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TLC5916 LED driver for TFT assistance

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Micro9900

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I'm going to be doing a project using the TLC5916 LED driver which will control a TFT LCD. However, I could use some assistance as to how to properly configure and setup the driver. The display I am using requires 40mA @ 19.2V. So, I would need 40mA and 19.2V coming out of one pin (AKA one channel) to drive the LED backlight for the TFT display.

Looking through the datasheet I was able to ascertain the following:

CM=current multiplier

If CM = 1 then Iout target = (1.25V/Rext)x15
if CM= 0 then Iout target = (1.25V/Rext)x5

Also, if CM is 0 then the current output is limited to 3 to 40mA, else it is limited to 10mA to 120mA.

Using CM=0, and the equation Iout target = (1.25V/Rext)x5 and solving for Rext I get:

Iout target = 40mA
Rext = (5*1.25V/40mA) => 156.25 ohms or ~156 ohms

In addition, going off of pg. 24 of the datasheet, the 8-bit configuration code looks like:

01111111

So, my question are as follows:
1. Is the method I am using correct. Do I have the correct resistance?
2. Now that I have 40mA output current how do I get 19.2V. Do I need to take additional steps?
3. When hooked up to a microcontroller using SPI, is /OE essentially /CS?
4. How do I configure the TLC5916 to only utilize 1 channel for the output

If anyone can answer any of the above questions that would be much appreciated. Thank you.

EDIT:
Sorry about that, this is to control the LED backlight of a TFT LCD display.
 
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the datasheet is very clear and has plenty of examples.
1. yes
2. what do you mean? IC has onboard current limiting and if you use proper configuration you are set - as long as you have power disipation in grip.
3' if that what you like to think of it
4. you only connect led(s) to one channel

your post is confusing, you mix LED and LCD or TFT for some reason. why?
this device is only supporting LEDs.
 
Why use a LED driver IC for a LCD display?
 
I apologize for the confusion. This will be used to drive the LED backlight of the TFT display. I asked the support members in the manufacturer's forum and they said it was fine. Also it was recommended in this forum thread:

https://www.eevblog.com/forum/projects/constant-current-lcd-backlight-driver-question/

Of course if there is a more suitable driver I should be using then I am all for it. This is what I have found online that some people have recommended and going by the datasheet it is able to supply 40mA and 19.2V for the LED backlight of the display. I'm just a little uncertain about properly configuring the chip.

It is a bit clearer to me from panic mode's post. So, I gather that if it is properly configured for 40mA then it will produce 19.2V once connected to the backlight pin of the display. I'm interested in having one channel active at a time. I'm not sure if it would be a good idea to have other pins spitting out 40mA on a PCB while they may not be connected (is that an acceptable design?). So, if I wanted only one channel, I make the one connection, and the other channels should be disabled?
 
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no......

these LED drivers are very common and allow you to drive multiple (8 in this case) LEDs or groups of LEDs with a constant current.
this 'constant' means you can specify it (usually set by resistor value or in this case by combination of resistor value and configuration).

voltage is not generated by the IC, voltage is something that you provide from suitable power source (power supply or battery for example).
but IC can control current flowing from power source, through LED(s) into IC outputs (here outputs are sink type).


clarification about voltage:
if you want to drive several LEDs at once, you can connect them in a string and use single resistor (or constant current source).
however adding more LEDs means that you need to increase supply voltage accordingly. for example if you have LEDs with forward voltage drop of Vf=2.5V and your voltage source is 19.2V, then you connect 7 LEDs in a string (resistor or constant current source will drop remaining voltage, in this case 7*2.5=17.5V then 19.2V-17.5V = 1.7V is the difference taken by resistor or IC).

many ICs used in such applications can drive single LED per output because the outputs only tolerate voltage up to 5V.
this IC can tolerate up to 20V so you can connect multiple LEDs into a string. all LEDs in a string work together and are connected to one output. since this IC has 8 outputs, you can connect 8 strings of LEDs. 20V/2.5V = 8 but because output has some voltage drop too, you can not have 8 LEDs per string, but 7 will be fine.

however, you are not driving strings of LEDs, you are driving Background Light of a LCD display. you can connect multiple outputs together, or all eight if you like. suppose you set current to 5mA then just by enabling outputs you can vary current through LED (background light) from 0-40mA in steps of 5mA (each output you turn on, adds 5mA in this case). of course you could also change CM configuration too but you get most out of it by changing Configuration Code.
 
