LDO Thermal Calculations 1 Abstract Low Dropout Regulators in present is the cheapest solution for precise voltage source with a few external components. However, the main disadvantage is loss conversion producing significant heat. Therefore proper thermal design is a key for good LDO performance in real applications. 2 2 • Template • Nov-09 Confidential Proprietary Agenda • • • • Thermal parameters standards, terminology and definitions Suitable packages for good thermal performance Thermal Calculators for LDOs Thermal Simulations Used in Silicon Design Phase 3 3 • Template • Nov-09 Confidential Proprietary Thermal Parameters Standards, Terminology and Definitions 4 4 • Template • Nov-09 Confidential Proprietary Standards • JEDEC specifications: – Find information related to θ and Ψ parameters definitions and other useful information at website: www.jedec.org in “General Folder” click on “Free Standards” and in “Search by document number:” text field write “JESD51” – Glossary of Thermal Measurement Terms and Definitions (JESD51-13) 5 5 • Template • Nov-09 Confidential Proprietary Thermal to Electrical Analogy Used Electrical Thermal Voltage (V) Temperature Difference (°C) Current (A) Dissipated Power (W) Resistance (Ω) Thermal Resistance (°C/W) I R XY 6 6 • Template • Nov-09 VXY = RXY PD = Confidential Proprietary TX − TY θ XY θ (Theta) - Thermal Resistance θXY - Thermal Resistance between X and Y points specifies: – The amount of heat that flows from point X to point Y if the temperatures at X and Y points are known (connected by the thermal resistance). The path the heat flows is known and it is completely determined by the resistance. TX − TY °C ⎞ ⎛ In general: PD = ⎜W = ⎟ θ XY °C/W ⎠ ⎝ Examples: PD = TJ max − TA θ JA Calculation of Power Dissipation for given Maximum Junction Temperature and Ambient Temperature. Thermal Resistance is specified for particular application conditions. – The temperature at point X if the temperature at point Y and the amount of heat are known. The path the heat flows is known and it is completely determined by the resistance. In general: TX = θ XY ⋅ PD + TY Examples: TJ = θ JC ⋅ PD + TC 7 7 • Template • Nov-09 ( °C = °C/W ⋅ W + °C ) Calculation of Junction Temperature for given Power Dissipation and Case Temperature (i.e. Tab temperature for D2PAK). Thermal Resistance is specified for particular package. (Important: almost all heat flows through the Tab) Confidential Proprietary Ψ (Psi) - Thermal Characterization Parameter ΨXY - Thermal Characterization Parameter between X and Y points specifies: – The temperature difference between point X to point Y if the total heat is known. The heat flowing along the specific path point X to point Y is NOT known. – This parameter typically serves for estimation of Junction temperature (TJ) at known total dissipated power (PD) inside the package when a temperature is measured at package perimeter (Lead, Exposed Pad, Board, etc.) In general: TX = ΨXY ⋅ PD + TY ( °C = °C/W ⋅ W + °C ) Examples: TJ = ΨJLn ⋅ PD + TLn Junction Temperature estimation by measuring nth Lead (Pin) Temperature (SO, DFN, etc.) TJ = ΨJB ⋅ PD + TB Junction Temperature estimation by measuring Board (Pad) Temperature (SO with Exposed Pad, DFN, etc.) 8 8 • Template • Nov-09 Confidential Proprietary Thermal Parameters in Datasheet (NCV4269) TJmax 9 9 • Template • Nov-09 Confidential Proprietary Thermal Parameters in Datasheet (NCV4269) θJA , ΨJLn, ΨJPad The θ and Ψ values are specified for particular application conditions (PCB parameters). Search in Application Section for other PCB parameters values, i.e. θJA vs PCB area. 10 10 • Template • Nov-09 