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LM138: Support Enquiry

William Lahan
Prodigy 10 points
Part Number: LM138
Other Parts Discussed in Thread: CSD18537NKCS

We are interested in using the LM138 in a constant current source application.  The device is used in a 1 second pulse mode for current delivery to the load 3.5 amps.  This mode is very low duty cycle (hours to days).

The one area for concern is the LM138 power dissipation @94 watts during the pulse.  The only specification that I can find is 50 watts and this appears to be a steady state condition.  Therefore, the concern is the die temperature rise during the pulse and is the rise within acceptable limits.

All support information on this issue is appreciated.  A different device can also be considered.

  • 0
    TI__Expert 7445 points
    Hi William,

    As highlighted in the datasheet the specifications are applicable for TO3 package device dissipation up to 50W. I would think if you dissipate 95W, it would result in exceeding the junction temperature of the device.
    Are you taking any steps to ensure Junction temperature of he device does not exceed its rating.
    As Junction to case bottom is spec'd at 0.7C/W. what is the board temperature in your application? Device is rated for 125C max junction temperature, so you do not want to exceed that spec.
    I will also follow up with our reliability group as to if they have some input on the package dissipation.
  • 0
    Intellectual 515 points

    Hello William,

    What is needed here is a 'Transient Thermal Impedance' chart, where the x-axis is time in seconds and the y-axis is Effective Zθ(J-C), for this particular package.

    While this 'Transient Thermal Impedance' chart is a fairly typical thermal chart for bipolar and MOSFET type transistors intended for pulsed operation, it's rarely seen for linear regulators where continuous steady-state operation is the major (only?) consideration.

    As an example, see the 'Single Pulse' line in Figure 1 of the CSD18537NKCS datasheet. 

    My opinion is that a 1 second pulse duration is a long enough that it could probably be considered as 'continuous', even in the TO-3/NDS package.