[6] Handling Guide
107
3.3.14 Thermal
Design
The failure rate of semiconductor devices is greatly increased as operating temperatures increase.
As shown in Figure 3.9, 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.
θ
ja
=
θ
jc
+
θ
ca
θ
ja
=
(Tj
−
Ta)/P
θ
jc
=
(Tj
−
Tc)/P
θ
ca
=
(Tc
−
Ta)/P
in which
θ
ja = thermal resistance between junction and surrounding air (°C/W)
θ
jc = thermal resistance between junction and package surface, or internal thermal
resistance
(°C/W)
θ
ca = 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)
3.3.15 Interfacing
When connecting inputs and outputs between devices, make sure input voltage (V
IL
/V
IH
) and
output voltage (V
OL
/V
OH
) 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.
Figure 3.9 Thermal Resistance of Package
Tc
θ
ca
Ta
Tj
θ
jc
