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LP87524B-Q1: How do I configure switching frequency?

Masatoshi Oguri
Expert 4780 points
Part Number: LP87524B-Q1


Hello,

How can a user control the switching frequency?

I could not find any information about the switching frequency in the datasheet and EVM user manual.

Almost only information is the device supports 1 to 24MHz external clock. But what is the resultant Fsw?

Oguri (TIJ automotive FAE)

  • 0
    TI__Mastermind 30310 points
    Oguri-san,

    Thank you for posting your question on the LP87524B-Q1 in the e2e forum. Your thread has been assigned to the expert within TI, who will reply to your question as soon as possible and attempt to resolve your issue.
  • 0 in reply to Brian Berner
    TI__Genius 16410 points
    Hello Oguri-san,

    The switching frequency of the device is fixed by OTP, and cannot be changed using I2C.
    The device can also sync to an external clock, and this range is 1MHz to 24MHz in 1MHz steps using the PLL_CTRL register. The internal PLL will synchronize to this frequency, and scale the clock to match the device switching frequency.

    For example, if Fsw = 2MHz, and external PLL is configured for 4MHz, then the switching frequency will be 1/2 of the external clock frequency.
    Note that if the external clock goes too far away from the configured PLL frequency, the device will switch back to internal Fsw.

    Regards,
    Karl
  • 0 in reply to Karl Wallinger
    TI__Expert 4780 points

    Hi Karl,

    Thanks.

    So, the datasheet spec of fSW (3.6 min, 4 typ, 4.4 max for 4MHz) is applicable both for internal oscillator and external clock?

    Regards,

    Oguri

  • 0 in reply to Masatoshi Oguri
    TI__Genius 16410 points
    Hello Oguri-san,

    The datasheet fSW spec is internal switching frequency only.

    The external clock spec items are under the "EXTERNAL CLOCK AND PLL" section of the spec table. The external clock can be 1MHz to 24MHz in 1Mhz steps, and requires -30%/+10% accuracy in order for the device to sync to it. And the nominal external clock frequency is configured by PLL_CTRL register.

    Regards,
    Karl
  • 0 in reply to Karl Wallinger
    TI__Expert 4780 points
    Hi Karl,

    Thanks.
    Then, what will the resultant Fsw be with external clock?
    Say, the external clock is 13.3MHz, and the Fsw is ...?
    Can it switch as fast as up to 24MHz?

    Regards,
    Oguri
  • 0 in reply to Masatoshi Oguri
    TI__Genius 16410 points
    Hi Oguri-san,

    The resulting Fsw will be set by the ratio of the OTP- defined switching frequency and the external clock.

    For example:
    fSW = 2MHz, PLL_CTRL.EXT_CLK_FREQ=13MHz - Then the actual switching frequency will be 2/13 * external clock frequency.
    So if external clock is 13.3MHz, then the switching frequency will be 2/13 * 13.3MHz = 2.05MHz.

    If fSW=2MHz, PLL_CTRL.EXT_CLK_FREQ=14MHz, then the actual switching frequency will be 2/14 * external clock frequency.
    So if external clock is 13.3MHz, then switching frequency will be 2/14 * 13.3MHz = 1.9MHz.

    And any frequency can be used from 1MHz to 24MHz (step size 1MHz) for the external clock.

    The external clock frequency must be within -30%/+10% of the PLL_CTRL.EXT_CLK_FREQ setting. So if you set PLL_CTRL.EXT_CLK_FREQ=13MHz, the clock must be between 9.1MHz, and 14.3MHz for the device to synchronize to it.

    Regards,
    Karl