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LMH6321: Calculating junction temperature given the case temperature

Simon Edwards1
Intellectual 270 points
Part Number: LMH6321
Other Parts Discussed in Thread: OPA2210

Hello,

I am using eight LMH6321 PowerSO 8 package devices to provide higher current output capabilities for eight op-amps which are used for transducer excitation (Wheatstone bridges etc); there are two devices used per transducer excitation channel.  My PCB has 6 layers of 1oz copper of which three are used for heat spreading.  Other layers are used for signals and have been flooded with copper where possible to increase the spreading copper area.  The board area is approximately 7 square inches and thermal vias are used to connect each plane together.

Using a thermocouple I have measured the case temperature as well as I can to be approximately 81°C with an ambient of 25°C when dissipating 1W of power.  Only one channel is on at a time but there are other components on the board that contribute to the heat within the PCB.  What I need to know is the junction temperature of the LMH6321.

The datasheet for the LMH6321 provides values for theta(JA) when dissipating 0.75W of power with various PCB stacks and copper areas (Table 2). Choosing the best case of around 82°C/W and multiplying by the power dissipated then adding on the ambient temperature gives a theoretical junction temperature of 107°C.  Working back for theta(JC) would give 26°C/W but I am not convinced this is a particularly accurate figure knowing that if I drop the power dissipation by half, I should get a junction temperature of 66°C yet I measure a case temperature of 70°C.

Reading SPRA953C I understand that given a psi(JT) value I can estimate the junction temperature to a reasonably accurate value using the thermocouple method.

Do you have a psi(JT) value for this device please?

If you have any tips on how I can get an accurate junction temperature value I'd be very interested.

Thanks,

Simon

  • 0
    TI__Genius 15555 points

    Hello Simon, 

    I can help with getting you the temperature specs for the PowerSO 8 package. I will get back to you within 1-2 weeks.

    Best,

    Hasan Babiker

  • 0 in reply to Hasan Babiker31
    Intellectual 270 points

    Hi Hasan,

    Thanks for your help, I'll look forward to receiving the specifications soon.

    Regards,

    Simon

  • 0
    Guru 140200 points

    Hi Simon,

    what are you needing the LMH6321 for? Are you performing a modulation of bridge excitation?

    Can you show a schematic?

    Kai

  • 0 in reply to kai klaas69
    Intellectual 270 points

    Hi Kai,

    Here's a small part of the schematic, not wanting to give too much away!

    The sense and output connections are joined together at the transducer so that any voltage drop along the connection is catered for (shown by the green line).  The bridge excitation can be a modulated or DC signal which is fed into the left hand of R54.  Decoupling is not shown on this schematic.

    I'm using the LMH6321 part to increase the current output capability of the OPA2210 by putting it into the feedback loop.

    Cheers,

    Simon

  • 0 in reply to Simon Edwards1
    TI__Genius 15555 points

    Hello Simon,

    Below is the thermal info for the part:

    Result- Theta JA-High K (standard datasheet value):

    37.8

    Result-Theta JC, top (standard datasheet value):

    51.6

    Result-Theta JB (standard datasheet value):

    11.7

    Result- Psi JT (standard datasheet value):

    2.5

    Result- Psi JB (standard datasheet value):

    11.7

    Result-Theta JC, bottom (standard datasheet value):

    3.6

    Best,

    Hasan Babiker

  • 0 in reply to Simon Edwards1
    Guru 140200 points

    Hi Simon,

    have you thought about using a transistor buffer instead of LMH6321 at the output of OPA2210? The transistors could be mounted onto a heat sink to provide much better cooling.

    By the way, I carried out a phase stability analysis of your circuit and found that a cap of about 100nF from EXC-OUTP to signal ground is necessary to keep the circuit stable.

    Kai

  • 0 in reply to kai klaas69
    Intellectual 270 points

    Hi Kai,

    I have thought about a push-pull pair on the output to increase the current drive capacity of the circuit and it is something I may prototype in the future.  Heat sinking to the case of the enclosure isn't an option as the design is a card/backplane one so no permanent contact to the enclosure can be made.

    With the temperature figures given by Hasan in the previous post I should be able to judge how close to the junction temperature limit I am when driving a full load and decide upon what cooling techniques I can use from there.

    Thank you for pointing out the stability issue with the design.  I had made several phase margin simulations on that circuit but you have made me go over them again.  There seems to be a point where between 5nF and 10nF capacitor load on the output brings the circuit close to instability - I simulated around 30 degrees PM.  So adding extra capacitance at the output will help; a 100nF capacitor on the output results in 60 degrees PM.

    Thanks again - superb support as always.

    Simon

  • 0 in reply to Hasan Babiker31
    Intellectual 270 points

    Thanks Hasan, that's very valuable information.

    To confirm, I can take the Psi JT value and use that to estimate the junction temperature knowing what power is being dissipated by the buffer IC and what the top temperature of the case is?

    Thanks,

    Simon

  • 0 in reply to Simon Edwards1
    TI__Genius 15555 points

    Hello Simon,

    That is correct, you can find more details in the following app note:

    https://www.ti.com/lit/an/spra953c/spra953c.pdf?ts=1593457842850

    Best,

    Hasan Babiker