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circuit
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Joined: 10/01/2016
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Posted: 02:34pm 23 Jan 2016

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I have an ILI9341 LCD operating with a Micromite Plus. The 2.8 inch display appears to be identical to that illustrated on Page 17 of the Micromite Manual (5.1) Images load well from the SD card and all is okay except...the LED backlight is not quite as bright as I would like it to be. Referring to the Micromite Manual page 18, I am instructed to connect the LED to 5V via a current limiting resistor and 18R is recommended as a typical value. It then continues "The value of this resistor can be varied to reduce the power consumption or to provide a brighter display." I am now looking for the minimum value that this current-limiting resistor can have; I am having trouble finding any reference to this.

Looking at the schematic for the LCD backpack, I note that there is a variable resistor which would allow, at the end of its travel, a direct connection to 5 volts. Can I just connect the LED pin to 5V or is a current-limiting resistor truly needed and, if so, what should be the minimum value?

palcal

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Joined: 12/10/2011
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Posted: 03:09pm 23 Jan 2016

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I have run them directly off 5v with no ill affect but I usually short out J1 and run directly from 3.3v
Paul.

"It is better to be ignorant and ask a stupid question than to be plain Stupid and not ask at all"

TassyJim

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Posted: 04:17pm 23 Jan 2016

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At least one of my displays has a resistor in series with the LED pin. I have measure ~4 ohms.
Before playing with lower value external resistors, I would hook up a meter and measure the current.

Jim

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bigmik

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Posted: 04:42pm 23 Jan 2016

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Hi Circuit,

The specs I have say this pin should be directly connected to 3v2.. (3v3 is close enough) and that it should draw 60-70mA. mine, as Jim stated, has a 3R9 resistor in series..

Using 65mA as the current and wanting to connect to 5V I would use 1.8/.065 = 27ohm...

I am sure that this can be 18 ohm safely and this would draw 100mA.

My backpack170 draws about 80mA with the circuit used and hasn't caused any harm..

Re. the shorting out the Vreg, I have never liked that as I believe that it is to lower the voltage to about 2.1V (or was that 2.8V) for the internal workings of the display module itself. Shorting J1 would put a 3v3 on the `innards' and could be the cause of some of these displays failing..

Regards,

Mick

Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<<

palcal

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Posted: 11:17pm 24 Jan 2016

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I was under the impression that the 'innards' run of Gnd and Vcc, and the voltage on the LED pin is for the backlight only.
Matherp originally suggested shorting J1 so maybe he can clear this up.
Paul.

"It is better to be ignorant and ask a stupid question than to be plain Stupid and not ask at all"

matherp
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Posted: 11:55pm 24 Jan 2016

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The regulator on the ILI9341 is a 3V LDO device, so it just regulates 3.3V to 3V. The spec on the ILI9341 controller gives an absolute maximum of 4.6V for the logic input and the supply voltage with normal maximum for operation of 3.3V.

The chip also uses a 2.0V supply for its core logic. This is nothing to do with the voltage regulator on the PCB and is generated by the chip itself in the same way that the PIC32 generates its internal 1.8V supply (stabilising this is what the VCAP capacitor is for on the PIC32).

So if you are using a 3.3V supply shorting out J1 is fine as you are staying within all normal operating conditions.

The backlight LED supply is completely separate, doesn't go through the regulator, and is sourced on the LED pin. There is an onboard current limiting resistor of 3.9ohm from the LED pin. I measure 40mA with a 3.3V supply connected to LED. For a brighter display you can connect 5V to LED direct to give 150mA which will probably shorten display life or for optimum brightness PWM a 5V supply onto the LED pin using a npn transistor to drive a p-channel mosfet.Edited by matherp 2016-01-26

OA47

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Joined: 11/04/2012
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Posted: 12:47am 25 Jan 2016

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Peter, Thanks for your explanation of the power and back light requirements for the ILI9341 as it should clear up some misconceptions that have been mentioned previously.
GM

bigmik

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Posted: 08:10pm 25 Jan 2016

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Hi All,

Sorry to go against all of the esteemed members here but I have re-read the specs for the display and unless I am reading something incorrectly the operating voltage of the Display itself (Not the logic board it is attached to) is 2v8 nominal with an absolute maximum voltage of 3v0. Shorting the 3V regulator is risking damaging the TFT module.

It took a while to find the specs as I had squirreled them away in the depths of my PC but here they are for perusal.

Voltage level highlighted

Zip of all of the files (I have photoshop enhanced for easier reading) associated with the 2.4" SPI boards

A link to the original files Link

Regards,

Mick

Here are each page zipped (sorry TBS only accepts 500KB file size max.

2016-01-26_063608_Board.zip

2016-01-26_063619_Ratings.zip

2016-01-26_063630_Schem.zip

2016-01-26_063638_Screen.zip

2016-01-26_063646_Specs.zip

Regards,

Mick

Mik

Edited by bigmik 2016-01-27

Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<<

matherp
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Posted: 10:23pm 25 Jan 2016

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Versions from the Adafruit website

bigmik

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Posted: 10:31pm 25 Jan 2016

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Peter,

Hmm,

That shows 3v3 Max voltage... are the RED sections your changes or on the sheet already? That could explain why some displays are failing....

Regards,

Mick

Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<<

matherp
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Posted: 10:45pm 25 Jan 2016

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Quote are the RED sections your changes or on the sheet already?

Unchanged, as downloaded from Adafruit.

On my display (2.8") I inadvertently powered it with 5V and it survived despite this exceeding the 4.6V absolute maximumEdited by matherp 2016-01-27

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