LEDs Switching; LED driver help needed

[adinogcas]

LEDs Switching

Hi all,

I need advices in this problem. I have an array of LEDs ,say 4-7 LEDs, I want to design a circuit that can turn on and off those LED alternatively. The turn on and off signals are from uC. Should I use IC switch? or use an array of transistor (or FETs)?

Each LED will have 100mA go through to turn it on, so using uC is impossible to turn on the LED so just use it to signal switches to connect LEDs and power supply (3.3V).

The turn on/off will be 500us for each LEDs, for example:
0-500us: LED 1 is on, others are off
500-1000us: LED 2 is on, others are off
1000-1500us: LED 3 is on, others are off
and so on...

I need the switch that can turn LED on and off COMPLETELY. If possible can you provide me the name of the parts? There are hundreds of components...so I need advice on choosing the switches too.

Really appreciate any help in this problem,

 

[IanP]

Re: LEDs Switching

You can use transistors as switches .. see attached picture ..
For 100mA current the good candidates are: 2N2222, BD135, ZTX450 and many others ..

Other option includes logic level ‘N’ channel MOSFETs, for example (STP36NF06) ..
**broken link removed**

Rgds,
IanP

 

[adinogcas]

Re: LEDs Switching

Thank you IanP for replying me so fast ,

The circuit I am going to build is this one (attached img.), but with 4-7 LEDs not just 2. However, with the part I am using for P-FET is AO3423 (not the one in schematic), I can not have it turn on/off completely. All the LEDs I am driving will have forward voltage around 2.5-3V (not over 3.3V) and forward current around 100mA. So that is why I am confusing in choosing the right switches!

Is there any guidelines in choosing FETs? (I prefer FET due to its easiness to calculate current)

 

[IanP]

Re: LEDs Switching

I can not have it turn on/off completely.

You may have problems with Ugs ..

Assuming you're looking for a p-channel MOSFET - Vishay/Siliconix has a number of them ..
Just look for "logic level" on their selector guide ..
For example, the IRF7425 which has 0.013 ohm Rds(on) at Vgs -2.5V ..

Rgds,
IanP

 

[adinogcas]

Re: LEDs Switching

Thanks for the reply,

So for switching application what parameters do I need to know when choosing a FETs? Like you said Rds (on). What else? And can you explain those parameters?

 

[IanP]

Re: LEDs Switching

If you use a MOSFET as a switch there are only 3 main parameters that matter: Uds (max voltage that it can be exposed to), Ugs (how much voltage do you need to switch it on and off) and Rds(on) - impedance/resistance of the channel while switched on - the higher the Rds the hotter it will become when you pass current through it - so, select those with lowest possible Rds(on) ..

Rgds,
IanP

 

[adinogcas]

Re: LEDs Switching

When looking for FETs in digikey.com, I saw there is an option "Rds On (Max) @ Id, Vgs". For example: 12mOhm, 11A, 4.5V. How do you interpret this option?

What I understand is: Vds=Rds*Id=12mOhm * 11A =0.132V

At the V-I curve of FET, the curve will "bend" at 0.132V (Vds) which is at 4.5V (Vgs). If I lower the curve to 3.3V (Vgs), there will be no difference (cause the curve is so steep). That also means, if I apply 3.3V to Vgs, the FET will move to saturation region (turn on) almost immediately, and if I apply 0V to Vgs, it will turn off completely.

Am I right?

P/S: The picture below is for symbolic only, the curve will much steeper near Id-axis.

 

[IanP]

Re: LEDs Switching

You are right, and the graph shows it very clear ..
At Ugs=0 the MOSFET is switched OFF, and at Ugs=4.5V the MOSFET is ON, sat current 11A at Rds(on)<0.012Ω ..
Of course at Ugs=3.3V the sat current will be lower, but if you don't need to go anywhere near sat current then it really doesn't matter much ..

I think I'm going into splitting hairs, so that's it ..

Rgds,
IanP