Hey, I am wondering if there is someone who can help me or maybe just direct me in the right direction. I am busy with a school project where i am building a board that connects to the arduino where i can control the brighness of seven rows, with 4 in series LED's in each row. Each row is a different color. I am using Avago Power PLCC-4 common cathode SMT LED's.
I have a Arduino Duemilanove 328 which is connected to a single TLC5940. For my testing I used single 5mm round LED's and I could turn the LED's on and off and dim them. I have since assembled my LED board with the SMT LED's, but now not one of the rows want to light up. I know that I need to add a bigger voltage source as the 5V from the arduino board wont cut it considering the voltage drop over each LED, max being 4V times 4 = 16V drop. Would a boosted voltage source work? I tried it with a 18V regulator I had lying here, but this is more then the max rated input voltage of the TLC5940 and don't want to risk blowing the chip, and it also doesn't work. I know the rows do work because when i set up the 18V source with a resitor the row lights up. So I don't know if I am doing something wrong, or just missing something simple or being really stupid...
Failing this is there any other driver chip you can recommend for me to use to control the current to the LED's? Any assistance or points into any direction would be greatly appreciated!
Do a Google for "SLVA280" or "Using TLC5940 With Higher LED Supply Voltages and Series LEDs " It is an application note.
max being 4V times 4 = 16V drop.
No the voltage drop on a single LED is about 2.2V so you should be able to light them up with a voltage over 8.8V and the TLC5940 is good to about 12V (check the data sheet).
don't want to risk blowing the chip, and it also doesn't work.
If you know it doesn't work you have probably tried it and blown up the chip.
Thanks for the reply Grumpy_Mike. I have read "Using TLC5940 With Higher LED Supply Voltages and Series LEDs" and now have an understanding of what i need to do.
If think i was mistaken when I said "max being 4V times 4 = 16V drop", i was under the assumption that the voltage drop is the forward voltage specified in the LED datasheet?
I know that a LED is more or less constant voltage. Once the threshold voltage is exceeded current rises rapidly with small increases in voltage, that's normally shown in the forward voltage vs forward current graph in the data sheet.
I had a look at the data sheet, for the white LED with a typical forward Voltage of 3.5V @ IF = 30 mA, with max being 4.35V, and the colored LED's having a typical forward voltage of 2.8V @ IF = 50 mA with max being 3.2V.
I am just a bit confused and maybe you could help me. I would be correct in using the graphs in the data sheets to determined what the forward voltage/drop would be per LED? But having done that, i am in this catch 22 situation, I don't want to make the power supply voltage to great and waste a lot power in unused voltage when i am just driving at low currents, but having said that i want to be able to drive the LED at a higher current if needed.
I know I probable sounds like an idiot, but I have never really played around with power electronics like this, so any advice or general direction you could give me would be greatly appreciated.
Where you are going wrong is in thinking you have to supply the LED with it's forward voltage.
You have to supply the LED with a voltage in excess of it's forward voltage and use other means to control the current.
For small currents this is usually done with a series resistor. However in the TLLC5940 you have a constant current generator so as long as it is above the joint thresholds, the TLC5940 will take care of limiting the current.
You didn't say it was a white LED, 4V is reasonable for that.
I have read through the "Using TLC5940 With Higher LED Supply Voltages and Series LEDs" and have been looking around to find a suitable n-channel MOSFET, but know fairly little about them or which manufacture makes better IC's. I know that need one with a Vds is lower then the 17V max specified, but have no idea where to start looking.
If possible could you maybe just guide me in the right direction, or what type MOSFET you would recommend or use?
I know that need one with a Vds is lower then the 17V max specified, but have no idea where to start looking.
Wrong way round. You need a FET with a Vds greater that that you are going to subject it to. The Vds is a rating of how much voltage it can withstand before it breaks down.
There are thousands of FETs about, there are no "bad" manufacturers to avoid. A lot of it depends on the sort of package you want.
Thank you so much for all your help, I have a couple more questions though, they might be stupid questions, but I just want to make sure I am on the right way.
I am using a 24V power source, so I need a FET with Vds greater then 24V. I have read that transistor needs to have a small Drain-to-Source On-Resistance, how important is it? And how big of an influence does it have on my circuit?
In the "Using TLC5940 With Higher LED Supply Voltages and Series LEDs" they connect the gate pin up to Vcc with a 100ohm resistor. How and where does the Vgs play a role?
I have a bunch of Fifth Generation HEXFETs from International Rectifier, IRLZ44N, with the following ratings,
Vdss = 55V
Rds(on) = 0.022Ohm
Id = 47A
Vgs(th) = 1.0 - 2.0 V
Vgs = -+ 16V
I know they would prob be a bit overkill for that the circuit i have in mind, but do you think they would work for the above mentioned circuit?
a small Drain-to-Source On-Resistance, how important is it?
This affects how hot the FET will get. The smaller the Ron (on resistance) is the less voltage it drops for a given current. As power dissipation is this voltage times the current to minimise the heat minimise the Ron.
they connect the gate pin up to Vcc with a 100ohm resistor
This is to limit the instantaneous current when the gate voltage is first applied. In effect the FET gate is a small capacitor and that resistor limits the charge current. Once charged up there is little or no current flowing from the gate driver.
How and where does the Vgs play a role?
It is the Vgs that turns a FET on. To turn a FET fully on, that is have the minimum Ron you have to have at least what is stated in the data sheet, usually shown as a condition for the Ron parameter. Any less than this and the FET isn't fully on and so it can get hot as stated above. Sometimes you might want a FET to work in this mode but not in your case.
but do you think they would work for the above mentioned circuit?