Question asked by RobertOl on Dec 11, 2015
Latest reply on Jan 19, 2016 by RobertOl

Hey all

I'm not happy with a current backlight solution I use on a custom board, it tends to blow up the driver IC for yet unknown reasons.

So I looked around for other LED drivers and found the ADD5211.

I've looked into the datasheet and also downloaded the calculation sheet, but now I'm not sure because I get completely different values than the example values in the datasheet and beside that I'm not sure which values I should use for calculation. I suppose I have to use the worst case values even if I might run the backlight at lesser current?

I have to drive 4 LED strings of a TFT backlight.

Each string runs at around 100 mA, max string voltage is 54V, at 100 mA probably less than 50V.

I also want to have an option to alter the string current if I have to use a different panel, so I might need more or less than 110 mA.

(In my current solution I just adjust the resistors that set the string current.)

As an inductor I want to use SRR1280-220M (22µH, Isat = 4.1A), which -if I'm not wrong- should be sufficient.

I selected SS5P9 as diode (If = 5A, Ifsm = 150A, 90V, could be replaced by SS5P10 if 100V would be better).

IRF7488 was chosen as NMOS switch (5A contin. drain current, 50A pulsed, Vds = 80V). Here I'm not that sure if it's okay to use this mosfet, but looking at the calculated values, it should be?

Switching frequency should be around 1000 kHz. (1070 kHz if Rfreq is 16 kOhm). I wouldn't mind changing the switching frequency, if anyone here says it's better to use more or less than that.

Minimum input voltage is around 23V.

Now, I'm not sure about the current sense resistor and the compensation parts.

Using the calculation sheet, Rcs is around 0.165 Ohm. What resistor should I use? 0.16 Ohm (are available as 1206 and 250 mW), or more/less than that?

If I calculate it correctly, Prcs is around 174 mW, so 250 mW should be safe?

About Cout: what values are suggested? I'm not going to use PWM dimming. Would it be better to use 4.7µF or rather 10µ or 22µF?

Altering the Cout capacity also alters the values of Rc and Ccomp. Rc's suggested value is 100 Ohm, is it okay to safely use other values such as 330 Ohm?

A quick calculation:

with Cout = 22µF, Rc would be 330 Ohm and Ccomp around 2.2µF. These values are common and available.

with Cout = 10µF, Rc would be 150 Ohm and Ccomp around 2.2µF, or 330 Ohm and 1µF. These values, too, are common and available.

But again, what Cout would be best and is it safe to use the just calculated values for Rc and Ccomp depending on Cout's value?

Whew, I hope this makes sense to you

I'd really appreciate some hints so I can design a better solution than my current one that causes troubles now and then ...

Thanks in advance and best wishes,

Robert