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LMR14030-Q1: Thermal information questions

Dongbao Zhou
Expert 4170 points
Part Number: LMR14030-Q1
Other Parts Discussed in Thread: LMR14030

Hi team,

For below thermal information provided in LMR14030-Q1 datasheet:

  • Why Rja （42.5) is lower than Rjc (56.1)? Typically, Rja equals to Rjc+Rca.
  • Since Rja is not so accurate for thermal evaluation as described in SPRA953, apart form Webench, how to do junction thermal calculation based on ambient temp and power dissipation?

Best regards,

Dongbao

  • 0
    TI__Expert 4170 points
    Another question is, could you explain why Tj=Ta+Rja*Ploss is higher than actual IC junction temp?
  • 0 in reply to Dongbao Zhou
    TI__Mastermind 34575 points
    Hi Dongbao,

    When test Rjc, only case top is thermal conduction path, all other surfaces is thermal insulated, LMR14030 only has thermal pad on case bottom, so the Rjc is higher than Rja. Rjc is more applicable for package with metal heat-sinking on case top surface.

    Tj=Ta+Rja*Ploss is higher than actual IC junction temp or not, it depends on the actual system Rja compared with data sheet spec 42.5C/W. in other words, if ctm PCB design has more layers(>4) and bigger size(> 114x76mm), then actual Rja maybe lower than 42.5C/W which result in a lower Tj.