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LP87524J-Q1: Spread-Spectrum Mode

Koshi Ninomiya
Expert 5455 points
Part Number: LP87524J-Q1
Other Parts Discussed in Thread: IWR1843

Tool/software:

Dear Team,

My customer is using the LP87524JRNFTQ1 for powering the IWR1843AQGABL.
This time, they are considering to use the Spread-Spectrum Mode of the LP87524J-Q1 to reduce the EMI emissions radiated.
Then, they have some questions below.

Q1: Is there any document that compares the EMI reduction effect of the 4MHz fixed mode and the Spread-Spectrum Mode?

Q2: Can you please tell us the range of frequencies that are spread in the Spread-Spectrum Mode?

Q3: LP87524J-Q1 setting timing.
         1. The IWR1843 start-up.
         2. Set the following address for LP87524J-Q1 in I2C.
               Address: 0x2B
                              PLL_MODE[1:0] : 0x2 ⇒ 0x0
               Address: 0x2C
                              EN_SPREAD_SPEC: 0 ⇒ 1
       Are there any restrictions on setting timing?
       Until now, the LP87524J-Q1 has been used with the default value, but are there any functions that will be hindered by changing the above?

Best Regards,

Koshi Ninomiya

  • +1
    TI__Genius 16940 points

    Hello,

    Koshi Ninomiya said:
    Q1: Is there any document that compares the EMI reduction effect of the 4MHz fixed mode and the Spread-Spectrum Mode?

    No, they need to make this comparison by enabling the SS. 

    Koshi Ninomiya said:
    Q2: Can you please tell us the range of frequencies that are spread in the Spread-Spectrum Mode?

    The spread is almost +/-10% of Fsw. It can be calculated with 21.75/24xFsw to get lowest frequency from spread spectrum and 24/21.75XFsw to get max spread frequency. For example for the 2.25MHz Fsw, the Fsw would spread between 2.04 MHz to 2.48MHz. 

    Koshi Ninomiya said:
    Q3: LP87524J-Q1 setting timing.
             1. The IWR1843 start-up.
             2. Set the following address for LP87524J-Q1 in I2C.
                   Address: 0x2B
                                  PLL_MODE[1:0] : 0x2 ⇒ 0x0
                   Address: 0x2C
                                  EN_SPREAD_SPEC: 0 ⇒ 1
           Are there any restrictions on setting timing?
           Until now, the LP87524J-Q1 has been used with the default value, but are there any functions that will be hindered by changing the ab

    Your sequence is correct for enabling the SS. There might be very short behavior change in the switching node since the PLL gets disabled and internal oscillator enabled. That can be observed on the oscilloscope with switch node but that might be so short that it doesn't affect anything. Other than that I do not think of any impact. 

    Regards,

    Ishtiaque