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I've been tasked to find a suitable backlight led driver for a Mitsubishi LCD panel (AA104XF12-cf1) which is PWM dimmable, but in my search I've only found confusion.

In nearly all datasheets it specifies a certain amount of LEDs the driver is designed for. Isn't it irrelevant how many LEDs you connect as long as the voltage is correct and the driver can supply or sink enough current?

Bonus question: Why do some ICs have multiple sink channels for separate strings when you can connect them together to increase the total current? I have 2 backlight strings of 120mA each, which could be connected to 8 channels of 30mA as long as they're current sinks.

EDIT:

As per request here are some examples of datasheets which specify the number of LEDS.

  1. LTM8042.

  2. BD6142AMUV.

Electric_90
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Nathan Prins
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  • *..datasheets it specifies a certain amount of LED's the driver is designed for.* It would be helpful if you include a link (to the datasheet) of an example of such a driver. Depending on the type of driver the number of LEDs might be irrelevant but that's not true for all drivers. Multiple sink channels: only if the LEDs are identical can you combine outputs. If the LEDs are not identical (for example: different color) then separate current sinks are needed. – Bimpelrekkie Feb 15 '19 at 09:03
  • Thank you for your reply :) A lot of the LED driver datasheets say 'x amount of strings with up to y leds'. But wouldn't the amount of LEDs be irrelevant since the power consumption of LEDs can vary widely? On the multiple sink channels: Okay makes sense, so if I took the IC from the second datasheet and give both 120mA strings 4 channels of 30mA, that would work fine? – Nathan Prins Feb 15 '19 at 09:44
  • The limit is in the **voltage** so the maximum number of LEDs is set by the total voltage needed to drive them. The driver can handle only so many volts so that sets the limit. The LEDs power consumption: it is not the LEDs which determine the power consumption, LEDs need to be driven with a certain **current** which is set by the driver. Given enough voltage and current an LED will destroy itself! So we must limit that. For LEDs controlling the current is most convenient as the voltage across a LED is somewhat unpredictable. – Bimpelrekkie Feb 15 '19 at 09:51
  • Hmmm. As far as I can see, the LTM8042 datasheet says "current" and "voltage" all over the place, but doesn't mention "number of LEDs" anywhere. – JRE Feb 15 '19 at 09:51
  • *second datasheet and give both 120mA strings 4 channels of 30mA, that would work fine?* Yes, you can combine channels to get a larger current. The currents simply add up. – Bimpelrekkie Feb 15 '19 at 09:53
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    *but doesn't mention "number of LEDs" anywhere* True, that's because it is unknown which LEDs you're using. Depending on color, age, temperature etc. a LED has a different voltage. The LED will have a nominal operating current specified so you make the LTM8042 drive that current (or less) through the LED. The LTM8042 will be able to do so as long as the LEDs do not need more voltage than the driver can supply. – Bimpelrekkie Feb 15 '19 at 09:57
  • It specifies the number of leds in the table below the description on the first page. They also mention the number of leds when looking on [TI's website](http://www.ti.com/power-management/led-drivers/backlight-led-drivers/products.html?pqs=paqs&familyid=3055#p238min=1.8;5&p238max=5;40&p634min=0;24&p634max=24;120&p1299=1;2&p2823=10;200) for example. But from what you guys are telling me my suspicions were true and those numbers are just an indication of what you can use the driver for. Thanks for the information! – Nathan Prins Feb 15 '19 at 10:07
  • @Bimpelrekkie: I understand that. The comment was about the OP's claim that the datasheets are all about the number of LEDs. The datasheet goes on for pages and pages about voltage and current (as is to be expected,) and I didn't see the claimed references to number of LEDs. Turns out there's one chart about number of LEDs that I missed (the only one about number of LEDs in the whole datasheet.) – JRE Feb 15 '19 at 11:31
  • That one table you missed was just one example that I could quickly find. I've been looking at datasheets for hours and thought that would be enough to get the point across. Also, I never claimed that the number of LED's is all that they're about, but since this is my first time choosing a LED driver I wasn't sure if that was a number I could safely ignore. I think I understand electronics decently, but I'm just not yet as confident in this field as I am in others. Hence the question. Anyway, thanks for answering it :) – Nathan Prins Feb 15 '19 at 15:01

1 Answers1

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Isn't it irrelevant how many LEDs you connect as long as the voltage is correct and the driver can supply or sink enough current?

Only the string max voltage and current are relevant not the max number of LEDs.
I don't know why they specify the output in terms of the number of LEDs.

Why do some ICs have multiple sink channels for separate strings when you can connect them together to increase the total current?

Flexibility to increase target market.

I have 2 backlight strings of 120mA each

And about 24-30V per channel.

Check out the TI LP8862, Allegro A8502

Misunderstood
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