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3 General Safety Precautions and Usage Considerations

3-8

3.3.8

Thermal design

The failure rate of semiconductor devices is greatly increased as operating temperatures

increase. As shown in Figure 2, the internal thermal stress on a device is the sum of the ambient

temperature and the temperature rise due to power dissipation in the device. Therefore, to

achieve optimum reliability, observe the following precautions concerning thermal design:

(1) Keep the ambient temperature (Ta) as low as possible.

(2) If the device’s dynamic power dissipation is relatively large, select the most appropriate

circuit board material, and consider the use of heat sinks or of forced air cooling. Such

measures will help lower the thermal resistance of the package.

(3) Derate the device’s absolute maximum ratings to minimize thermal stress from power

dissipation.

θja = θjc + θca

θja = (Tj–Ta) / P

θjc = (Tj–Tc) / P

θca = (Tc–Ta) / P

in which

θja = thermal resistance between junction and surrounding air (°C/W)

θjc = thermal resistance between junction and package surface, or internal thermal

resistance (°C/W)

θca = thermal resistance between package surface and surrounding air, or external

thermal resistance (°C/W)

Tj = junction temperature or chip temperature (°C)

Tc = package surface temperature or case temperature (°C)

Ta = ambient temperature (°C)

P = power dissipation (W)

θca

θjc

Figure 2 Thermal resistance of package

3.3.9

Interfacing

When connecting inputs and outputs between devices, make sure input voltage (VIL/VIH) and

output voltage (VOL/VOH) levels are matched. Otherwise, the devices may malfunction. When

connecting devices operating at different supply voltages, such as in a dual-power-supply system,

be aware that erroneous power-on and power-off sequences can result in device breakdown. For

details of how to interface particular devices, consult the relevant technical datasheets and

databooks. If you have any questions or doubts about interfacing, contact your nearest Toshiba

office or distributor.

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