INTEGRATED CIRCUITS AN2021 Thermal considerations for FAST logic products 1995 Mar 13 Philips Semiconductors 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 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. the Pdyn becomes: 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 Philips Semiconductors Product specification Thermal considerations for FAST logic products AN2021 Data sheet status Data sheet status Product status Definition [1] Objective specification Development This data sheet contains the design target or goal specifications for product development. Specification may change in any manner without notice. Preliminary specification Qualification This data sheet contains preliminary data, and supplementary data will be published at a later date. Philips Semiconductors reserves the right to make chages at any time without notice in order to improve design and supply the best possible product. Product specification Production This data sheet contains final specifications. Philips Semiconductors reserves the right to make changes at any time without notice in order to improve design and supply the best possible product. [1] Please consult the most recently issued datasheet before initiating or completing a design. Definitions Short-form specification -- The data in a short-form specification is extracted from a full data sheet with the same type number and title. For detailed information see the relevant data sheet or data handbook. Limiting values definition -- Limiting values given are in accordance with the Absolute Maximum Rating System (IEC 134). Stress above one or more of the limiting values may cause permanent damage to the device. These are stress ratings only and operation of the device at these or at any other conditions above those given in the Characteristics sections of the specification is not implied. Exposure to limiting values for extended periods may affect device reliability. Application information -- Applications that are described herein for any of these products are for illustrative purposes only. Philips Semiconductors make no representation or warranty that such applications will be suitable for the specified use without further testing or modification. Disclaimers Life support -- These products are not designed for use in life support appliances, devices or systems where malfunction of these products can reasonably be expected to result in personal injury. Philips Semiconductors customers using or selling these products for use in such applications do so at their own risk and agree to fully indemnify Philips Semiconductors for any damages resulting from such application. Right to make changes -- Philips Semiconductors reserves the right to make changes, without notice, in the products, including circuits, standard cells, and/or software, described or contained herein in order to improve design and/or performance. Philips Semiconductors assumes no responsibility or liability for the use of any of these products, conveys no license or title under any patent, copyright, or mask work right to these products, and makes no representations or warranties that these products are free from patent, copyright, or mask work right infringement, unless otherwise specified. Copyright Philips Electronics North America Corporation 1998 All rights reserved. Printed in U.S.A. Philips Semiconductors 811 East Arques Avenue P.O. Box 3409 Sunnyvale, California 94088-3409 Telephone 800-234-7381 print code Document order number: yyyy mmm dd 10 Date of release: 10-98 9397-750-05219