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3.3V LCD Displays with 5V back-light

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Most of 3.3V LCD displays come with 5V back-light and one way of powering it in such displays is using step up converter. There are many available, like LT1300 from LT or MAX756/856 from Maxim.

For my display, I am using LT1300 which requires 4 additional parts only:

L1 - 22uH inductor (10uH-22uH will work)

C1,C2 - 47uF (47uF-100uF)

D1 - 1N5817 Schottky diode





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Current Draw?

Wouldn't that be the backlight current divided by the smps regulator efficiency? For example, divide a 40-ma backlight current by an smps regulator efficiency of 89% and you get 45-ma. Just plug in the correct values for your particular backlight current and regulator efficiency.

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From what I can see, quiescent current is 150uA, and then ~300mA under ~75mA load.

My meter must be introducing pretty large error, because that would mean efficiency <50%.

Also, I will try to increase caps to 100uF and see if I get better results.

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  • 2 weeks later...

What should the input voltage of an AC-DC SMPS be? I have built an smps in a simulation programme (Multisim) but when trying to measure the input voltage (which should be the mains 240V AC) i can only get a DC volts reading on the multimeter, when on AC it reads 0V.


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