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Original question:

TPS54540: Compensator check

WEBENCH® Tools/TPS54540: Junction temperature by SW frequency

user5784956
Prodigy 20 points
Part Number: TPS54540

Tool/software: WEBENCH® Design Tools

Hi All,


I am designing a convertor (24V->15V) with TPS54540.

At this time I tried to use the tool WEBENCH and I am puzzled with the title issue.  Maybe without the tool the issue also would put me in a maze.


The WEBENCH recommends the resistance Rt (connected to the RT/CLK pin) to be 127kOhm.


According to the Equation 8 on the datasheet, the SW frequency may be 760kHz.  In that case the figure 5 shows the junction temperature of MOSFET seems to be very high, much higher than its rating temperature.


Is it premised on using heatsink?


It seems strange that the SW frequency margin is very tight for the junction temperature in the Fig. 5 on the datasheet.  And it has -25, -50 degreeC in the figure.  Why??


How should I read the junction temperature charateristics?

  • 0
    TI__Mastermind 25590 points
    User,

    Figure 5 is showing you the frequency drift as temperature changes. So this is not saying that an operating frequency of X will make the IC run at Y temperature. It's saying that if you chose a frequency X, it will drift this much as Y changes.

    This figure shows how the frequency changes if you've selected ~500kHz at room temp but this trend can be extended to any switching frequency.

    -Sam
  • 0 in reply to Samuel Jaffe
    Prodigy 20 points

    Hi Sam,


    I appreciate you for the quick reply.  I could be completely understood.

    What simple the issue is, once I know it!

    Thank you,

    Mina