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TC1306 Datasheet(PDF) 6 Page - Microchip Technology |
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TC1306 Datasheet(HTML) 6 Page - Microchip Technology |
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TC1306 DS21527C-page 6 © 2007 Microchip Technology Inc. 5.0 THERMAL CONSIDERATIONS 5.1 Thermal Shutdown Integrated thermal protection circuitry shuts the regulator off when die exceeds approximately 160°C. The regulator remains off until the die temperature drops to approximately 145°C. Thermal shutdown is intended to protect the device under transient accidental (fault) overload conditions. Thermal Shutdown may not protect the LDO while operating above junction temperatures of 125°C continuously. Sufficient thermal evaluation of the design needs to be conducted to ensure that the junction temperature does not exceed 125°C. 5.2 Power Dissipation The amount of power the regulator dissipates is primarily a function of input and output voltage, and output current. The following equation is used to calculate worst case actual power dissipation. EQUATION 5-1: The maximum allowable power dissipation (Equation 5-2) is a function of the maximum ambient temperature (TAMAX), the maximum allowable die temperature (125°C), and the thermal resistance from junction-to-air ( θJA). The MSOP-8 package has a θJA of approximately 200°C/W when mounted on a four layer FR4 dielectric copper clad PC board. EQUATION 5-2: Equation 5-1 can be used in conjunction with Equation 5-2 to ensure regulator thermal operation is within limits. For example: Given: VINMAX = 3.8V ± 5% VOUT1MIN = 1.8V ± 2.5% VOUT2MIN = 3.0V ± 2.5% ILOAD1MAX= 60mA ILOAD2MAX= 120mA TJMAX = 125°C TAMAX = 55°C θJA = 200°C/W Find: 1. Actual power dissipation 2. Maximum allowable dissipation Actual power dissipation: PD ≈ [(VINMAX – VOUT1MIN)] x ILOAD1MAX + [(VINMAX – VOUT2MIN)] x ILOAD2MAX [(3.8 x 1.05) – (1.8 x .975)] x 60 x 10-3 + [(3.8 x 1.05) – (3.0 x .975)] x 120 x 10-3 = 256mW Maximum allowable power dissipation: In this example, the TC1306 dissipates a maximum of 262mW; below the allowable limit of 350mW. In a similar manner, Equation 5-1 and Equation 5-2 can be used to calculate maximum current and/or input voltage limits. For example, the maximum allowable VIN is found by substituting the maximum allowable power dissipation of 350mW into Equation 5-1, from which VINMAX = 4.5V. 5.3 Layout Considerations The primary path of heat conduction out of the package is via the package leads. Therefore, layouts having a ground plane, wide traces at the pads, and wide power supply bus lines combine to lower θJA and therefore increase the maximum allowable power dissipation limit. Where: PD ≈ (VINMAX – VOUT1MIN)ILOAD1MAX + PD VINMAX VOUT1MIN ILOAD1MAX = Worst case actual power dissipation = Minimum regulator output voltage1 = Maximum output (load) current1 = Maximum voltage on VIN (VINMAX – VOUT2MIN)ILOAD2MAX VOUT2MIN ILOAD2MAX = Minimum regulator output voltage2 = Maximum output (load) current2 PDMAX = (TJMAX – TAMAX) θJA Where all terms are previously defined. PD = (TJMAX – TAMAX) θJA = (125 – 55) 200 = 350mW |
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