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LM25143: TPS40140 External SYNC frequency range

Minkle Eldho Paul
Prodigy 30 points
Part Number: LM25143
Other Parts Discussed in Thread: TPS40140

Tool/software:

Hello,

The 'External SYNC frequency range' of +/-20% is specified for an input range of 8V to 18V (page 11 of datasheet) . What happens to this range when V_IN is close to 42V?

Also, does the +/-20% range account for variation in the internal oscillator clock frequency set by the RT resistor (195 kHz to 245 kHz when R_RT = 100 kOhm: section 8.6 on page 12.)

The application under consideration requires external synchronization capability with the external clock frequency being any value in the range of 180 kHz to 260 kHz. Can LM25143 handle that when R_RT is set to 100 kOhm?

Thank you,

Eldho

  • 0
    TI__Mastermind 22280 points

    Hi Eldho,

    The device should be OK at 42V. The condition listed in the datasheet is just where the specification was tested at and where you will see the best performance in relation to that specification.

    The +/-20% will be for whatever the switching frequency measured on the device is when you set RT=100kHz. You will need to account for the variation in the internal clock to calculate your worst case scenarios for switching frequency.

    The LM25143 is capable of meeting that requirement, but you need to calculate the worst case possible variation in frequency and account for it in your design.

    Beat regards,

    Ridge

  • 0 in reply to Ridge Lahti
    Prodigy 30 points

    Hi Ridge,

    Thank you very much for your response.

    Okay. Then, if I understood it correctly, the reliable external SYNC frequency range, for R_RT = 100 kOhm is only from 196 (=245*0.8) kHz to 234 (=195*1.2) kHz, once the internal clock frequency variation is taken into account.

    Could you please recommend some workways to make the solution work reliably, within the external SYNC frequency range of 180 kHz to 260 kHz?

    Thank you and kind regards,

    Eldho

  • +1 in reply to Minkle Eldho Paul
    TI__Mastermind 22280 points

    Hi Eldho,

    These are specifications of the variance within the device, and there are not workarounds for these internal parameters.

    The device is capable of meeting the application conditions, but if you are concerned about possible worst case frequency range, you have to calculate it based on the limits listed in the datasheet. 

    Best regards,

    Ridge

  • 0 in reply to Ridge Lahti
    TI__Mastermind 22280 points

    Hello,

    I am closing this thread due to inactivity.

    Best regards,

    Ridge

  • 0 in reply to Ridge Lahti
    Prodigy 30 points

    Hi Ridge,

    Once again, thank you very much for your quick response.

    Okay. I understand.

    Kind regards,

    Eldho

  • 0 in reply to Minkle Eldho Paul
    TI__Mastermind 22280 points

    Hi Eldho,

    Since this issue seems resolved, I will close this thread. If you have more questions, you can re-open this thread by replying or create a new thread.

    Best regards,

    Ridge

  • 0 in reply to Ridge Lahti
    Prodigy 30 points

    Hi Ridge,

    we are thinking of using TPS40140 for this application (even though it is an old part and no longer recommended by TI for new designs. Unfortunately the alternative suggested TPS40322 doesn't support the Vin range!). Does TPS40140 has any limitations on the external SYNC frequency range? The datasheet doesn't mention anything of that sort!

    Thank you and kind regards,

    Eldho

  • 0 in reply to Minkle Eldho Paul
    TI__Mastermind 22280 points

    Hi Eldho,

    You should not use devices which are not recommended for new designs. 

    If you are looking for newer devices in the mid-vin range, I will transfer this thread to the relevant product line.

    The best device for a wide-vin range is still the LM25143.

    Best regards,

    Ridge

  • 0 in reply to Ridge Lahti
    Prodigy 30 points

    Hi Ridge,

    Thank you for your quick response.

    I understand. LM25143 does fit the bill in every aspect except one. As mentioned earlier, LM25143 doesn't seem to support the external sync frequency range of 180 kHz to 260 kHz (or at least 190 kHz to 250 kHz), needed for the application under consideration. Hopefully there are some ways to ensure that LM25143 can be reliably operated within that frequency range. Would highly appreciate, if you could transfer this thread to the relevant product line. 

    Kind regards,

    Eldho

  • 0 in reply to Minkle Eldho Paul
    TI__Expert 4970 points

    Hi Eldho,

    TPS40140 can support an external clock pulse stream at 6 or 8 times the Master PWM frequency. However, we do not recommend this device for newer designs.

    Regards,

    Maha

  • 0 in reply to Maha Noor
    Prodigy 30 points

    Hi Maha,

    Thank you for the reply.

    I understand that TPS40140 is no longer recommended for new designs. Nevertheless, it is the only device that seems to meet our requirements!

    Since the internal oscillator is disabled in the external clock mode of TPS40140 (by connecting the RT pin to BP5), I assume that the external SYNC/CLOCK frequency range is independent of the internal oscillator clock frequency (unlike in LM25143) and can be 6 or 8 times the 'Phase frequency set range of 150 kHz to 1000 kHz. Isn't that the case?

    Kind regards,

    Eldho

  • +1 in reply to Minkle Eldho Paul
    TI__Expert 4970 points

    Hi Eldho,

    Yes, the internal clock is disabled so the external clock is independent of the internal clock.

    Regards,

    Maha

  • 0 in reply to Maha Noor
    Prodigy 30 points

    Hi Maha,

    Okay, thank you for the confirmation.

    Kind regards,

    Eldho