Confidential Proprietary Thermal Parameters in Datasheet (NCV4269) θJA vs PCB area For given package the θJA decreases: - with increased PCB area - with increased Cu thickness 11 11 • Template • Nov-09 Confidential Proprietary Suitable Packages for Good Thermal Performance 12 12 • Template • Nov-09 Confidential Proprietary Selecting Suitable Package There is no ideal option. There are always trade-offs among Thermal Performance, PCB footprint size and Cost. To choose package with sufficient thermal performance, consider mainly: – Power dissipation (Steady state and pulsed) – PCB parameters (Area, Cu thickness, Number of layers) – Temperatures in particular points (Ambient, PCB) If the package cost is a key factor, try to consider: – Eliminating other heat sources on PCB which will effectively increase temperature of the PCB – Using multilayer PCBs larger copper areas and thicker Cu layers. Use thermal vias to improve PCB thermal performance 13 13 • Template • Nov-09 Confidential Proprietary LDO Packages Relative Performance Comparison Package Type D2PAK (3, 5, 7 leads + Tab) DPAK (3, 5 leads including Tab) SOT-223 (3 leads + Tab) SO NB (8 leads) SO NB Fused Leads (8 leads) SO EP (8 leads + Exposed Pad ) SO NB Fused Leads (14 leads) SO EP (16 leads + Exposed Pad) SO WB Fused Leads (16, 20 leads) Micro8 (8 leads) TSOP, SOT-23 (5, 6 leads) DFN (6, 8, 10, 20 leads + Exposed Pad) Thermal Performance PCB footprint Cost ++ ++ + -+ + --+ -+ + + --++ ++ ++ + ++ + -+ -+ ++ - + means relative advantage - means relative disadvantage NB – Narrow Body WB – Wide Body EP – Exposed Pad (At bottom side of package) Fused Leads – Leads connected to the leadframe 14 14 • Template • Nov-09 Confidential Proprietary Thermal Calculators for LDOs 15 15 • Template • Nov-09 Confidential Proprietary Disclaimer • • These tools do not provide “guaranteed” results since real application conditions are different in most cases (i.e. PCB routing) and this factor has influence on results. There could also be other factors in real applications such as airflow, other heat sources, etc which affect the thermal behavior. These tools serve as a guide to show directions for proper thermal design. The models are limited for certain range of operating conditions (i.e. PCB area) 16 16 • Template • Nov-09 Confidential Proprietary Thermal Calculator Important Facts • PCB Cu Area – The area number units are mm2 – The larger is area the lower is thermal resistance • PCB Cu Thickness – The thickness units are oz (1 oz is 35 µm, 2 oz is 70 µm) – The thicker is Cu layer the lower is thermal resistance • • • The Single Layer PCB model is used Ambient Temperature, Power, Pulse With and Duty Cycle can be varied to see effect on Junction Temperature Results can be viewed for multiple packages if it is applicable for particular parts 17 17 • Template • Nov-09 Confidential Proprietary Thermal Calculator Example (NCV4269) SOIC-14 w/ 6 thermal leads analog NCV4269 Package Device Product active area Die thk Die X Die Y Die attach Thick Die Attach Cond PCB Cu Area PCB Cu thk T_junction MAX T_ambient Power Pulse ON Time Duty Cycle Theta JA Psi LA Psi B-A Psi B-top-A Psi J-B-top Psi J-L Tjunc (DC) R(0.3sec) Tjunc (single pulse) R(0.3sec, 10.0%) Tjunc (pulsed) Foster Network 1 2 3 4 5 6 7 8 9 10 18 18 • Template • Nov-09 50.0 1.0 150.0 25.0 0.8 0.3 10% 127.1 111.0 105.5 87.7 39.5 16.1 126.7 0.0 25.0 0.0 25.0 Theta JA R C/W 0.089 0.192 0.608 1.155 3.64 3.10 0.80 10.0 29.0 77.0 mm mm mm mm W/MK mm^2 oz °C °C W sec C/W C/W C/W C/W C/W C/W °C C/W °C C/W mils mils sqmils mils Rth C/W C m1 m2 R^2 Foster Network R1 R2 Junction C1 C2 Copper area and Copper