TPS92511: The Switching Wave from at LX pin Issue

Reddy

Prodigy 140 points

Part Number: TPS92511

Hi,

I have used TPS92511 for Driving two 3V LEDs at 350mA.

The Operating frequency was set as 300kHz using 33.2kHz Resistor

inductor used was 68uH.

The Switching waveforms at the LX pin doesn't seems to be perfect. There is some kind unwanted noise in it.

It would be helpful if some one can help in fixing this issue.

Thanks

over 6 years ago

0 Irwin Nederbragt over 6 years ago

TI__Mastermind 28655 points

Hello Rahul-R,

I'm not sure what the first picture is. What is the saturation current of the 68 uH inductor?

It's running around 31 KHz, I would check your resistor value. It is running discontinuous because the frequency is too low.

Best Regards,

0 Reddy over 6 years ago in reply to Irwin Nederbragt

Prodigy 140 points

The resistor value used is 33.2 Kohm, it is supposed to Work at 300kHz. The first wave form was captured when Input voltage is 12V.
The second wave form was captured when Input voltage is around 8V.

0 Reddy over 6 years ago in reply to Reddy

Prodigy 140 points

I changed the FS resistor to 20K, so that the frequency can be 500khZ, but still their is no change in the waveform.

First Picture when Input is 12V, Second Picture when Input is 30V

0 Irwin Nederbragt over 6 years ago in reply to Reddy

TI__Mastermind 28655 points

Hello,

What is your inductor part number/manufacturer? Something is not working correct, perhaps a schematic may help figuring out what is going on.

There is a change in the waveform. It almost looks like it's hitting current limit, what is connected to your output, two leds in series? Is there any capacitance?

Best Regards,

0 Reddy over 6 years ago in reply to Irwin Nederbragt

Prodigy 140 points

Inductor Used is Coilcraft 68uH

LPS8045B-683MRB

and another board with 33uH Bourns Inductor

Both of these are giving similar waveforms

0 Irwin Nederbragt over 6 years ago in reply to Reddy

TI__Mastermind 28655 points

Hello,

It appears the inductor should be fine. What are the values for R1, R2, C1, C2, C3 and D3 (part number). Is any thing connected to DIM? Or any other connections?

Best Regards,

0 Reddy over 6 years ago in reply to Irwin Nederbragt

Prodigy 140 points

R1 4.3 Ohm, R2 20kOhm, C3 10uF, C2 10uF, D3 PMEG6010CEJ,115 60V, 1A, nothing is connected to DIM

0 Irwin Nederbragt over 6 years ago in reply to Reddy

TI__Mastermind 28655 points

Hello,

Quick question, R1 is 4.3 ohm or 4.3 Kohm? It needs to be 4.3 Kohm. Diode looks fine.

"The integrated MOSFET has a fixed current limit of 1.2A to protect the application circuit during critical operation conditions like
short circuit of the LED string. Once the limit is hit, the integrated MOSFET turns off immediately for 34 μs to let
the inductor discharge."

It looks like this is about how long the converter is off before trying to run again.

Best Regards,

0 Reddy over 6 years ago in reply to Irwin Nederbragt

Prodigy 140 points

Got the problem, The resistor value was reading just 4.3 Ohm when measured. I replaced it with the 5K ohm and Its working Good Now. Attached are the waveform s at 10V and 30V Input.

Power management

Power management forum

TPS92511: The Switching Wave from at LX pin Issue

LPS8045B-683MRB