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TPS79901-EP: Thermal resistance Junction to Ambiant

Pascal Geyre
Prodigy 40 points
Part Number: TPS79901-EP

Hi all,

We are trying to add a drop-in alternate to TPS79901-EP for purchasing timeline.

the idea is to select TPS73401DRVR as the alternate to TPS79901-EP.

the LDO is converting the 3.3V input to 1.50V output (max 150mA at output) to power a FPGA core.

as the FPGA core 1.5V voltage input is within 1.425 & 1.575, the drop-in replacement will move the nominal 1.5V power voltage from 1.50V to 1.52V which is an acceptable value for the FPGA core. the TPS73401 can source more current than TPS79901 (250mA) and LDO dropout is in the same order order of magnitude for the use case.

the only concern (except the temperature screening @-55°C must be done for the  TPS73401DRVR as the use cas is within the military temperature range: -55°C to +125°C) is to verify the thermal features of  TPS73401 and the TPS79901 are in the same order of magnitude.

Datasheet of TPS73401 indicates Junction to ambiant  JθA (jedec High-K) of 65°C/W.

Datasheet of TPS79901-EP indicates Junction to ambiant  JθA  of 74.2°C/W but here there is no indication of jedec High-K/Low-K setup, therefore it is not clear to state whether TPS73401 JθA  is better than TPS79901-EP JθA or not. Can someone provide clarification on thermal comparison between these 2 Parts ?

Many thanks,

Pascal

  • 0
    TI__Genius 14245 points

    Hi Pascal, 

    Unless explicitly stated our LDO thermal metrics are taken using the standard JEDEC High-K board. We've tried to make that clear in our newer datasheets so some of our older datasheets didn't explicitly state that it used the standard high-k board, but that was what was used. 

  • 0 in reply to Kyle Van Renterghem
    Prodigy 40 points

    Hi Kyle,

    Thank you for confirming TPS79901-EP JθA  of 74.2°C/W is the value using the standard JEDEC High-K board. 

    Therefore from the datasheet comparison  TPS73401with JθA (jedec High-K) of 65°C/W has likely better thermal performance than the TPS79901-EP. 

    In order to definitively confirm this statement and close this topic, I would like you provide clarification on the power derating of TPS79901-EP.

    TPS73401 datasheet indicates a derating factor above +25°C of 15.4mW/°C and 1.54W for temperature below +25°C.

    What is the derating factor of TPS79901-EP  (datasheet does not provide it) ? this is un important characteristic too for thermal performance comparison...

    Many thanks,

    Pascal

  • 0 in reply to Pascal Geyre
    TI__Genius 14245 points

    Hi Pascal, 

    The derating factor listed in TPS73401 datasheet is not included into modern datasheets because we want to provide as much flexibility to customers as possible and so instead of dictating the power dissipation, what is important is keeping the junction temperature at or below the max specified temperature. Since every PCB design has a unique Rja (we have an App Note that shows how much it can vary with different package/PCB layouts) we don't set a maximum power dissipation since PCB designs that are very good at offloading heat will have a lower derating and vice versa. 

    What are you actually trying to do with this information? Maybe we can provide some guidance. 

  • 0 in reply to Kyle Van Renterghem
    Prodigy 40 points

    Hi Kyle,

    Before closing this topic, we need to be sure the TPS73401DRVR as a drop-in alternate to TPS97701-EP (same location in the PCB use case), is not providing issue due to thermal features. From our discussion so far, we can consider there will be no regression by this change due to power management,  as  JθA  of TPS73401DRVR is a bit better than JθA  of TPS97701-EP  with the same thermal setup.

    If you concur with this statement, we can conclude this issue is resolved.

    Many thanks,

    Pascal

  • +1 in reply to Pascal Geyre
    TI__Genius 14245 points

    Hey Pascal, 

    Yes after reviewing the thermal resistance for each part, the TPS73401DRVR has better thermal resistance and so we would expect the junction temperature of TPS73401DRVR to remain cooler than TPE79901-EP for the same thermal setup (PCB layout and power dissipation).