
http://drazzy.com/package_drazzy.com_index.json
ATtiny breakouts, mosfets, awesome prototyping board in my store http://tindie.com/stores/DrAzzy

jshields

Thank you for your help.  The SOT 23 sounds familiar but I must have forgotten in among all of the options.  Your breakout boards look appealing but I don't see the reason for the connection between the gate and source.  I will be opening the gate with a microcontroller and the source will be straight from the batteries.  If you can explain how your circuit works I would love to skip mounting these components for my prototype.

MorganS

If you're already using a driver chip, look for one that has an ENABLE or SLEEP input pin. I looked at a datasheet for the AMC7135 and it might work if you drive Vdd low but it doesn't say what happens in that state - it may lock on fully. It only draws 200μA on that pin in normal operation so it should be OK to hook directly to an Arduino output.
"The problem is in the code you didn't post."

DrAzzy

The resistor between gate and source makes sure that when the gate is not connected to anything, it will be off. Otherwise, if the pin is floating (for example, connected to an AVR chip, but you're loading a sketch onto it, or the chip is in reset, or something like that), it will turn on and off randomly due to ambient electomagnetic fields. So my breakouts integrate the resistor you almost always need.
ATTinyCore and megaTinyCore for all ATtiny, DxCore for DA/DB-series! github.com/SpenceKonde
http://drazzy.com/package_drazzy.com_index.json
ATtiny breakouts, mosfets, awesome prototyping board in my store http://tindie.com/stores/DrAzzy

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