UT63M1XX Series Transceiver Consideration (1/99)

UTMC APPLICATION NOTE
UT63M1XX SERIES TRANSCEIVER: THERMAL CONSIDERATIONS
INTRODUCTION
To operate UT63M1XX Series Transceivers over the upper
end of the data sheet temperature range, thermal protection
is recommended. The following discussion will define an
electrical analog model used to analyze thermal systems
consisting of a packaged integrated circuit, thermally
conductive mounting material, and heat sink.
Thermal Resistance
The heat generated within a packaged integrated circuit will
conduct away from its sources (transistor junctions and
resistors) to the case. Heat conduction results in a
temperature gradient between the case and junction
proportional to the power dissipated by the device. The
proportionality factor is a term that represents the resistance
to heat transfer and is defined as thermal resistance, Q JC.
Eq. 1
ΘJC = (TJ - TC)/PD (°C/W)
Where:
TJ = device maximum junction temperature (°C)
TC = maximum case temperature (°C)
PD = device power dissipation (W)
The thermal resistance of the heat sink and mounting
material also represents heat transfer resistance in degrees
Celsius per watt. Thermal conductivity or K-factor for the
heat sink or mounting material is specified, by the
manufacturer, in watts per centimeter-Celsius or (W)/(cm°C). Thermal resistance of a material is defined as:
Eq. 2
ΘM = W/(A x K) (°C/W)
Where:
W = material thickness (cm)
A = heat transfer area (normal to heat flow)
K = thermal conductivity (K-factor)
Electrical Analog
Since thermal resistance is defined as a temperature gradient
proportional to power dissipation (i.e., °C/W), it is useful
to say:
Eq. 3
∆TJC = PD x ΘJC
The electrical analog is derived from the above equation
since the heat generated at the junction flows through the
package, the mounting material and into the heat sink. Each
material’s thermal resistance results in a temperature rise
starting at ambient temperature (TA). Figure 1 shows the
1
From the electrical model an equation is written as follows:
TJ
Eq. 4
∆TJC
QJC
TC
Eq. 5
QCS
∆TCS
PD
TS
∆TSA
TA
ΘJC is the thermal resistance
junction to case;
ΘCS is the thermal resistance case to heat sink;
ΘSA is the thermal resistance of heat sink
to ambient.
Figure 1. Electrical Analog of Thermal System
2
TJ = PD(ΘJC + ΘCS + ΘSA) + TA
To perform a worst-case analysis of this system, enter the
transceiver’s maximum junction temperature along with the
maximum system ambient temperature, ΘJC, and the
maximum power dissipation (PD).
Conclusion
QSA
Where:
TJ = ∆TJC + ∆TCS + ∆TSA + TA
Where:
∆TJC = PD x ΘJC
∆TCS = PD x ΘCS
∆TSA = PD x ΘSA
The thermal impedance of the mounting compound and heat
sink is calculated from equation 2. Size the mounting
material and heat sink (i.e., thickness and surface area) to
solve equation 5 for TJ. Refer to UTMC’s UT63M1XX Bus
Transceiver data sheet for the maximum limits used in
equation 5.