TPS61185: inrush current and direct PWM dimming question

Ann_Lien

Part Number: TPS61185

Hi expert,

My customer used TPS61185 for their panel backlight application.

They found out there is inrush current at 5Vin. We would like to know if it is normal operation when we used direct PWM dimming which is switching from 0A to Iset. And also we would like to know if LED current will drop to zero when PWM is keep high. As we know, it should keep Isat when PWM is high. Would you please help on it? Thanks a lot!

CH1:Vout(34V), CH2: direct PWM signal, CH3: LED negative side, CH4: total output current from Boost stage.

Best regards,

Ann Lien

over 5 years ago

0 Sean Zhou over 5 years ago

TI__Mastermind 22745 points

Hi Ann,
For my understand , your inrush current is means the spike on the Ch4? If my understand is right, this is the normal phenomenon.
And in direct pwm dimming , the output loed current will follow with the pwm signal, it should keep high when your pwm is high.
Regards
Sean

0 Ann_Lien over 5 years ago in reply to Sean Zhou

TI__Genius 12380 points

Hi Sean,

Thanks for feedback!! For inrush current, we know this is normal behavior.
But for PWM high period, why LED current would drop to GND? For our understanding, current would be always Iset point when PWM signal is high. It should not be zero when PWM signal is high. Could you kindly explain? Thanks a lot!!

Best regards,
Ann Lien

0 Sean Zhou over 5 years ago in reply to Ann_Lien

TI__Mastermind 22745 points

Hi Ann,
Could you provide a zoom out waveform with pwm dimming? I don't find the current drop to GND when the PWM, you can also send me email if attachment is too large.
Regards
Sean

0 Ann_Lien over 5 years ago in reply to Sean Zhou

TI__Genius 12380 points

Hi Sean,

Thanks for reply!!

Please see below picture for current down to zero, thanks!

Best regards,

Ann Lien

0 Sean Zhou over 5 years ago in reply to Ann_Lien

TI__Mastermind 22745 points

Hi Ann,
Could you upload the schematic for this tets, I think your board maby be is not in direct PWM mode, it maybe in frequency programmmed mode. And why this is a messga ein right shows the Duty of channel 2 is 50.71% from you waveforms.
Regards
Sean

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TPS61185: inrush current and direct PWM dimming question