Selection of LED Driver

[FreshmanNewbie]

I have a display whose backlight section has 7 LEDs in each of its 4 strings. So, total LEDs are 28. Typical and Maximum LED Driver voltge is 21.3V and 24V respectively.

The total current consumption of the backlight as mentioned in the datasheet is 200mA, typ.

My question is, whether this LED Driver would be suitable?

Asking this because, eventhough the above LED Driver satifies my current rating requirement, in the circuit application images on page 1 of the datasheet, only 1 string of LEDs is connected?
I am coming to a conclusion that this LED driver would be fine because I looked at the maximum switch current rating in the datasheet which is 500mA. Since, this is the peak current through the inductor, and this current is greater than my required LED string current of 200mA, I think this will work. Is my understanding correct? If not, how to verify? Also, does switch current parameter should be considered to arrive this conclusion?

What should I do if I have 4 strings? Can I short all the 4?

Just to clarify, that 200mA is a typical current for all the 4 strings, 28 LEDs together. I had heard that That is a fundamentally bad ideas because small differences in Vf can lead to big differences in the light output of the different strings. What are your thoughts on this?

 

[D.A.(Tony)Stewart]

It depends on your LEDs. YOu can balance or normalize the strings by adding a series Rs to prevent thermal runaway. E.g. A white LED might vary 0.1V to 0.2V in a batch or less, so adding a series Rs, resistor that drops greater than this variation, reduces the variation in current sharing and NTC effects , but to get perfect sharing, you increase the Rs to make the total Rs more fixed.

All the Vf variations in white LEDs are due to Rs internal variations that cause the Vf rise above ~ 2.85V which may be a good reference for most LEDs . More powerful LEDs with better efficiency will be closer to 2.6V

 

[FreshmanNewbie]

It depends on your LEDs. YOu can balance or normalize the strings by adding a series Rs to prevent thermal runaway. E.g. A white LED might vary 0.1V to 0.2V in a batch or less, so adding a series Rs, resistor that drops greater than this variation, reduces the variation in current sharing and NTC effects , but to get perfect sharing, you increase the Rs to make the total Rs more fixed.

All the Vf variations in white LEDs are due to Rs internal variations that cause the Vf rise above ~ 2.85V which may be a good reference for most LEDs . More powerful LEDs with better efficiency will be closer to 2.6V

Thank you. Whether the MIC2287 is OK?

 

[KlausST]

Hi,

We don't see the datasheet, so we need to guess...
If I'm not mistaken, you have 4 strings, each 24V and 200mA.

So if you use one supply you have these options:
* 4S which means all 4 in seriesresulting in 96V, 200mA
* 4P, all 4 in parallel, 24V, 800mA
* 2S2P ... 48V, 400mA

2 supplies:
* 2P each
* 2S each

4 supplies ... each for a string

Don't know how you decided to connect them.

Klaus

 

[FreshmanNewbie]

Hi,

We don't see the datasheet, so we need to guess...
If I'm not mistaken, you have 4 strings, each 24V and 200mA.

So if you use one supply you have these options:
* 4S which means all 4 in seriesresulting in 96V, 200mA
* 4P, all 4 in parallel, 24V, 800mA
* 2S2P ... 48V, 400mA

2 supplies:
* 2P each
* 2S each

4 supplies ... each for a string

Don't know how you decided to connect them.

Klaus

this is the configuration. Can you please clarify better now?

 

[D.A.(Tony)Stewart]

It seems to support 24V @ 500 mA at 85% d.f. so 425 mA avg. but that's at max junction temp with good thermal design so 200 mA seems reasonable. I see your backlight already has normalizing Rs in each string, which answers my concern about thermal runaway and is pre-wired for 24V which answers Klaus's concern.

It's always good to have a 2nd source part in your design and use soft-start with quality low ESR caps and Vin >=5V.

 

[FreshmanNewbie]

It seems to support 24V @ 500 mA at 85% d.f. so 425 mA avg. but that's at max junction temp with good thermal design so 200 mA seems reasonable. I see your backlight already has normalizing Rs in each string, which answers my concern about thermal runaway and is pre-wired for 24V which answers Klaus's concern.

It's always good to have a 2nd source part in your design and use soft-start with quality low ESR caps and Vin >=5V.

Can you tell me what do you mean by having a 2nd source?

 

[D.A.(Tony)Stewart]

Consider alternative designs and compare efficiency, temperature rise, EMI, Vin, cost with your design goals, which are (?)

e.g. https://www.ti.com/product/TPS92360

 

[FreshmanNewbie]

Consider alternative designs and compare efficiency, temperature rise, cost.

Sure thank you for the clarification. But please see below,

I followed the calculations as mentioned in this App note - https://www.ti.com/lit/an/slva372d/...76335&ref_url=https%3A%2F%2Fwww.google.com%2F

--- Updated Nov 1, 2023 ---

Consider alternative designs and compare efficiency, temperature rise, cost.

Sure thank you for the clarification. But please see below,

I followed the calculations as mentioned in this App note - https://www.ti.com/lit/an/slva372d/...76335&ref_url=https%3A%2F%2Fwww.google.com%2F

 

[D.A.(Tony)Stewart]

I guess that won't work with 5V in. Try 10V in or some other IC.