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#### gudurix

• Posts: 1
« on: December 22, 2012, 20:19:55 PM »
Hi there:

I have read the blog entry about this led driver. It's quite interesting. I have been reading also the datasheet but some doubts that I have in my mind have not been solved and I guess if someone can help.

1.- What it means a valley current controler? does it means that the chip is meauring current flowing through R1 and it's is all the time keeping the current over that valley peak?or what?

2.- What about switching frecuency?. The components values concerning a typical configuration for these driver modify the Ton and Toff as far as I have read, specially R1. So, I guess this frecuency we are talking is just the frecuency of the current "peak to peak", the "natural current" the driver "generate", with a certain set of  selected values. Is that  correct?

3.- So, what happen when I introduce a PWM dis signal? Are all of these calculations modified?Do the chip use then the average current taking into account the Disabled time of the DIS pin? Which is the minimum frecuency (I mean input PWM frecuency into DIS)  affordable with this driver?could it reach some few Hertz? There is something to have into account when a low frecuency PWM signal is introduced in DIS pin?

4.- Which voltage drop are these driver working? Can I reach for example 3 V with an input of 24V? which component (R, C,L) handle this?

5.- Excel design tool gives a good idea of the right choosing of pasive components. SO, when I introduce the values into de blue cells, I can have an idea of which is the final result of my personal selection. BUT, I can't see the resultant led assembly voltage. Why

Can someone help?

Regards

#### kam

• Hero Member
• Posts: 1849
« Reply #1 on: December 22, 2012, 22:51:15 PM »
1.- What it means a valley current controler?
If you see the output current, you will see that there are spikes that looks like mountains, underneath of them is the valley. Pretty poetic... don't you think? So, the mountain is the peak (peak voltage or peak current) and the bottom of the mountain is the valley (valley voltage or valley current). It is just a name...

2.- What about switching frecuency?. The components values concerning a typical configuration for these driver modify the Ton and Toff as far as I have read, specially R1. So, I guess this frecuency we are talking is just the frecuency of the current "peak to peak", the "natural current" the driver "generate", with a certain set of  selected values. Is that  correct?
The buck regulator oscillates at a frequency to "charge" the inductor. This charge is measured by the regulator to maintain it stable (through the sense resistor). So, according to your current needs and your parts selection, the regulator will oscillate at the selected frequency and will set the duty cycle so that it provides the current that you need. The frequency has to do with part selection and size generally: Higher frequency means smaller parts, so you aim usually for the highest frequency. But it is not always possible to provide the current you need at high frequency...

3.- So, what happen when I introduce a PWM dis signal? Are all of these calculations modified?Do the chip use then the average current taking into account the Disabled time of the DIS pin? Which is the minimum frecuency (I mean input PWM frecuency into DIS)  affordable with this driver?could it reach some few Hertz? There is something to have into account when a low frecuency PWM signal is introduced in DIS pin?
From my experience i found out that the response to PWM signal input is pretty much linear (PWM duty cycle to current and thus brightness). But i cannot tell what happens inside the chip. I also cannot tell you the best frequency for PWM, 1KHz works fine for me

4.- Which voltage drop are these driver working? Can I reach for example 3 V with an input of 24V? which component (R, C,L) handle this?
Theoretically, a buck regulator can achieve whichever voltage drop you want at a very high efficiency. I managed 3V at much higher input during my tests

5.- Excel design tool gives a good idea of the right choosing of pasive components. SO, when I introduce the values into de blue cells, I can have an idea of which is the final result of my personal selection. BUT, I can't see the resultant led assembly voltage. Why
That is the funny part. A constant current driver does not care about the LED voltage. It will provide as much voltage required as possible to reach the desired current. Cool right?