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LM5005 / Rt

Sho Ogane
Genius 5575 points
Other Parts Discussed in Thread: LM5005

Hi all

I have two questions about LM5005.

1.Datasheet P.16 R1,R2,C12
I see the following text.
"when using an external set-point divider it may be necessary to clamp the SD pin at high input voltage conditions."
I don't know high input condition.
How much is this input voltage?

2.Datasheet P.10 Oscillaror and Sync Capability
We use the following equation to calculated Rt value.
Rt=((1/F)-580*10^-9)/135*10^-12
What is the value(135*10^-12)?
I think this value is concerned with Error Amplifier Gain/Phase(Figure 7).

※I understood 580*10^-9 is minimam off time.
Is my understanding correct?

Best Regards,
Sho

  • 0
    TI__Guru 52732 points

    Hello Sho,

    The abs max rating for SD is 7V for the LM5005, hence the recommendation to use a 6.2V zener if the SD voltage could go higher than this specification. This depends on the input voltage voltage and the resistor divider ratio from VIN to SD that confgures the input UVLO.

    For the RT equation, those are just constants that set the frequency - no relation to the loop parameters or Toff(min).

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Genius 5575 points

    Hi Tim san

    Thank you for your answer.
    Sorry my customer would like to know the detail information of the RT equation,
    I can't understand your comment "those are just constants that set the frequency"

    What are the things that the constants refers?

    Best Regards,
    Sho

  • 0 in reply to Sho Ogane
    Genius 5575 points

    Hi Tim san

    Thank you for your support.

    Could you please help us on our customer question ?
    We would appreciate it if you could advise us by 17th november.

    Best Regards,
    Sho

  • 0 in reply to Sho Ogane
    TI__Guru 52732 points

    Hi Sho-san,

    We have an updated version of the LM5005 datasheet that is pending publication. The equation for the frequency set resistor has been simplified in terms of the constants. In general, the resistance vs. freqeuncy curve is measured experimentally and and the constants are derived by suitable curve fitting.

    Regards,

    Tim