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AM26LS31: The ship is very hot , what`s the reason

aiwen chen
Intellectual 625 points
Part Number: AM26LS31
Other Parts Discussed in Thread: ISO5451, , AM26LV32, AM26C31

The picture above is the circuit diagram design, and the chip is hot when working. The 5V for power supply, the current is very small, about 50mA, I have the following questions:
1、What causes the chip to be hot?
2、 Is there a problem with the circuit diagram design?
3、How to improve the problem of fever?
4、Are there any other better design recommendations?

  • 0
    TI__Mastermind 45867 points

    Hi Aiwen,

    I have a few questions real quick to clarify a few things

    1. Where are you measuring the 50mA - is that ICC or is that output current from the drivers? 

    2. Can you show the loading of the drivers - I don't see any major concerns with what is shown - but I would like to see what teh driver is communicating with if at all possible.

    3. What is the ambient temperature during operation? Is there air-flow or a heat sink attached to the device

    This device will typically  gain ~ 64C for every Watt of power dissipated - depending on environment and loading will greatly influence how much power the device can handle without heating up.

    Please let me know on those three questions so I can check if this is more or less expected in this application.

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Intellectual 625 points

    Hi  parker；

    The VCC CURRENT is  50mA 。The input termination is an IGBT module, the model is ISO5451, and the FLT pin of ISO5451 is connected to AM26LS31.The output of AM26LS31 is connected to AM26LV32 through a 70CM cable. We use the differential output to be converted to single-ended to increase the immunity to interference, and currently our ambient temperature is 30°C to test the temperature of the chip at 64°C, and the temperature rise is 34°C. Please help assess whether this temperature rise is normal? Is there a way to lower the temperature？

    AM26LV32 does not get hot. The AM26LV32 is followed by a DSP. In addition, I switched R83 and R84 to 22R, and the temperature dropped by 1°C. Is there any other way to bring the temperature down?

  • 0 in reply to aiwen chen
    TI__Mastermind 45867 points

    Hi Aiwen,

    So based on the results you are seeing and the fact that changing that resistor changes your heating (essentially increasing the resistance on those resistors is decreasing the amount of current - from a very quick approximation - I'd say ~4mA reduction in current based on your results - I don't see a major issue. If there 

    This part consumes a lot of power when the outputs are disabled (up to 80mA - in just quiescent current - without any driving - which - and your outputs are enabled which will increase the current consumption ) and they are actively driving a load (which includes the cabling and device). You are basically increasing the junction temperature from VCC/ICC alone about 15C to 20C that plus the output drivers is going to greatly increase your self heating of the device. 

    On the note that the receiver isn't heating up as much is because receivers are the less power intensive side of the bus - they act as a resistance to ground on the bus input - which is high enough to not create too much self heating. That plus no driving capabilities usually result in less power (therefore less self-heating) on receiver side.

    On the note to decrease heating - there are a few options.

    1. Increase air-flow over IC; this will decrease the effective thermal to ambient impedance - which will result in less self heating for the same power applied. 

    2. Attach a heat sink to IC which will prevent the IC from heating as quickly. 

    3. You can increase those series resistors - but this could degrade your differential output which could result in communication failure - however on a 70CM cable you may be able to bump those up to 100 Ohms without much issue as that is generally considered a short bus with respect to frequency. 

    Here is a link to a application note that goes into great depth on what impacts the thermal impedance - as the effective is what matters at the end of the day - not the listed value (as these are very variable based on the system setup)  https://www.ti.com/lit/an/spra953c/spra953c.pdf?ts=1689273733372 

    That being said - if you see any other functional concerns please let me know- at this point however I do think the heat rise that you are seeing is more or less expected. 

    Best,

    Parker Dodson

  • 0 in reply to aiwen chen
    Guru 263650 points

    The AM26LS31 is a very old, bipolar device. To reduce its quiescent supply current, replace it with a CMOS device like the AM26C31.