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LM140K: Max DC Input Voltage

Aaron Spaete
Expert 4290 points
Part Number: LM140K

Team,

My customer is asking for help with a question regarding Absolute Maximum Conditions and interpretation of the datasheet.

The TI datasheet for the LM140K lists the Maximum DC Input voltage as 35V.  Is there any guidance that can be provided as far as transient overshoots that may result in a spike above the 35V DC Limit?

Thanks,

Aaron

  • 0
    TI__Expert 4270 points

    Hi Aaron,

    Thanks for reaching out. Let me look into this and get back to you with an update by the end of the week.

    Regards,

    Kelsey

  • 0 in reply to Kelsey Farr
    TI__Expert 4290 points

    Kelsey, 

    Specifically, customer is looking for:  

    Can you please evaluate the LM140K with an input transient of 45V for 60mS.  If the LM140K cannot sustain 45V/60mS, please evaluate at 40V/40mS.  The operational ambient temperature range of the LM140K is -18C to +70C. 

     

    Thanks,

    Aaron

  • 0 in reply to Aaron Spaete
    TI__Expert 4270 points

    Hi Aaron,

    I took a look at this now, and LM140K does not support input voltages >35V absolute maximum. If more than that is required, we don't have a great alternative at that level of output current. Is the full 1.5A required?

    Regards,

    Kelsey

  • 0 in reply to Kelsey Farr
    TI__Expert 4290 points

    Here is the feedback.

    I am working with an existing circuit, we are locked-in to a fixed 5V linear regulator in a TO-3 package.  My task is to replace an obsolete Zener that is acting as over-voltage clamp ahead of the LM140, however the best Zener, (that I can choose from a limited list), will clamp 2-3 volts above 35V in my simulation.  (I padded the 2-3 volt exceedance for margin when I told you 40V/40mS)

     

    Is there a fixed +5V linear with a 40V or more absolute input voltage that is a drop-in replacement for LM140K?  The typical load current is about 50mA.  As I mentioned earlier, the ambient at the regulator is -18C to +70C.  The steady-state nominal input voltage to the regulator is 28VDC.

    Regards,

    Aaron

  • 0 in reply to Aaron Spaete
    TI__Expert 4270 points

    Hi Aaron,

    I'm not sure which OPN they're using, but LM140K only seems to be available in a TO-CAN-2 package (NDS). We do not have a recommended p2p replacement matching both specs and package. Instead, I'd recommend TPS7B8150DGNR.

    If the operating conditions are 28V to 5V at 50mA, that is 1.15W of power dissipation, which corresponds to an LDO die temperature increase of 72°C on top of ambient conditions, based on the JEDEC-standard thermal resistance value (RθjA) for this device. If the ambient temperature of the system is 70°C, the LDO die will be 142°C. This is still under the rated operating junction temperature of the device, but I recommend some thermal optimization of the PCB is done to reduce the effective RθjA.

    RθjA can be affected significantly based on the PCB layout, and we have an App Note that shows actual board measurements and quantifies the changes in RθjA for different types of board layouts. As a general rule, an optimized PCB layout can see a reduction of 35%-55% in the RθjA listed in our datasheets. App note on that HERE.

    Regards,

    Kelsey