And I have experimented with TLC4950 on my bread board. If you apply 12V to a single led attached to one of the channels then the TLC4590 can't completely shut off the LED. It's logic circuit runs of 5V and can only apply 5V to the channels. Same principal as trying to turn off a PNP transistor with the voltage applied to the base less that the voltage applied to the emitter.
Hi,
It´s not a problem of the TLC4950, it´s a problem of your circuit that it can´t shut down the LEDs completely.
--> The TLC4950 is designed to properly work with up to 17V LED voltage. And it can drive up to 120mA. Enough to supply 8 LEDs with 15mA each.
You overcomplicate the circuit. Causing much trouble.
Calculate the reisitors for 15mA at 3,9V (across LED and resistor). Supply the whole circuit with 5V and you are done.
No need for 12V, no need for an extra voltage regulation circuit, no need for extra BJTs, reduced heat. Where is a drawback?
This causes about 2W per IC of power dissipation. Thus I recommend a proper thermal PCB layout and additionally I´d glue a heatsink onto each driver IC.
Klaus
Hi,
please use a simulation tool, or use a pencil, paper and (mostly) ohm´s law.
I already spent a lot of time. But it´s not my project.
If you come back with your calculations and found out that it does work, then I will be here to find the mistake.
Klaus
See, you have already tied the base BC516 or BC517 to the ground. The transistor cannot work if you tie the base to the ground.
The output channels in the LED drivers are constant current sources - you need to lift the base of the driving transistor from the ground and apply suitable voltage so that some reasonable current goes in the base that can turn on the transistor hard.
I am not sure how your circuit is going to work at all.
Because your simulation is not realistc.It works at this level at least.
Use two LEDs. Run one LED with 15mA and the other with 18mA.Nor can I avoid the transistors on the TLC4950 channels. 8 x RGB blue at 18mA = 144mA. The TLC4950 can sink a max of 120mA per channel.
Hi,
Because your simulation is not realistc.
I don´t want to repeat the issue with the constant current driver outputs of the TLC4950...
Connecting Q1, Q3 and Q4 to the TLC4950 will result in increased emitter voltage of those transistors.
Thus you need higher voltage at the LEDS (referenced to GND)
and thus you need higher voltage at emitter of Q2
As soon as this voltage exceeds about 6.1V you may get in trouble with your Q5 circuit.
Additionally Q2 will become hot. Thus V_BE will decrease.. the output voltage referenced to GND will increase...
****
Use two LEDs. Run one LED with 15mA and the other with 18mA.
How big is the difference in brightness. Maybe you can see a difference when they are next to each other, but are you able to see the difference when they are 10 cm apart?
And did you do a current measurement of the LEDs in your complete circuit when you expect it to be 18mA.
Klaus
Why guessing?23R is my best guess from the datasheet what the impedance is of the TLC4950 output pins.
This makes the voltage problem of Q2 emitter even worse.And Q2 in hindsight does not need to be a darlington so I changed it to TIP31.
I already told you that higher voltage increases the problem with Q2.But I also realised I might be better of making that LM317 circuit a regular high current LM317 straight from the data sheet, which would give me access to the full range of the 12V input.
Hi,
Why guessing?
--> the datasheet tells you it is a "current source".
in post#11 I told you this.
in post #15 and #30 c_mitra told you the same.
Hard to help, if you ignore this all.
This makes the voltage problem of Q2 emitter even worse.
Hard to help if you don´t do a simulation on your own.
I already told you that higher voltage increases the problem with Q2.
in post#21 I recommended to omit the complete voltage regulation circuit.
Hard to help if you ignore this.
Don´t get me wrong. You are free to design your circuit as you like.
But I ask myself why you ask for help but ignore all the recommendations and informations we give.
I see no progress, neither with the discussion, nor with your application.
Klaus
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?