LCD Backlight Driver Design

[themaccabee]

Hi ,

I need to develop a LCD module back light drvier circuit, Its a string of LEDs and the datasheet says we need to take care of temperature derating.
Can some body help me to get a starting circuit?
Ive only 5V supply onboard.
I suppose i need to do a 19.2V boost converter.
Then i need a constant current source circuitry to maintain the current through the string at < 15mA
Shall i need to take care of any protection like failing one LED (open or short?) in the series string?
And how to make the current temperature independent or compensate it?

Thanks & Regards




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[rk433]

You can design a very good current source or sink by using band gap & resistor
i.e vbg / r .
In the above equation, band gap is constant but resistor varies a lot. to minimize the resistor variation across temperature, use two resistors (one with positive temp. coefficient & another one with negative temp. coefficient) , you will get good current across temperature. For process variation use trimming.

Bye

 

[themaccabee]

You can design a very good current source or sink by using band gap & resistor
i.e vbg / r .
In the above equation, band gap is constant but resistor varies a lot. to minimize the resistor variation across temperature, use two resistors (one with positive temp. coefficient & another one with negative temp. coefficient) , you will get good current across temperature. For process variation use trimming.

Bye

Sorry rk433, ive no idea about the band gap..
Isnt bandgap refernces are used in ICs?? can we implement band gap with discrete components??
Please elaborate a little bit..im a novice
Thanks

 

[betwixt]

I think what is being asked is how to produce 19.2V at about 15mA when the supply is only 5V, not how to temperature compensate it.

The simplest answer is to use a DC-DC converter module with 5V input and dual 12V output. Join the outputs in series to give 24V and use a series resistor of 330 Ohms.
Unless you plan to use the LCD in both sub zero temperatures and also in mid-day desert conditions, don't worry about stabilizing the current.

Brian.

 

[dick_freebird]

Do you really need to design this? Because most of the
major industrial analog IC companies have families of LED
drivers that do exactly what you describe, with app notes.

 

[bobcat1]

Hi

Simply use the TI TPS61040/41 (depend on how many led in series)
Don't forget to add Zener diode to protect the output transistor when the circuit works with open load

All the best

Bobi

 

[shryaskumar]

Hi bobcat1,
Thanks for recommending Zener Diode.