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LM1085: Current limit conditions vs output voltage

Part Number: LM1085

When assessing the current limit spec at Vin = 26.25, can we expect this device to supply the min 200mA, even when we set the output to 1.25 V - ie resistor R2=0 volts??  Is it valid to expect this spec to hold under the conditions f VIn =26.25 V, and R2=0 Volts??  The datasheet does not seem to address this condition., 

  • Hi Andrew,

    LM1085 is rated for a maximum junction temperature of 125C. You can calculate the power that will be dissipated by the LDO as:

    Pd = (Vin - Vout) x Iout
    = (26.25 - 1.25) x 0.2
    = 5 W

    By using the thermal information table of the datasheet you can find that RθJA for the NDE package of LM1085 is 22.8 C/W. You can use this information to calculate the approximate junction temperature of the LDO:

    Tj = Pd x RθJA + Ta
    = 5 x 22.8 + Ta
    = 114 C + Ta

    As you can see the ambient temperature does impact this equation. For an example lets say your ambient temperature is 25 C. This means that your junction temperature under your conditions would be 139 C and therefore exceeds the maximum junction temperature rating of the LDO. If your ambient temperatuer was 11 C or less, you would be within the operating conditions for LM1085.

    As a protection measure, LM1085 will limit the power dissipated by limiting the current; however, this is meant as a protection feature and not as a substitute for proper application design.

    In your case you have ample headroom (Vin - Vout). You can reduce the power dissipated in the LDO by stepping down the input voltage before the LDO with a DCDC in order to conserve power, another LDO in order to boost your system PSRR, or even a power resistor in series before the LDO to simply spread the dissipated power between the resistor and the LDO.

    Very Respectfully,
    Ryan
  • Thanks for the prompt response, Ryan.  One follow up question, do you know if there have been any design changes that would affect the current limiting feature of this LM1085?  Another observation we have is that the current material we have handles a bit differently than some samples we have from Circa 2005.  If there are any PCN related to a design change on this part, can you provide?

    Thanks

  • Hi Andrew,

    I apologize for the delay in my second response.

    There can be some process variation that marginally affects performance from lot to lot; however, we test the devices to ensure that they meet our datasheet specifications before shipment. I am not aware of of any design changes on this part.

    Very Respectfully,
    Ryan