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TLV757P: Clarification of Thermal specifications

Renan Santos Adriano
Genius 16500 points
Part Number: TLV757P

Hello Team, 

Thermal specs for TLV757P are described in datasheet section 6.4

RθJB Junction-to-board thermal resistance =  64.4 °C/W for SOT23-5 package
RθJB Junction-to-board thermal resistance  = 64.3 °C/W for WSON package
...=> RθJB for SOT23-5 and WSON have same value (within 0.1);
...=> indicates that both packages are equal at conducting heat into PCB;
...=> indicates that junction temp shoud be same for both packages.

RθJA Junction-to-ambient thermal resistance = 231.1 °C/W for SOT23-5 package;
RθJA Junction-to-ambient thermal resistance = 100.2 °C/W for WSON package;
...=> RθJA for SOT23-5 is approx 2.31 x WSON;
...=> indicates that junction temp for SOT23-5 will be hotter than WSON for same power dissipation;

Can you explain why the RθJB ratio is the same for both packages but the RθJA ratio is 2.31?

Regards,

Renan

  • 0
    TI__Mastermind 37945 points

    Hi Renan,

    If you use RθJB to measure the junction temperature, you have to measure the temperature of the board near the DUT. The temperature of the board near the DUT will not be the same between the packages because heat leaves the DUTs differently. The RθJB's being basically the same mean that the junction temperatures of each package will be about the same amount higher than the measured board temperature, but again the board temperature will be different so the junction temperatures will be different. You see a large difference in the RθJA because the ambient is more constant than the board temperature since it does not change when you start dissipating energy in the devices. 

    Does that make sense?

    Regards,

    Nick

  • 0 in reply to Nick Butts
    TI__Genius 16500 points

    Hello NIck,

    It's still not clear to them how RθJB ratio for Package types can be ~ 1:1..but RθJA can be ~ 2:1?

    Also they added that Psi_JB ratio for both Package types is ~ 1:1 (64.4: 64.3);
    They assume Psi_JB values have been simulated/measured based on same Pcb and Environment ?
    Psi_JB ratio should be closer to application level with heat flow from Package both into PCB and Air ?
    How do you explain ratio for Psi_JB ratio is the same...but RθJA can be ~ 2:1 ?

    Regards,

    Renan

  • 0 in reply to Renan Santos Adriano
    TI__Mastermind 37945 points

    Hi Renan,

    As I described in my previous comment, the difference is that using RθJB uses the board temperature as the reference to calculate the junction temperature, which is not constant. RθJB and Psi_JB being about the same between the packages says that the junction temperature will be about the same temperature above the board temperature. The board temperature depends on the ambient temperature and amount of power being dissipated and spread into the board, which is to say that the board will be at a different temperature for the different packages because heat leaves the packages differently. 

    In contrast, the RθJA uses the ambient temperature as the reference, which is more constant. The SOT-23 package is not as effective at removing heat from the part and so has worse thermal performance than the WSON, which is reflected in the RθJA.

    All of the thermal metrics are simulated with JEDEC standardized PCB layouts, and yes the characterization parameters are closer to actual applications and are considered much less dependent on board layout. 

    Regards,

    Nick