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Topic: Brightness control MM5451 (Read 984 times)previous topic - next topic

007good

Aug 12, 2015, 08:26 pm
Hello,

I would like to understand how the brightness controle of the MM5451 works.

http://www.micrel.com/_PDF/mm5450.pdf

If I understand correctly the datasheet. input 750µA make an output of 15-25mA ?? Why so large difference ?

So as the arduino output is 5V, and the MM5451 is Vout = 1.8V
U = R*I => R = U/I => (5-1./0.000750 = 4 266 Ohm So I must have a 4.7kOhm resistor to have between 15-25mA of output ?

Thanks for yor help

Grumpy_Mike

#1
Aug 13, 2015, 01:04 am
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Why so large difference
Look at the block diagram, you will see this goes through a triangle, that is an amplifier.
Not much good at emoticon math myself.

Wawa

#2
Aug 13, 2015, 08:01 amLast Edit: Aug 13, 2015, 08:20 am by Wawa
I understand from the datasheets of the chip and the Kingbright display that the IC's supply voltage has to be exactly 10volt (regulated).
And that the LED current (brighness) is 20x the current into pin 19.
And that that current has to come from the 10volt supply.
A 100uA current into pin 19 is 20mA LED current.

I would try an opto coupler.
Opto LED driven via 1k current limiting resistor from a PWM pin.
Opto transistor and a 100k trimpot in series between pin 19 and 20 of the LED driver.
Set the trimpot to max LED current (20mA).
Leo..

Grumpy_Mike

#3
Aug 13, 2015, 11:54 am
I understand from the datasheets of the chip and the Kingbright display that the IC's supply voltage has to be exactly 10volt (regulated).
Leo you need to learn how to read data sheets. Page 4 at the top left in bold print it says:-
Quote
Supply voltage (VDD - VSS).......................... +4.75V to +11V
Quote
And that the LED current (brighness) is 20x the current into pin 19.
Yes.

Quote
And that that current has to come from the 10volt supply.
No

Quote
I would try an opto coupler.
Opto LED driven via 1k current limiting resistor from a PWM pin.
Opto transistor and a 100k trimpot in series between pin 19 and 20 of the LED driver.
Set the trimpot to max LED current (20mA).
No.

Wawa

#4
Aug 13, 2015, 12:50 pmLast Edit: Aug 13, 2015, 12:52 pm by Wawa
Absolute max supply for the IC is 12volt. Kingbright display is ~8volt@20mA.
Read somewhere in the datasheet that voltage difference has to be kept under ~2volt, otherwise the IC could overheat. So 10volt seemed the best option. Close to the IC's max, so regulated.

Could not find the voltage needed on pin19. The datasheet only states current.
And the drawings show a 100k pot between the IC's supply (pin20) and the curent input (pin19).
Opto isolaton seemed the easy way, unless you have another solution.
A simple yes or no is not going to help OP.
Leo..

Grumpy_Mike

#5
Aug 13, 2015, 12:52 pm
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A simple yes or no is not going to help OP.