Ivan,
I'm a little confused as to what you are trying to do.
If I understand you correctly you want to vary the percieved brightness of the LED strips by a PWM?
However you want to set the current available to the LED strips via another device that only has a current capability upto a few milliampers?
The obvious question is why do you want this device to control the current to the LED strips, why not do it independantly.
That is make yourself a constant current powersupply connect the LED strip to this then have a power fet or whatever from the LED strip to ground. And drive the gate of this FET from the PWM output?
There is no way to do something like that with your chip, you have 16 outputs that can provide a constant current to the driver
but you can't control the output current based on a constant driver current and there is no way in your application to add external load sense in the 16 outputs.
Alex
I don't want to controll current through LED strip (as it is designed to work directly of 12V), but instead I want to PWM the supply to it.
You don't have a control over the PWM duty cucle, you said in your first post "current will be constant (defined by TLC external resistor)"
The frequency is not high but the PWM steps are too much and in an extreme position (duty cycle)
the mosfet would have to switch on and off in 1ms * (1/4096)= 0.24n sec,
You need a dedicated high current mosfet driver to achieve such a speed and I think it will still be difficult to do it that fast.
Alex
The frequency is not high but the PWM steps are too much and in an extreme position (duty cycle)
the mosfet would have to switch on and off in 1ms * (1/4096)= 0.24n sec,
You need a dedicated high current mosfet driver to achieve such a speed and I think it will still be difficult to do it that fast.
Alex
Hm, I think that in this case time is not but 0,24 ns but 0,24 uS or 240 ns. First I took you number and I was in dispear where I'm gonna find such quick FET. With 240ns on/off in worst case, I feel more relax
ooops... yes you are correct , the correct calculation is 240ns but also note that this will be the duration that the mosfet will stay on, the turn on and off delay (time it takes for the mosfet to turn on/off) should be a small percentage of this time.
Alex
5v 12v
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| LED STRIP
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PULLUP |
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| D
OUTx -------------------------------G||
S
\
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GND
Well the Gate of a MOSFET has capacitance, so every time we apply a pulse from the TLC5940, this capacitor is charged. Now, we know that when we apply voltage to a capacitor we see a peak in current (where voltage across the cap is 0) which tails off (and voltage increases) as it becomes charged.
Depending on your MOSFET this very short blip of current can be pretty large, enough to upset or stress out the outputs of the TLC5940.
So out first thought is "No problem, I'll put a resistor between the TLC5940's output and the Gate". Well, this brings about a second issue, gate charge time. We know that the voltage across the gate dictates the conduction state of the transistor, so whilst it is charging the transistor is not fully on. This means there is power dissipation over the transistor and, with a large load, this could quickly cause problems.
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