MC68HC05J3 Datasheet, PDF (74/92 Pages) Motorola, Inc – High-density Complementary Metal Oxide Semiconductor (HCMOS) Microcomputer Unit

9.2
Thermal characteristics and power considerations
The average chip junction temperature, T_J, in degrees Celsius can be obtained from the following
equation:
T_J = T_A + (P_D • θ_JA)
[1]
where:
T_A = Ambient temperature (°C)
θ_JA = Package thermal resistance, junction-to-ambient (°C/W)
P_D = P_INT + P_I/O (W)
P_INT = Internal chip power = I_DD • V_DD (W)
P_I/O = Power dissipation on input and output pins (User determined)
An approximate relationship between P_D and T_J (if P_I/O is neglected) is:
P_D = _T----J----₊--K---2---7----3--
[2]
Solving equations [1] and [2] for K gives:
K = P_D • (T_A + 273) + θ_JA • P_D2
[3]
where K is a constant for a particular part. K can be determined by measuring P_D (at equilibrium)
for a known T_A. Using this value of K, the values of P_D and T_J can be obtained for any value of T_A
9
by solving the above equations. The package thermal characteristics are shown in Table 9-2.
Table 9-2 Package thermal characteristics
Pins
PA0–7, PB0–5
Characteristics
Thermal resistance
– 20-pin SOIC package
– 20-pin DIL package
Symbol Value Unit
θ_JA
60 °C/W
θ_JA
60 °C/W
R1
R2
C
3.26kΩ 2.38kΩ 50pF
V_DD = 4.5 V
R2
Test
point
C
R1
MOTOROLA
9-2
Figure 9-1 Equivalent test load
ELECTRICAL SPECIFICATIONS
TPG
MC68HC05J3