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LM51551-Q1: Switching frequency tolerance

DY Ko
Expert 5110 points
Part Number: LM51551-Q1

Hello Power team,

My custoemr asked about the switching tolerance of LM51551-Q1.
According to the datasheet, the fsw spec is 1980kHz(min)/ 2200kHz(typ)/ 2420kHz(Max) as capture below.
But my customer want know about the accuray of the switching tolerance of LM51551-Q1 itself (or internal oscillator variation spec) when setting the fsw=2200kHz using Rt=9.09kohm+/-0.1% resistor tolerance.

Best regards,
DY

  • 0
    TI__Intellectual 1700 points

    Hi DY,

    appollogies, but please expect delay during chinese new year.

    I will get in contact with the dedicated specialist and get you the answer by the end of the week.

    regards,

    Moritz

  • 0 in reply to Moritz.
    TI__Guru* 78070 points

    Hi DY, 

    The datasheet curve and limits exclude the effects of external resistor tolerances. When you use 0.1% resistor, the worst case frequency tolerance can basically increase by a little more than 0.1% due to additional resistor variations like aging and temperature coefficient.  Check your resistor datasheet to get aging and temperature coefficient. 

    Thanks,

    Youhao Xi, Applications Engineering.

  • 0 in reply to Youhao Xi
    TI__Expert 5110 points

    Hi Youhao,

    Thanks for your response.

    My customer wanted to know the intenal oscillator frequency accuracy of LM51551-Q1 itself excluding external RT resistor. Because the MAX15005 has +/-7% tolerance @ Fosc=1MHz as capture below. So please check the intenal oscillator frequency accuracy of LM51551-Q1 for Fsw=1MHz and 2.2MHz, respectively.

    Best regards,
    DY

  • 0 in reply to DY Ko
    TI__Expert 5110 points

    Hello Power expert team,

    My customer is still waiting for the accuracy spec of LM51551-Q1's internal switching frequency.

    Best regards,
    DY

  • 0 in reply to DY Ko
    TI__Guru* 78070 points

    Hi DY, 

    The frequency accuracy at 2.2MHz is already given in the datasheet, it is 1980 to 2420 KHz.  In other words, it is +/-10% about 2.2MHz.  At 100kHz, it is +/- 15%. I don't have the char data but it should be close to +/- 12.5%.  Note that these are datasheet parameter limits.  Actual device frequency tolerance would be much tighter but the datasheet just gives out the worst case parameter spans and you should not easily see such extreme numbers in real circuit. 

    Thanks,

    Youhao