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TC1263 Datasheet(PDF) 5 Page - Microchip Technology |
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TC1263 Datasheet(HTML) 5 Page - Microchip Technology |
5 / 14 page © 2002 Microchip Technology Inc. DS21374B-page 5 TC1263 4.0 THERMAL CONSIDERATIONS 4.1 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. 4.2 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 4-1: The maximum allowable power dissipation (Equation 4-2) 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 4-2: Table 4-1 and Table 4-2 show various values of θJA for the TC1263 packages. TABLE 4-1: THERMAL RESISTANCE GUIDELINES FOR TC1263 IN 8-PIN SOIC PACKAGE *Pin 2 is ground. Device is mounted on topside. TABLE 4-2: THERMAL RESISTANCE GUIDELINES FOR TC1263 IN 3-PIN TO-220/DDPAK PACKAGE *Tab of device attached to topside copper Equation 4-1 can be used in conjunction with Equation 4-2 to ensure regulator thermal operation is within limits. For example: Given: VINMAX = 3.3V ± 10% VOUTMIN = 2.7V ± 0.5% ILOADMAX = 275mA TJMAX = 125°C TAMAX =95°C θJA =60°C/W Find: 1. Actual power dissipation 2. Maximum allowable dissipation Actual power dissipation: PD ≈ (VINMAX –VOUTMIN)ILOADMAX = [(3.3 x 1.1) – (2.7 x .995)]275 x 10–3 = 260mW Maximum allowable power dissipation: In this example, the TC1263 dissipates a maximum of 260mW; below the allowable limit of 500mW. In a similar manner, Equation 4-1 and Equation 4-2 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 500mW into Equation 4-1, 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 Where: PD ≈ (VINMAX –VOUTMIN)ILOADMAX PD VINMAX VOUTMIN ILOADMAX = Worst case actual power dissipation = Minimum regulator output voltage = Maximum output (load) current = Maximum voltage on VIN PDMAX =(TJMAX –TAMAX) θ JA 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 125 sq mm 2500 sq mm 2500 sq mm 35°C/W PDMAX =(TJMAX –TAMAX) θJA = (125 – 95) 60 =500mW |
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