
Philips Semiconductors Application note
AN2021
Thermal considerations
for FAST logic products
1995 Mar 13 2
INTRODUCTION
Thermal considerations by both supplier and user require more
attention as package sizes shrink and operating frequencies
increase. This is because an increase in junction temperature (Tj)
can adversely affect the long term operating life of an IC. Some of
the variables that affect Tj are controlled by the IC manufacturer
while others are controlled by the system designer.
With increasingly frequent use of Surface Mount Device (SMD)
technology, management of thermal characteristics becomes a
growing concern. Not only are the SMD packages much smaller, but
the thermal energy is concentrated more densely on the printed
circuit board.
FAST PRODUCTS IN SSOP PACKAGE
The FAST product family is a high performance Bipolar Logic
Family. In the SMD packages, such as SSOP, it is necessary to
estimate operating junction temperatures of the FAST products in
the system environment. The information provided herein should
assist the system designer with thermal management
considerations.
POWER DISSIPATION
The power dissipation equations, definition of terms and the
assumptions made in estimating power dissipation are shown below.
The total power is the sum of the static power and dynamic power.
Ptotal = Pstatic + Pdynamic.
The equation for static power dissipation is,
Pstat = (VCC × ICC)
but since ICCH, ICCL and ICCZ are different values, the equation
becomes,
Pstat = VCC[DCen (NH × ICCH/NT+NL×ICCL/NT)+(1-DCen)ICCZ]
where: DCen = % duty cycle enabled
NH = number of outputs in high state
NL= number of outputs in low state
NT= total number of outputs.
The equation for the dynamic power dissipation is,
Pdyn = [DCen x Nsw x VCC x f1 x (VOH – VOL) x CL]
+[DCen × Nsw × VCC × f2 × (ma/MHZ/bit)] × 10-3
where: DCen = % duty cycle enabled
Nsw = total number of outputs switching
f1 = operating frequency (in Hz)
f2 = operating frequency (in MHz)
CL= external load capacitance (in F)
mA/MHz/bit = slope of the ICC vs frequency curve.
Thermal Resistance (ja)
The ability of a package to conduct heat from the IC chip inside the
package to the environment is expressed in terms of “thermal
resistance”. It is measured in degrees Centigrade per watt of power
dissipated by the chip. Table 1 lists some thermal resistance values
for selected FAST products in SSOP packages. The values listed
were measured in still air with no traces attached (worst case
environment).
Table 1. FAST Products in SSOP Package
PRODUCT PIN
COUNT ja mA/MHz/bit (unloaded)
74F245 20 125 0.158
74F244 20 127 0.125
74F2244 20 127 0.045
74F373 20 125 0.158
74F374 20 125 0.102
74F543 24 118 0.512*
74F827 24 121 0.125
74F240 20 124 0.275
74F299 20 121 0.183
74F533 20 124 0.129
74F657 24 113 0.202
* The 74F543 ICC vs Frequency slope increases above 20 MHz. From 20 MHz to 30 MHz, slope = 1.64. From 30 MHz to 40 MHz, slope = 2.55.