TPS92641: TPS92641 Schematic review & UDIM

Hi Sheree,

I am out of office today. I will check this thread tomorrow.

BR,

Steven

Kindly waiting for your valuable reply, do the needful at the earliest..

Hi Sheree,

Sorry for missing this thread. I will have a check today.

BR,

Steven

Hi Sheree,

Sorry for the delay.

• In general, PWM dimming frequency (the UDIM frequency you mentioned) should be at least one order of magnitude lower than the steady state switching frequency to prevent aliasing. However, for good linear response across the entire dimming range, the dimming frequency may need to be even lower.
• Regarding your schematic, your current setting is around 6.67A. Consider using a 0.033 ohm sense resistor to achieve the 6A target.

Best Regards,

Steven

Any formula to calculate UDIM Frequency?

Bcz, I am getting oscillation in leds at some frequency like 200Hz,1.2KHz etc.., 

Hi Sheree,

There is no formula. The UDIM frequency is based on your application / spec requirement / performance requirement.

Regarding the oscillation you mentioned, do you have any oscilloscope capture for it? Where do you probe? Do the other parts of the system work well?

Best Regards,

Steven

Above LED line graph is captured through high speed camera & for 1.22KHz it get oscillate & for 25KHz no oscillation,

in put through Function generator & frequency varied randomly as per line graph visual inspection 

Hi Sheree,

So the graphs are captured from an instrument that can measure light component (R/G/B) intensity. Right?

I would say 25kHz is too much for TPS92641 PWM dimming with UDIM pin. Normally a dimming frequency <= ~1kHz is appropriate to maintain good dimming linearity.

For the 1.22kHz oscillation you mentioned, can you measure the voltage across the sense resistor / output current with an oscilloscope and send me the capture picture?

Best Regards,

Steven

Hi Steven,

       When I try to operated led current  between 4 to 5.5 A,for all UDIM  frequency led gets oscillated (can'table to arrest the oscillation), so kindly support in this regards...

Hi Steven,   

Kindly refer the video & voltage measured for various Led current (ILed) & kindly support in this regards for 4 to 5.5A load also

Hi Sheree,

• What do you mean by saying "oscillation"? Do you mean the LEDs are blinking?
• How do dim the LED? using UDIM or SDIM pin? Do you test the circuit on the TPS92641EVM?
• One concern is that your Vin = 52V, Vout = 49V, the only 3V dropout voltage may be too low for TPS92641 to function well. 

BR,

Steven

Hi Steven,  

     In Line Graph, the waveform gets oscillated (means leds are blinking), but 1.22KHZ,25Khz we can't able see visually.

     yes, By using TPS92641EVM UDIM pin i am dimming the led below components are changes

Mosfet : STD100N10F7

Inductor : 22uH

C2 : 10uF

R10 : 0.03 Ohm

R1 : 38.1 K

R7 : 445 K

Vin : 52 V (if recommended, i can adjust 53V max)

Iin : 7A Iled : Max 6A (Kindly share your recommendation)

Vled : Min 49V (Kindly share your recommendation)

Hi Sheree,

•  Your Vin = 52V, Vout = 49V. The only 3V dropout voltage may be too low for TPS92641 to function well.
• Please kindly answer the below questions:
  • Where does this line graph come from? From what instrument? This is important as it will let me know how you judge the oscillation.
  • What is the oscillation frequency?
• Please be noticed that when you use the UDIM pin to dim the LED, the LED is PWM dimmed. That is to say, the LED is turned on and off repeatedly. I am not sure if this is the oscillation you mentioned. Just a kind reminder.

BR,

Steven

Hi Steven,

       (1) The Led Strip with diffuser is directly view through High speed camera & the view through frame grabber & its software. (depending on led brightness line graph will be plotted) 

       (2)  Oscillation frequency depends led load  for 2A Led strip oscillate at 1.22Khz & Okay frequency is 25Khz  / 48.5V

        for 3.8A Led strip oscillate at 1.22Khz & Okay frequency is 8.5Khz  / 49.2V

      (3)  Yes, I am using UDIM pin (TPS92641EVM) & feeding PWM through Waveform generator.

If we use PWM for SDIM pin, issue can be solved?

If we use SDIM we need to add below components (or) any new to be added

(a) Mosfet  (Q3) : STD100N10F7

(b) R11 : 1.21MΩ (along with existing R9)

(c) C9 : 1uF

Sheree, 

Our experts in China are on holiday this week.  Please expect a delay in response.

Best regards,

Dave

Hi Sheree,

Thanks for your patience.

What I mean is that could you measure the blinking frequency from the frame grabber?

Or could you measure the voltage across the sense resistor / output current with an oscilloscope and then check the oscillation frequency?

BR,

Steven

Hi Sheree,

For 13A application, you can still take a look at TPS92641 / TPS92640.

BR,

Steven