thickness curve fit coeff. Theta JA Psi LA Psi B-top-A tau_9 tau_10 tau_10 Theta JA Psi LA Psi B-A LN fit LN fit LN fit LN fit R_Ln R_Ln R_Ln 297.17 337.09 361.93 265.84 22.01 27.44 40.05 -0.220117 -0.2840 -0.3151 -0.2836 -0.2049 0.1576 0.0804 -0.190036 -0.2512 -0.2376 -0.2319 0 0 0 97% 97% 97% 98% 100% 99% 99% 125.6 111.0 105.5 87.7 9.9 50.8 54.8 Ln fit = C*Cu_area^m1*Cu_oz^m2*Pl_thk^m3 silicon mold compound k 100 0.7 W/MK R3 Rn p 2E+06 2E+06 g/M3 Cp 0.79 0.89 W-s/gC C3 Cn Const 95.79 1245.56 C/Wt^-.5 sqr(t) max 1.8E-03 2.1E-01 sec each rung is exactly characterized by its RCproduct time constant; amplitudes are the resistances Ambient (thermal ground) °C Tau Sec 1.0E-06 1.0E-05 1.0E-04 5.3E-04 0.004 0.028 0.200 0.45 9.87 50.8 C (calculated) W-Sec/C 1.1E-05 5.2E-05 1.6E-04 4.6E-04 1.1E-03 9.0E-03 2.5E-01 4.5E-02 3.4E-01 6.6E-01 Psi L-A R C/W -0.45 1.20 8.80 27.00 -21.00 95.44 Tau Sec C (calculated) 0.023 0.095 1.45 14.40 48.00 54.85 -5.1E-02 7.9E-02 1.6E-01 5.3E-01 -2.3E+00 5.7E-01 W-Sec/C Confidential Proprietary 5.3E-04 Die effect correction composite 40% of tau 8.5E-02 sec -1.5 C/W 88.95 C/Wt^-.5 Thermal Calculator Results (NCV4269) SOIC-8 std package NCV4269 1.0 oz SOIC-8 std package NCV4269 2.0 oz SOIC-14 w/ 6 thermal leads NCV4269 1.0 oz SOIC-14 w/ 6 thermal leads NCV4269 2.0 oz SOIC-20 w/ 8 thermal leads NCV4269 2.0 oz SOIC-20 w/ 8 thermal leads NCV4269 2.0 oz 200 180 160 Theta JA (C/W) 140 120 100 80 60 40 20 0 0 100 200 300 400 Copper heat spreader area (mm^2) 19 19 • Template • Nov-09 Confidential Proprietary 500 600 700 Thermal Calculator File (NCV4269) NCV4269 Thermal Calculator 20 20 • Template • Nov-09 Confidential Proprietary Thermal Simulations used in Silicon Design Phase 21 21 • Template • Nov-09 Confidential Proprietary Thermal Simulation Tools at Silicon (Die) Level • The tools are important to predict thermal behavior of Silicon and provide guidelines for proper design and layout, mainly for temperature sensitive circuitries • Three possible simulation tools in Silicon Design phase – Thermal Tool • Steady-state thermal simulations • Three-layer die level simulation, linear system • LabView environment – Synopsys TCAD • Steady-state as well as transient thermal simulations • Nonlinear system, calculations based on the finite element method (FEM) – HeatWave • Steady-state, transient, thermal and electro-thermal simulations • Nonlinear system, calculations based on the finite element method (FEM) • The most precise and the most complex calculations 22 22 • Template • Nov-09 Confidential Proprietary Thermal Tool 23 23 • Template • Nov-09 Confidential Proprietary Synopsys TCAD 24 24 • Template • Nov-09 Confidential Proprietary HeatWave 25 25 • Template • Nov-09 Confidential Proprietary References 1. Electronics System Thermal Design and Characterization (Presentation 8th July 2007), Roger Stout, P.E., ON Semiconductor 2. Psi or Theta: Which One Should You Choose? (Article in Power Electronics Technology, May 2008), Roger Stout, P.E., ON Semiconductor 3. Application Note: AND8220/D How To Use Thermal Data Found in Data Sheets http://www.onsemi.com/pub_link/Collateral/AND8220D.PDF 4. Thermal Simulations (Presentation 1st July 2009), Andrej Vrbicky, Bratislava Development Center, ON Semiconductor 5. Website http://www.jedec.org/ 26 26 • Template • Nov-09 Confidential Proprietary For More Information • View the extensive portfolio of power management products from ON Semiconductor at www.onsemi.com • View reference designs, design notes, and other material supporting automotive applications at www.onsemi.com/automotive 27 27 • Template • Nov-09 Confidential Proprietary