LM3410XMF OUPUT CURRENT CHANGES WITH DUTY CYCLE

WhiteHorse

Other Parts Discussed in Thread: LM3410

E2e,

We have a customer using the LMX3410XMF to drive 21 LEDs configured as 3 parallel strings of 7 series LEDs. They pulse width modulate the signal on the LM3410 pin4 at 20KHz to adjust the display brightness.

The LM3410 is set to deliver about 209mA at 100% duty cycle and it does this as expected. However, it delivers only 16.6mA at 44% duty cycle. 16.6mA is only 7.6% of the max current. Can you explain the reason the current changes with duty cycle and how to correct it?

Thanks for your help.

Regards,

John Wiemeyer

over 9 years ago

0 Clinton Jensen over 9 years ago

TI__Mastermind 25500 points

Could you provide operating points such as Vin and Vout? It would also help to have a schematic and BOM. Is there any way to get a scope shot of the output current during dimming? If you are using a large output capacitor the rise and fall times of the current may just be way too slow for such high frequency dimming.

0 WhiteHorse over 9 years ago in reply to Clinton Jensen

Genius 4430 points

Hi Clinton,

Vin is 5V. I need to check on Vout. I can provide a schematic, but need to email to you to keep private. Can you provide your email address?

Regards,

John Wiemeyer

0 Clinton Jensen over 9 years ago in reply to WhiteHorse

TI__Mastermind 25500 points

I will not post it here but I'll send you a friend request so you can contact me privately.

0 WhiteHorse over 9 years ago in reply to Clinton Jensen

Genius 4430 points

Hi Clint,

Vout = 9.9V

Vin = 5V

The customer has a 10uF capacitor on the output at the diodes. Could this be the problem? I emailed the customer's schematic to Issac Hsu.

Thanks for your help.

John Wiemeyer

0 Clinton Jensen over 9 years ago in reply to WhiteHorse

TI__Mastermind 25500 points

Yes, that is most likely the problem. It is difficult to PWM dim a boost converter at 20kHz unless you have a very small output capacitor. That's the thing with boost converters, the output current isn't continuous like it is in a buck, so an output cap is required. With a 10uF capacitor the current rise and fall times are too long so the current profile becomes very non-linear at high frequencies. With a 10uF capacitor you could probably only dim well up to about 500Hz to maybe 1kHz at most.

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LM3410XMF OUPUT CURRENT CHANGES WITH DUTY CYCLE