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TPS922055: Schematic review for dimming 12x Cree XP-G4 LEDs

Sam Lane
Prodigy 10 points
Part Number: TPS922055

Tool/software:

Dear TI,

I would like to request a review of the following design. The goals of the design are as follows:

  • Dimming of 12x Cree XP-G4 High Intensity LEDs in series
  • 42V input
  • Hybrid dimming (ADIM/HD pulled to ground)
  • 1kHz PWM signal
  • 1.5A max current
  • Smooth low end dimming with no flickering

I have closely followed the second application example provided in the datasheet as it seems to fit my needs. All values have been calculated from equations provided.

Questions:

  1. What switching frequency should be chosen to achieve this (currently I am using 1MHz, which gives a 10uH inductor, is this wise?)
  2. Are there any guidelines for choosing an appropriate diode.
  3. Are my UVP resistors suitable
  4. Is any additional input capacitance required?
  5. Cflt is 2.2uF on the evaluation board, and 1nF on the datasheet. This is a very large discrepancy. Why?
  6. Ccomp is 10nF on the evaluation board, but 1nF on the datasheet. What is the reason for this discrepancy?
  7. With 0603 components, the passive capacitors aren't that close to the IC (see the first picture), is this an issue?

Notes:

  • All passive components are 0603, except for the 1uF output capacitor (0805) and the input electrolytic capacitor.
  • Fault is pulled to ground.
  • ADIM/HD is pulled to ground to enable hybrid dimming.

  • 0
    TI__Mastermind 25850 points

    Dear Sam,

    Pls kindly let me review your schematic and get back to you during these days. Thank you!

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    Prodigy 10 points

    Dear Lucia,

    Thank you very much. I look forward to reading your response.

    Please find updated layout and schematic below:

    I have not finished adding vias to this layout but it gives an idea of routing and placement.

  • 0 in reply to Sam Lane
    TI__Mastermind 25850 points

    Hi Sam,

    Thanks for your update. I will review the latest schematic and layout you sent me and will give you feedback before tomorrow. Thank you!

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    Prodigy 10 points

    Hi Lucia,

    Thank you, have you had time to review this design?

    Kind regards,

    Sam

  • +1 in reply to Sam Lane
    TI__Mastermind 25850 points

    Dear Sam,

    Sam Lane said:
    • What switching frequency should be chosen to achieve this (currently I am using 1MHz, which gives a 10uH inductor, is this wise?)
    • Are there any guidelines for choosing an appropriate diode.
    • Are my UVP resistors suitable
    • Is any additional input capacitance required?
    • Cflt is 2.2uF on the evaluation board, and 1nF on the datasheet. This is a very large discrepancy. Why?
    • Ccomp is 10nF on the evaluation board, but 1nF on the datasheet. What is the reason for this discrepancy?
    • With 0603 components, the passive capacitors aren't that close to the IC (see the first picture), is this an issue?

    Yes, it is wise to choose 10uH inductor if you are using 1MHz switching frequency.

    The choose of suitable diode mainly depends on the rated voltage and current. You can share me the part number of the diode and I can check for you.

    We have a FAQ discusses about how to choose suitable UVP resistors, pls refer to: 

    For the input capacitor, I would recommend you can follow the User's guide with 68uF/100V + 2* 22uF/100V and add an additonal 100nF cap which is close to VIN pin for high frequency filtering.

    Pls follow the datasheet and choose 1nF for Cfit. We are revising the User's guide.

    Pls folow the datasheet and choose 1nF for Ccomp. We have FAQ talks about how to select compensation components. Pls refer to: 

    I would recommend that you can place it as close as possible to the device.

    For the layout, you can refer to datasheet figure 8-26. And in the schematic, fault is connected to ground. However, in the layout, looks like its floating. 

    Thank you!

    BRs

    Lucia