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TC1263-5.0VOATR Datenblatt(PDF) 8 Page - Microchip Technology |
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TC1263-5.0VOATR Datenblatt(HTML) 8 Page - Microchip Technology |
8 / 18 page TC1263 DS21374C-page 8 © 2005 Microchip Technology Inc. 5.0 THERMAL CONSIDERATIONS 5.1 Thermal Shutdown Integrated thermal protection circuitry shuts the regulator off when the die temperature exceeds 160°C. The regulator remains off until the die temperature drops to approximately 150°C. 5.2 Power Dissipation The amount of power the regulator dissipates is primarily a function of input and output voltage and out- put 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 (TJMAX) and the thermal resistance from junction-to-air ( θJA). EQUATION 5-2: Table 5-1 and Table 5-2 show various values of θJA for the TC1263 package types. TABLE 5-1: THERMAL RESISTANCE GUIDELINES FOR TC1263 IN 8-PIN SOIC PACKAGE TABLE 5-2: THERMAL RESISTANCE GUIDELINES FOR TC1263 IN 5-PIN DDPAK/TO-220 PACKAGE Equation 5-1 can be used in conjunction with Equation 5-2 to ensure regulator thermal operation is within limits. For example: Find: 1. Actual power dissipation 2. Maximum allowable dissipation Actual power dissipation: Maximum allowable power dissipation: In this example, the TC1263 dissipates a maximum of 260 mW below the allowable limit of 500 mW. 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 500 mW into Equation 5-1, from which VINMAX = 4.6V. Copper Area (Topside)* Copper Area (Backside) Board Area Thermal Resistance (θJA) 2500 sq mm 2500 sq mm 2500 sq mm 60°C/W 1000 sq mm 2500 sq mm 2500 sq mm 60°C/W 225 sq mm 2500 sq mm 2500 sq mm 68°C/W 100 sq mm 2500 sq mm 2500 sq mm 74°C/W * Pin 2 is ground. Device is mounted on top-side. Where: VINMAX VOUTMIN ILOADMAX PD = Worst-case actual power dissipation = Minimum regulator output voltage = Maximum output (load) current = Maximum voltage on VIN P D V IN MAX V OUTMIN – ()I LOA DMAX = Where: VINMAX VOUTMIN ILOADMAX PD = Worst-case actual power dissipation = Minimum regulator output voltage = Maximum output (load) current = Maximum voltage on VIN P DMAX T JMAX T AMAX – θ JA --------------------------------------- = Copper Area (Topside)* Copper Area (Backside) Board Area Thermal Resistance (θJA) 2500 sq mm 2500 sq mm 2500 sq mm 25°C/W 1000 sq mm 2500 sq mm 2500 sq mm 27°C/W 125 sq mm 2500 sq mm 2500 sq mm 35°C/W * Tab of device attached to top-side copper Given: VINMAX =3.3V ± 10% VOUTMIN =2.7V ± 0.5% ILOADMAX =275 mA TJMAX =125°C TAMAX =95°C θJA = 60° C/W (SOIC) P D V INMAX V OUTMIN – ()I LOADMAX ≈ P D 3.3 1.1 × () 2.7 .995 × () –275 10 3 – × = P D 260 mW = P DMAX T JMAX T AMAX – θ JA --------------------------------------- = P DMAX 125 95 – () 60 ------------------------- = P DMAX 500 mW = |
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