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Thank you panic mode for the response. There is no mention of each LED's Vf. It does however mention that the LED backlight is comprised of 12 LEDs and in order to drive the backlight you need 19.2V @ 40mA. So, from what I can gather I have to provide a power source of 19.2V. Am I better off using an amplifier to get 19.2V from say 5V or 3.3V? I'm trying to avoid batteries and power supplies and stick to USB to obtain the voltage needed.
 
why don't you post link to a datasheet of your display?
it would be simpler if thread started with this and simple question "how to control backlight?"
 
Your back light is built to run at 40mA. The voltage will be some where near 19.2 volts but could be as high as 22 volts. The data sheet does not have a min. voltage but my guess is 14 volts. So the back light voltage could be 14 through 22 volts depending on temperature and varies from unit to unit.

You should not apply 19.2 volts. LEDs are current devices not voltage devices.

If you are playing with a $0.20 LED do what you want. You are playing with a $50.00 display. Don't burn out the back light.

Look at LM3410. Note the OVP that turns off the part when the LED is disconnected.
 
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Thanks ronsimpson for the response. I'm trying to get it right the first time and get everything figured out beforehand which is why I could use some help.

You should not apply 19.2 volts. LEDs are current devices not voltage devices.

So, I can use the LT3510 in place of the TLC5916? It stays it "uses current mode control to regulate the output." So, I can input 3.3V or 5V and the LT3510 is capable of outputting 19.2V-22V @ 40mA? I'm a bit confused because it says "step-down." Shouldn't it be step-up DC/DC or is it capable of doing both?
 
Sorry LM3410. I will go back and edit my post.
 

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Very cool. Thank you ronsimpson. Just to be sure, with this I can connect the output (once configured to 19.2V and 40mA) of the LM3410 to the backlight pin of the TFT LCD? Thanks again.
 
The LM3410 will try to increase the voltage until 40mA flows. Assuming you pick the right resistor. The voltage is not important as long as it is above 5V (input voltage) and below 27??? (there is a limit) Read about OVP in the data sheet.

There are similar parts with the OVP built in.
 
I believe the input voltage can be between 2.7-5V even with OVP.

I'm looking at the datasheet pg. 31/49 (LM3410Y SOT-23 Design Example 5: 5 x 1206 Series LED String Application with OVP):

https://www.electro-tech-online.com/custompdfs/2013/07/lm3410.pdf

I'm going to try and configure it for 20V @ 40mA.

So far I have found Rset, according to the datasheet:

ILED=40mA
VFB= V feedback =190mV, VFBmax = 202mV

-using VFBmax

Rset=VFBmax/ILED => 202mV/40mA => 5.05 ohms

I believe this is correct. I'm am trying to figure out inductor and capacitor values now.

__________________

Also, here is the part on digikey, it is the LM3410Y:

https://www.digikey.com/product-detail/en/LM3410YMF/NOPB/LM3410YMF/NOPBCT-ND/1778508

EDIT:

Diodes D1 capacitors C1 and C2 seem to be reusable for my project, so I may go with those values/ratings. The zener diode should be rated at 25V to be safe.
 
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To experiment I would use cheep LEDs not a display. 5 of the white LEDs would give you about 15V and a good place to start. Any LED good for 50 to 100mA is good. The one watt LEDs are good for 300mA with a heat sink. Maybe 100mA with out.
 
Do not use a resistive load. Use a Zener diode. Better yet use a LED or maybe five. The supply is built to 'hit a wall' like a Zener or LED. You can get a 1 watt zener.
 
YES, It will get very hot. You can solder some copper to the leads for a heat sink. At first you can reduce the current until the supply is working well.
 
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