AN4209 datasheet(8/19 Pages) STMICROELECTRONICS

8 / 19 page

STRVS parameters

AN4209

8/19

DocID023949 Rev 2

infrared camera and calculate the junction temperature using Equation 6. The lead is the

most interesting reference point to evaluate accurately the average operating junction

temperature Tjavg. The thermal resistance value Rth(j-l) depends only on the package

proprieties unlike Rth(j-a).

1.2.2

Transient parameter

Transient thermal impedance (ZTH(j-Ref)(tp))

The transient thermal impedance ZTH(j-Ref)(tp) is the temporary variation of thermal

resistance from an input power step function up to reaching a stable value as defined

Equation 7.

Equation 7

Transient thermal impedance can be calculated using Equation 8.

Equation 8

Where:

Junction to ambient, ZTH(j-a) and junction to lead, ZTH(j-l) transient thermal impedance

curves are provided in the datasheet. However ZTH(j-l) should be preferred to evaluate the

peak junction temperature to get a better assessment.

Thus from Equation 8, the dynamic change in temperature over the time can be determined

using

Equation 9

The transient thermal impedance diagram provides a quick and simple method to estimate

the rise of junction temperature under transient conditions. Figure 6 below illustrates

transient thermal impedance diagrams of the SMB package. Note that thermal impedances

increase until they reach their asymptotic limit corresponding to their thermal resistance

Rth(j-l) and Rth(j-a).

ZTH(j-Ref)

Junction to reference transient thermal impedance expressed in °C/W.

Pulse duration of the step power expressed in s.

PStep

Step power function applied to the die expressed in W.

Tj(tp)

Junction temperature over the time width, expressed in °C.

TRef

Temperature of the reference point expressed in °C.

lim tp

Æ Zth(j-Ref) (tp) = Rth(j-Ref)

Step

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(

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(

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Step

T+

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