AN533 datasheet(16/22 Pages) STMICROELECTRONICS

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Surface mount packages

AN533

16/22

Surface mount packages

2.1

Thermal resistance

The thermal resistance of a semiconductor device characterizes the device’s capability to

dissipate the heat generated by the chip during operation. This parameter allows us to

calculate the junction temperature, taking into account the device environment (load current,

ambient temperature, mounting conditions etc...).

For surface mounted devices (SMDs), the thermal resistance between junction and ambient,

called Rth(j-a), depends on the copper surface used under the tab. Figure 15 shows curves

giving the relation between Rth(j-a) and the copper surface under the tab for an FR4 board

- 35 µm copper thickness.

Figure 15.

Rth(j-a) versus copper surface under tabs (FR4 board -

copper thickness = 35 µm) for several typical packages

2.2

Thermal impedance

When dealing with short duration pulses, the thermal impedance must be considered to

calculate the junction temperature. Depending on the time scale, the following elements are

thermally prevalent:

●

tp < 10 ms: die influence

●

tp < 0.1 s: package influence

●

tp < 10 s: PCB influence

●

above 10 s: thermal exchange board-air (example: with / without forced cooling)

Figure 16 shows the Zth / Rth ratio for SMD packages and Figure 17 shows Zth(j-a) for DPAK

and D2PAK.

012

345

100

110

120

130

S(Cu) (cm²)

Rth(j-a) (°C/W)

SOT-223

DPAK

D2PAK

02468

100

S(Cu) (cm²)

Rth(j-a) (°C/W)

1620

242832

3640

S(Cu) (cm²)

Rth(j-a) (°C/W)

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