LM3463: analog dimming 400 high power LEDs

Hi Florian,

Florian Schindler said:

I am designing a board with about 400 power LEDs on it. The main voltage is 48V and the LED current is up to 3A. An important requirement is, that the LEDs must be dimmable but can never be completely off when doing so. So I am looking for analog dimming not PWM (except maybe smoothing out the PWM). I am now mainly looking for a component that's suited for this application.

My understanding is that you want to avoid any LED flickering. Am I correct?

Florian Schindler said:

I thought about using the LM3464 and adding an inductor and a diode to smooth out the PWM. Would that be possible? If so, what's the highest PWM frequency the LM3464 can handle? I couldn't find any information about that in the datasheet.

Another option could be the LM3463 with true analog dimming. Did I understand correctly, that the LM3463 always has a PWM output except when you adjust the voltage on the IOUTADJ pin? So there's no option for a cascade operation with analog output? Could I as an option put the same voltage signal on the IOUTADJ pin of every LM3463 I use (up to 10 units)?

Do you need a need a linear-type LED driver like LM3464 and LM3463? Or is a switching-type LED driver also acceptable? Do you need to control the 400 LEDs individually? Or do you need to control them as a group? What is the forward voltage of the LED (string) you are going to drive?

Best Regards,

Steven

Hi Steven, thanks for the reply! It's not about flickering, at least not for the human eye. The off time could be some nano seconds at most.
A switching-type LED driver would be acceptable. The LEDs are dimmed as a collective, preferably with only one input signal (PWM or analog). I plan on making strings of 10 LEDs which would have a maximum forward voltage of 42V.

Thanks for the recommendations! I tend more to the TPS922054/5. Dimming below 20% isn't really necessary. Could I use it with 8 LEDs in series (max 33.6V)? As an alternative, could I also use 10 LEDs in series with a supply voltage of 60V?
But I'd actually prefer an IC that can drive multiple strings. Is there no option for that?

Hi Florian,

Yes. TPS922054/5 can work in Vin = 48V, Vout = 33.6V, Iout = 3A if good thermal dissipation methods are implemented.

I do not recommend you use TPS922054/5 with Vin = 60V and load = 10 LEDs since 60V is very close to the border of the recommended operating conditions of this device. Please refer to Section 7.3 Recommended Operating Conditions from the datasheet.

For an IC to drive multiple strings with 3A current, LM3463 is a possible solution. However, the efficiency of such a linear LED driver is normally lower than a switching-type LED driver like TPS922054/5.

Best Regards,

Steven

Thanks for all the answers and sorry for making this thread so long! But could you explain, how do I know the maximum number of LEDs in a string? Respectively how much must Vin be bigger than Vout. Because it looks like the smaller this difference is, the more efficient it gets (see Figure 7-12 on TPS92205x datasheet).

Hi Florian,

You are welcome.

There are lots of factors that determine the maximum number of LEDs TPS92205x can drive under a specific input voltage, for example - 

• Thermal consideration - Driving larger number of LEDs means higher switching duty cycle, which leads to higher conduction loss and may lead to thermal shutdown.
• System performance - You can refer to this E2E thread - TPS922055: Maximum output duty cycle (minimum difference between Vin and Vout) - Power management forum - Power management - TI E2E support forums.

You can estimate the thermal performance of the device with some calculation, or by referring to this calculation tool - [FAQ] Is there a calculation tool for TPS922055 / TPS922054 / TPS922053 / TPS922052? - Power management forum - Power management - TI E2E support forums.

Here is a list of FAQs you can refer to - [FAQ] List of FAQs for TPS922055 / TPS922054 / TPS922053 / TPS922052 LED driver - Power management forum - Power management - TI E2E support forums.

Best Regards,

Steven