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Original question:

LM1117: Meaning of datasheet items and Operating LM1117.

LM1117: Calculate of junction temperature for the heatsink

WANJIN KO
Prodigy 20 points
Part Number: LM1117

I calculated the junction  temperature as follows.

T(junction) = T(ambient) + R(Junction to ambient thermal) * (Vin - Vout) * I(Load)

if T(ambient) is -32c,

and Packaging is SOT-223, than, R(Junction to ambient thermal) is may 61.6(by manual page 4).

and i input 12V and output is 5V, and load current is 1.6mA.

Tj at this condition, it may calculated as  -32 + 61.6 * (12 - 5) * (1.6/1000) = -31.31

Is this the correct calculation method?

  • 0
    TI__Guru 55940 points
    Hi Wanjin,

    Yes this is the correct method to use the Rtheta_ja metric to calculate the junction temperature. Please keep in mind that the thermal metrics are modeled on the JEDEC Hi-K board, as such you may find different performance in your specific application. The primary heatsink for LM1117 in the DCY package is the Vout plane. As such, in order to maximize your thermal performance, maximize the Vout copper local to the linear regulator.

    Very Respectfully,
    Ryan