Issue with LM3464 LED driver

Vincenzo Miceli

Hello,

I'm working on a driver for 48 white LEDs (1 W each). I'm using 4 channels with 12 LEDs per channel, driven at 350mA. I'm also using the DHC feature by driving a 120W DC-DC step up converter. I use a microcontroller to dim the array via PWM.

Everything works fine as long as my PWN duty cycle is not 100%. I can set DC at 98% and it works great all the way down to 2%. However, if I send a 100% duty cycle wave, ie a fixed logic high signal, or if I connect the PWM pin to VCC without using the micro-controller then the array will light up as normal but it fades quickly within a second or two to a very low output. I can't figure out why this happening. Any ideas from the community?

Thanks!

Vincenzo

over 10 years ago

0 Victor Chen_HK over 10 years ago

TI__Expert 5130 points

That’s a strange phenomenon. If you reduce the duty cycle after the arrays fade to a low output, will they come back to normal?

I think it may have something to do with the thermal foldback control. Have you tried it on TI’s EVM board?

Do you mind attaching your schematic?

0 Vincenzo Miceli over 10 years ago in reply to Victor Chen_HK

Prodigy 30 points

Hello Victor,

yes I can go from 100% DC to 98% and the LEDs will go back to very bright. I'm not using an EVM board. I'm attaching the schematic I'm using with the actual components, it is a sketch I made...

Please let me know what you think,

Vincenzo

0 Victor Chen_HK over 10 years ago in reply to Vincenzo Miceli

TI__Expert 5130 points

Hello Vincenzo,

Could you please provide the following information?

What is the rail voltage during PWM dimming?
What is the LED stack voltage at full current?
The PWM dimming waveforms.
The dim pin voltage when the LED current drops to very low.

Thanks and best regards,

Victor

0 Vincenzo Miceli over 10 years ago in reply to Victor Chen_HK

Prodigy 30 points

Hi Victor,

I tried another chip and everything works as designed. I'm sure at this point that something must have been wrong with my soldering of the original part.

Thanks for your help!

Vincenzo

0 Vincenzo Miceli1 over 10 years ago in reply to Vincenzo Miceli

Prodigy 10 points

Hello,

I figured what fixes it. I was leaving the Thermal pin floating. So I just connected Thermal to VCC and the behavior at 100%DC is gone, the driver works perfectly.

I wonder is this is by design or just some weak link in my design. Anyway that's the fix.

Vincenzo

0 Victor Chen_HK over 10 years ago in reply to Vincenzo Miceli1

TI__Expert 5130 points

Glad to know it's fixed.

Since the thermal pin can also control the brightness of the LED array, you can't just leave it floating.

Regards,

Victor

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Issue with LM3464 LED driver