Philips Semiconductors Application note Thermal considerations for FAST logic products AN2021 The equation for static power dissipation is, 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. Pstat = (VCC x ICC) but since ICCH, ICCL and ICCZ are different values, the equation becomes, Pstat = VCC [DCen (NH x ICCH/NT +NL x ICCL/NT)+(1-DCen)ICCZ] where: 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. DCen NH NL NT = % duty cycle enabled = number of outputs in high state = number of outputs in low state = 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 x Nsw x VCC x f2 x (ma/MHZ/bit)] x 10-3 FAST PRODUCTS IN SSOP PACKAGE where: 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. 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). 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. 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. 1995 Mar 13 2 Philips Semiconductors Application note Thermal considerations for FAST logic products AN2021 FACTORS AFFECTING ja the Pdyn becomes: For a given package and lead frame, some factors which affect the thermal resistance (ja) in the application include (1) the die size of the IC chip, (2) the length of the printed circuit board traces attached to IC package on the system board and (3) the amount of airflow across the package. Pdyn = [(50%)(4)(5.25)(25x106)(3.4-0.4)(50x10-12)] + [(50%)(4)(5.25)(25)(0.26)](10-3) = (0.0394) + (0.0682) = 0.108 watts Figure 1 through Figure 7 provide ja information for the 20 and 24 pin packages as a function of die size, airflow and trace length. Ptotal = 0.433 + 0.108 = 0.541 watts The junction temperature estimation then becomes: Tj A SAMPLE CALCULATION = (127)(0.541) + Tamb = 69 + Tamb If the system ambient temperature is 55C, then An example for the 74F244. Junction temperature is estimated from the equation: Tj Tj = (ja x P total) + Tamb Ptotal = Pstat + Pdyn = 69 + 55 = 124 C. With the junction temperature of a device established for a given system environment the expected operating life of the IC can be determined from the graph in Figure 6. Assuming the number of outputs High = 4, VCC at 5.25V, the enable duty cycle (DCen) = 50%, and worst case ICC's (ICCL = 90 mA, ICCH = 60 mA, ICCZ = 90 mA) the static power calculation is: SYSTEM CONSIDERATIONS The manner in which an IC package is mounted and positioned in its surrounding environment will have significant effects on operating junction temperatures. These conditions are under the control of the system designer and are worthy of serious consideration in the PC board layout and system ventilation and airflow features. Pstat = (5.25){(0.50)[(4)(0.060)/8 + (4)(0.090)/8] + (1-0.50)(0.090)} = (5.25)[(0.0150) + (0.0225) + (0.045)] = 0.433 watts Forced-air cooling will significantly reduce thermal resistance. Assuming the following, DCen = f1 = f2 = CL = 1995 Mar 13 50% 25 x 106 Hz 25 MHz 50 pf (50 x 10-12F) NSW = VCC = ma/MHz/bit = VOH = VOL = Package mounting can affect thermal resistance. Surface mount packages dissipate significant amounts of heat through the leads that attach to the traces. Trace length is another significant factor. 4 5.25V 0.26 3.4V 0.4V Thermally conductive adhesive under the surface mount packages can lower thermal resistance by providing a direct heat path from the package to the board. 3 Philips Semiconductors Application note Thermal considerations for FAST logic products AN2021 SSOP20 ja vs Die Size 10/94 150 Test Conditions Test Ambient Power Dissipation - Still Air - 0.70 Watt - Philips Semiconductors SVL - SSOP Thermal Test Board - (40.0x19.0x1.6mm) - 15% Test Fixture 140 Accuracy ja ( C/W) 130 120 110 100 90 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Die Size (sq. mm) SF00650 Figure 1. 1995 Mar 13 4 Philips Semiconductors Application note Thermal considerations for FAST logic products AN2021 SSOP20 ja vs Airflow 10/94 0 Test Conditions Test Ambient Power Dissipation Test Fixture -5 Accuracy - 200, 400, 800 LFPM Airflow - 0.7 Watt - Philips Semiconductors SVL SSOP Thermal Test Board (40.0x19.0x1.6mm) - 15% Percent Change in ja -10 -15 -20 -25 -30 0 200 400 600 800 1000 Airflow (LFPM) SF00651 Figure 2. 1995 Mar 13 5 Philips Semiconductors Application note Thermal considerations for FAST logic products AN2021 SSOP24 ja vs Die Size 8/94 140 Test Conditions Test Ambient Power Dissipation Test Fixture 130 - Still Air - 0.70 Watt - Philips Semiconductors SVL SSOP Thermal Test Board (40.0x19.0x1.6mm) - 15% Accuracy 120 2.7mm x 3.5mm PAD 3.85mm x 4.5mm PAD ja ( C/W) 110 100 90 80 70 60 0 2 4 6 8 10 12 14 16 18 20 Die Size (sq. mm) SF00652 Figure 3. 1995 Mar 13 6 Philips Semiconductors Application note Thermal considerations for FAST logic products AN2021 SSOP24 ja vs Airflow 8/94 120 Test Conditions Test Ambient Power Dissipation Test Fixture - 200, 400, 800 LFPM airflow - 0.70 Watt - Philips Semiconductors SVL SSOP Thermal Test Board (40.0x19.0x1.6mm) - 15% 110 Accuracy 2.7mm x 3.5mm PAD 3.85mm x 4.5mm PAD ja ( C/W) 100 90 80 70 60 100 200 300 400 500 600 700 800 900 Airflow (LFPM) SF00653 Figure 4. 1995 Mar 13 7 Philips Semiconductors Application note Thermal considerations for FAST logic products AN2021 Effect of Trace Length on ja 10/94 +10 Test Conditions Die Size Power Dissipation Test Ambient Test Board +5 - 18,445 sq. mils - 1.0 Watt - Still Air - Philips PCB (2.24" x 2.24" x 0.062") Traces 27 mil wide 1 oz. soft copper % CHANGE IN ja ( C/W) 0 -5 -10 -15 -20 -25 -30 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Average Trace Length (Inches) SF00654 Figure 5. 1995 Mar 13 8 Philips Semiconductors Application note Thermal considerations for FAST logic products AN2021 FAST IN SSOP - ESTIMATED ONSET TO FAILURE (0.1% CUMULATIVE) 190 Molding Compound Characteristics Activation Energy Ea = 1.54 ev 180 170 Junction temperature ( C) 160 Maximum Safe Junction Temperature = 150C (Per Data Book) 150 140 130 120 110 100 90 80 1000 10000 Operating Lifetime (hr) 1 year = 8760 Hrs. Figure 6. 1995 Mar 13 9 100000 1000000 SF00655