TPS92512: TPS92512 flickering problem

Jishnu Prakash Nambiar

Prodigy 120 points

Part Number: TPS92512
Other Parts Discussed in Thread: TPS92515, TPS92640, LM3409

hi,

i am using TPS92512 ,for dimming purpose

i am using 24v supply and 4 X20 led matrics (4 series)

i get the flickering during the period when current in between 600mA and 800mA

i am using the same circuit given in the data sheet please give me a solution

total voltage 24

volatge across led 11V

no of led series 4

over 4 years ago

0 Irwin Nederbragt over 4 years ago

TI__Mastermind 28655 points

Hello,

It appears you are PWM dimming this circuit. What PWM frequency and duty cycle are you using?

Can you get oscilloscope pictures of Vin, Vled, the switch node and inductor current?

Best Regards,

0 Jishnu Prakash Nambiar over 4 years ago in reply to Irwin Nederbragt

Prodigy 120 points

we are varying duty cycle from 0 to 100% at frequency 500 Hz

Vin is 24 V

VLed 11V

indiactor current 0-3mA

we are getting smooth dimming if risense is high value lke 1ohm but the current is low like 350mA

our application needs atleast 1A maximum

flickering occures when current is between 750mA anf 800mA, we dont have problem when current in between

0-500mA and 800mA-1A which is between duty cycle 60-80%

0 Irwin Nederbragt over 4 years ago in reply to Jishnu Prakash Nambiar

TI__Mastermind 28655 points

Hello Jishnu,

It sounds like possibly a noise issue. I cannot help without oscilloscope pictures showing waveforms asked for above. I also don't know what indiactor current is? If this is inductor current, 0-3 mA with a 800 mA current set point will not work.

Best Regards,

0 Jishnu Prakash Nambiar over 4 years ago in reply to Irwin Nederbragt

Prodigy 120 points

hi this is image of Vin at 50% PWM

0 Jishnu Prakash Nambiar over 4 years ago in reply to Irwin Nederbragt

Prodigy 120 points

Vled at 50% PWM

0 Jishnu Prakash Nambiar over 4 years ago in reply to Irwin Nederbragt

Prodigy 120 points

PWM SIGNAL

0 Irwin Nederbragt over 4 years ago in reply to Jishnu Prakash Nambiar

TI__Mastermind 28655 points

Hello,

I'm not sure what the Vin oscilloscope picture is, it states 50V/div and shows about 2.5 divisions. Regardless, if Vin looks like that it won't work, it needs to be DC. Why is Vin unstable? Do you have an input filter for EMI? What does the power source voltage look like?

Best Regards,

0 Jishnu Prakash Nambiar over 4 years ago in reply to Irwin Nederbragt

Prodigy 120 points

hi,

these noises appear when the flickering occurs else it is perfect DC

0 Irwin Nederbragt over 4 years ago in reply to Jishnu Prakash Nambiar

TI__Mastermind 28655 points

Hello,

Doesn't matter, the input should not do that. Or let me rephrase, the flickering occurs when the noise appears. If the input moves that much it will flicker. BTW can you get oscilloscope pictures of Vin with the correct voltage range. I don't think your input is near 125 volts so I don't know if you have an offset or the V/div is wrong. If it is there is too much impedance which can cause instability. I can see instability in the waveforms. Get a low impedance power source and see what happens, or bypass your EMI filter if that's what is causing this.

Best Regards,

0 Jishnu Prakash Nambiar over 4 years ago in reply to Irwin Nederbragt

Prodigy 120 points

vin when pwm signal is not applied sorry for image quality

0 Jishnu Prakash Nambiar over 4 years ago in reply to Irwin Nederbragt

Prodigy 120 points

vin when pwm is applied & we are not using any emi filter

0 Irwin Nederbragt over 4 years ago in reply to Jishnu Prakash Nambiar

TI__Mastermind 28655 points

Hello,

Okay, so what does the SW node and output/inductor current look like now? Vin needs to be solid and stable to have an output current that is also stable.

Best Regards,

0 Jishnu Prakash Nambiar over 4 years ago in reply to Irwin Nederbragt

Prodigy 120 points

hi,
now the flickering issue is solve but , it takes some time to reach the designed maximum output current
i designed for maximum 680mA but when i switch on the device for first time it only reaches maximum current of 280mA
then over time i reaches maximum designed current

0 Jishnu Prakash Nambiar over 4 years ago in reply to Irwin Nederbragt

Prodigy 120 points

also what kind of thermal relief you would recommend,

0 Irwin Nederbragt over 4 years ago in reply to Jishnu Prakash Nambiar

TI__Mastermind 28655 points

Hello,

What is the time constant of C2 and R3? If this is not the reason I would measure the input and output voltage to make sure they aren't close together. You have 24V input and 11V led which should be fine. Make C2 a much smaller value, maybe 100 pF which should speed up the current set-point

Not sure what you mean by thermal relief? The datasheet will have recommendations for PCB layout. Page 21 of the datasheet shows an example.

Best Regards,

0 Jishnu Prakash Nambiar over 4 years ago in reply to Irwin Nederbragt

Prodigy 120 points

hi,
can you suggest another chip for our purpose,
we are hopping here to develop a led driver which can drive up to 20 w
at 24 v, we need pwm to analog dimming method

0 Irwin Nederbragt over 4 years ago in reply to Jishnu Prakash Nambiar

TI__Mastermind 28655 points

Hello,

Have you tried changing the time constant on Iadj?

There are a lot of parts that will work, TPS92515, TPS92640/641, LM3409, TPS92512/13.

Best Regards,

0 Jishnu Prakash Nambiar over 4 years ago in reply to Irwin Nederbragt

Prodigy 120 points

thank you

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TPS92512: TPS92512 flickering problem