Reducing 3.3V down to 2.8V

[Yatoto73]

Hello everyone,

I need a bit of help, and I hope everyone will understand me.
My goal is to build a pcb for a bare display ILI9341 .

I'm fairly confident I got the traces correct just few things that are bugging me- Reducing the voltage down.
My microcontroller board is ESP32 and it has 3.3V out
In the datasheet of the display it says it needs 2.8V VCC. So I thought to use a diode for 0.5V drop 1N4007, and one person suggested to me to place a capacitor 100nF to filter/ stabilize the voltage.

-Now I used Easy EDA to draw my first schematic, and saw that there is an Simulation option. So i decided to draw a circuit with 100nF capacitor and 1N4007 diode, battery, and led diode and test it. The issue is the voltage was way below 2.8V.

-Now I think that this idea with diode wont work at all, because simulation showed way low result...

The other issue, the backlight operating voltage is 3.2V-3.4V. - So i decided to use two resistors 1.1k +32ohm. Again when I made a circuit for simulation it didn't work at all.

I have no experience in building circuits, but can someone have a look at my schematic


 
 

[DavidAlfa]

You don't need 2.8V, 3.3V is fine, see the ILI9341 datasheet (Page 236).
Also the ER-TFT032-3.1 datasheet shows this (Page 10).

I don't understand the 1K1 resistor in the backlight circuit.
The datasheet states the backlight start conducting at 3.2V, so the voltage you need to care about is 3.3-3.2 = 0.1V.
For 90mA the resistor will be 0.1/0.09 = 1R.
But the working voltage is so close that it might barely light up at 3.3V, even with no resistor.

Better to power the backlight from the ESP 5V power input, calculating the resistor as (5-3.2)/0.09 = 20R = ~22R

[Yatoto73]

Thank you David.

I was worried about running at max voltage shortening its life.

Now I used Ohm Law for backlight LED (90mA-100mA 3.2-3.4V)
(3.3V-3.2V)/0.095A =~1 ohm resistor


I will run a simulation as you suggested with 100R and lower.


EDIT: I will test with 5v as suggested

This is what i got with 5V

[DavidAlfa]

Bad simulation, a red led is not a white led.

Red leds have 1.6-1.8Vf (Showing 2V in your simulation), while white leds have 2.9-3.2V.
So for the red led: (5-2)/22=136mA
But again, for the backlight: (5-3.2)/22=82mA

Also don't exceed 100mA, it's thw maximum allowable as stated in the datasheet.

[Yatoto73]

Thank you once again!
i didn't even know about that i thought it was all the same 

I think I will have to probe that with different resistors when I make the board. This Easy EDA doesn't offer white led, and when i change between color that they offer blue, red, green, yellow the values are all the same.

[BTO]

Thank you once again!
i didn't even know about that i thought it was all the same 

I think I will have to probe that with different resistors when I make the board. This Easy EDA doesn't offer white led, and when i change between color that they offer blue, red, green, yellow the values are all the same.

Maybe something to try, When i was much younger in electronics, i got a
White     LED
Red        LED
Green    LED
Orange LED
Yellow   LED
Blue      LED
Now, Use ohms law to calculate the resistance that gives you say 20mA of current through the LED (say the RED one)
Now use that same resistor for all the rest of them and see what happens.
They're not all the same, i can assure you

[DavidAlfa]

Just start with 47R, measure the current, then try lower values until getting around 90mA.

[Yatoto73]

Thank you! i will order different types of resistors with the board, and try to measure with different types of leds

[DavidAlfa]

You can't push 90mA into a normal led, will instantly burn it, just test the backlight, no need to simulate anything.

[tooki]

I was worried about running at max voltage shortening its life.

3.3V is the maximum of the recommended voltage. If you look at the Absolute Maximums, the values beyond which damage is likely, it's 4.6V, far beyond 3.3V.

Also, I've used that display before, and it's perfectly happy with 3.3V.

[tooki]

You can't push 90mA into a normal led, will instantly burn it, just test the backlight, no need to simulate anything.

It's not a "normal LED", it's 5 LEDs in parallel, as stated in the datasheet, 90-100mA recommended current.

And also, 100mA LEDs aren't rare anymore. Heck, I've used 1.5A single-die LEDs before.

[Irilia]

Starting with a LCD without the adapter board is fun, really recommend to have a programmable power supply. Also I recommend to have a component tester. For the LEDs blue white green, red yellow orange, infrared, ultra Violet (different groups = different voltage. 

[tooki]

Bad simulation, a red led is not a white led.

Red leds have 1.6-1.8Vf (Showing 2V in your simulation), while white leds have 2.9-3.2V.
So for the red led: (5-2)/22=136mA
But again, for the backlight: (5-3.2)/22=82mA

Also don't exceed 100mA, it's thw maximum allowable as stated in the datasheet.

The display’s datasheet says Vf = 3.2-3.4V. No need to use guesstimates when you have specs.

[DavidAlfa]

It's not a "normal LED", it's 5 LEDs in parallel, as stated in the datasheet, 90-100mA recommended current.

I know. But going for this. He's trying something else:

Thank you! i will order different types of resistors with the board, and try to measure with different types of leds