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TC1264-3.0VAB データシート(PDF) 7 Page - Microchip Technology |
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TC1264-3.0VAB データシート(HTML) 7 Page - Microchip Technology |
7 / 18 page © 2006 Microchip Technology Inc. DS21375C-page 7 TC1264 5.0 THERMAL CONSIDERATIONS 5.1 Thermal Shutdown Integrated thermal protection circuitry shuts the regulator off when 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 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 tem- perature (TJMAX) and the thermal resistance from junc- tion-to-air ( θJA). EQUATION 5-2: Table 5-1 and Table 5-2 show various values of θJA for the TC1264 packages. TABLE 5-1: THERMAL RESISTANCE GUIDELINES FOR TC1264 IN SOT-223 PACKAGE TABLE 5-2: THERMAL RESISTANCE GUIDELINES FOR TC1264 IN 3-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 TC1264 dissipates a maximum of 260 mW; below the allowable limit of 508 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 508mW 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 45°C/W 1000 sq mm 2500 sq mm 2500 sq mm 45°C/W 225 sq mm 2500 sq mm 2500 sq mm 53°C/W 100 sq mm 2500 sq mm 2500 sq mm 59°C/W 1000 sq mm 1000 sq mm 1000 sq mm 52°C/W 1000 sq mm 0 sq mm 1000 sq mm 55°C/W * Tab of device attached to topside copper Where: VINMAX VOUTMIN ILOADMAX PD = Worst-case actual power dissipation = Minimum regulator output voltage = Maximum output (load) current = Maximum voltage on VIN PD VINMAX VOUTMIN – ()I LOADMAX = PDMAX = (TJMAX – TAMAX) θJA Where all terms are previously defined. 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 125sqmm 2500sqmm 2500 sq mm 35°C/W * Tab of device attached to topside copper Given: VINMAX =3.3V ± 10% VOUTMIN = 2.7V ± 0.5% ILOADMAX =275 mA TJMAX = 125°C TAMAX =95°C θJA = 59°C/W (SOT-223) PD VINMAX VOUTMIN – ()I LOADMAX ≈ PD 3.3 1.1 × () 2.7 .995 × () – 275 10 3 – × = PD 260 mW = PDMAX TJMAX TAMAX – θ JA --------------------------------------- = PDMAX 125 95 – () 59 ------------------------- = PDMAX 508 mW = |
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