Ref.CE-P469 04/09

3. Thermal design outside the package

The thermal resistance outside the package Rc-a [deg C/W], which is the combination of those of the

heat-dissipative grease (adhesive) and the heat sink, is limited by the input power Pd [W], the ambient

temperature Ta [deg C], and the thermal resistance of the package Rj-c [deg C/W], i.e.,

Tj

= (Rj-c + Rc-a) x Pd + Ta → Rc-a = (Tj - Ta) / Pd - Rj-c

The formula can be converted into the function of Tj as follows:

Rc-a

= -Ta / Pd ＋ Tj / Pd - Rj-c,

which indicates the straight line with the slope of -1 / Pd and the intercept of Tj / Pd - Rj-c.

Fig. 2 is the chart on the CL-L251-C4 package that shows the relationship between the ambient

temperature Ta and the thermal resistance outside the package Rc-a with variations in the driving

current, where Tj is assumed to be 120°C, the absolute maximum rating value in the specifications.

The higher the ambient temperature Ta and the larger the driving current, the smaller the allowable

thermal resistance outside the package Rc-a = Rb + Rh.

This means that the grease (adhesive) and heat sink with smaller thermal resistance (in other similar

words, better heat dissipation) are required in order to keep Tj from exceeding the absolute maximum

rating in the specifications of 120°C, if the ambient temperature becomes higher and/or the driving

current is larger. Therefore, use Fig. 2 as a guide when selecting the external heat radiation part, and

conduct thermal verification on actual devices at the end.

For reference, Fig. 3 shows the equivalent chart on the CL-L251-C6 package.

Fig. 3

Fig. 2