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TC1223-XXVCT Datasheet(PDF) 5 Page - Microchip Technology |
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TC1223-XXVCT Datasheet(HTML) 5 Page - Microchip Technology |
5 / 14 page © 2002 Microchip Technology Inc. DS21368B-page 5 TC1223/TC1224 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). The 5-Pin SOT-23A package has a θJA of approximately 220°C/Watt. EQUATION 4-2: Equation 4-1 can be used in conjunction with Equation 4-2 to ensure regulator thermal operation is within limits. For example: Given: VINMAX = 3.0V ±10% VOUTMIN = 2.7V – 2.5% ILOADMAX = 40mA TJMAX = 125°C TAMAX = 55°C Find: 1. Actual power dissipation 2. Maximum allowable dissipation Actual power dissipation: PD ≈ (VINMAX –VOUTMIN)ILOADMAX = [(3.0 x 1.1) – (2.7 x .975)]40 x 10–3 =26.7mW Maximum allowable power dissipation: In this example, the TC1223 dissipates a maximum of 26.7mW; below the allowable limit of 318mW. In a similar manner, Equation 4-1 and Equation 4-2 can be used to calculate maximum current and/or input voltage limits. 4.3 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 –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) θJA Where all terms are previously defined. PDMAX =(TJMAX –TAMAX) θJA =(125 – 55) 220 = 318mW |
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