how STLD40D Driver work?

[MSm]

Hi everyone

I'm going to use STLD40D Driver IC for my TFT LCD. For driving LCD backlight, I'v chosen this Driver from ST. But I can't figure out the function theory of this driver.

could you please explain for me the STLD40D function theory and how to use that?

thank you in advance.

[Peter BENSCH · Accepted solution]

Well, the STLD40D is a boost converter. The function is also described in the data sheet, section 5.1.

It must be taken into account that the converter not only has a min off time, but also a max on time.

The switch-on moment is particularly interesting: initially there is no FB signal, so the power stage is switched on (SW to GND), which charges the boost inductor. As there is not yet enough output voltage to drive a current through the LEDs, pin FB remains zero. However, the max on time now kicks in and switches the power stage off again, whereby the energy stored in the boost inductor is delivered to the LEDs at a high voltage and results in a corresponding current and a signal to FB. From now on, FB can regulate normally.

Does it answer your question?

Regards

/Peter

[Peter BENSCH]

If the problem is resolved, please mark this topic as answered by selecting Select as best. This will help other users find that answer faster.

Regards

/Peter

[MSm]

Thank you Peter for your complete answer.

Another question. Am I right in this scenario?

My LCD Backlight needs 40 mA according to datasheet. For setting this current, I should divide FB voltage (in this case 165 mV) by 40 mA and calculate Rled? and how can I find the best value for Rset?

[Peter BENSCH]

Correct, Rled is calculated this way.

Rset sets the maximum peak current through the inductor, which depends not only on the LED current, but also on the length of your LED chain (sum of all LED voltages), the minimum input voltage and some other parameters. As you can see from the data sheet, the current can be set with 12...100kOhm in the range 200...1000mA (see fig. 9). Please also have a look at AN2333, which deals with the design.

Additional note about the inductor: there are values for saturation current and rated current. Please make sure that the saturation current is higher than the maximum peak current. Otherwise, if the peak current exceeds the saturation current, the inductance will suddenly collapse, which will usually lead to the coil burning out the next cycle the power stage is switched on, and in the worst case, to the destruction of the regulator.

Good luck!

Regards

/Peter

P.S. please don't forget to mark this topic as answered by selecting Select as best under your preferred answer. Thank you.