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LM431: Case temperature vs operating temperature

murali kandasamy
Intellectual 350 points
Part Number: LM431
Other Parts Discussed in Thread: TL431

Hi,

In our application, the regulator case temperature is measured to be 98deg. C. We are using a TO92 package LM431, with a max current of 75mA and output is 10V.

1. Is a heatsink required?

2. The case temperature exceeds the operating temperature of -40-85 deg. C. Is this acceptable?

Thanks & Regards,

Murali K    

  • 0
    TI__Mastermind 19370 points
    Hi Murali,

    That is a lot power going through the device.

    1. This depends on the configuration but if the device is heating up higher than the ABS max temperature. I would recommend one or use a different circuit configuration.
    Might you be able to show me the configuration of the device?
    Also what package is this device in?
    Is 75mA both the current through the device and the load or only the device? Is this when the load is off?

    2. I would not recommend going above the ABS max temperature. We do not guarantee any performance above that temperature.

    -Marcoo
  • 0 in reply to Marcoo Z
    Intellectual 350 points

    Hi,

    The above is our application circuit. This is based on section 9.2.1 Shunt Regulator of the LM431 datasheet. A regulated output of 10V is derived from 15V. The R39 was increased from 47R to 68R  to keep the cathode current within the 100mA limit but we observe that the temperature is still greater than 85deg. C. The typical load is 175Ohm when R39 can be maximum 82R and minimum load is 0 when R39 can be minimum 48R. What is the best value for R39 to have safe operation?

  • 0 in reply to murali kandasamy
    TI__Mastermind 19370 points
    Hi Murali,

    Your circuit is correct. One thing that is commonly over looked that needs to be take into account when designing a circuit is the Junction-to-ambient thermal resistance that is on section 6.4. By using this value and your package you can see if you need a heat sink if the temperature the device violates the ABS max temperature of the device. Another option is to change the circuit into something like Figure 19, as the current into the device is controlled and the device won't experience such a high temperature increase due to the load being off.

    Have you tried looking into the TL431. The TL431 specifies a Operating virtual junction temperature of 150C which means the device can work as long as the internal temperature of the device is below 150C. the LM431 and TL431 are extremely similar devices.

    -Marcoo