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TC1173-2.5VOA Datasheet(PDF) 4 Page - TelCom Semiconductor, Inc |
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TC1173-2.5VOA Datasheet(HTML) 4 Page - TelCom Semiconductor, Inc |
4 / 7 page 4 TC1173 300mA CMOS LDO With Shutdown, ERROR Output, And Bypass TC1173-2 2/2/00 Bypass Input A 470pF capacitor connected from the Bypass input to ground reduces noise present on the internal reference, which in turn significantly reduces output noise. If output noise is not a concern, this input may be left unconnected. Larger capacitor values may be used, but results in a longer time period to rated output voltage when power is initially applied. Thermal Considerations Thermal Shutdown Integrated thermal protection circuitry shuts the regula- tor off when die temperature exceeds 150 °C. The regulator remains off until the die temperature drops to approximately 140 °C. Power Dissipation The amount of power the regulator dissipates is prima- rily a function of input and output voltage, and output current. The following equation is used to calculate worst case actual power dissipation: PD ≈ (VINMAX – VOUTMIN)ILOADMAX Where: PD = worst case actual power dissipation VINMAX = maximum voltage on VIN VOUTMIN = minimum regulator output voltage ILOADMAX = maximum output (load) current Equation 1. The maximum allowable power dissipation (Equation 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 ( JA). The 8-Pin SOIC package has a JA of approximately 160 °C/Watt, while the 8-Pin MSOP package has a JA of approximately 200 °C/Watt; both when mounted on a single layer FR4 dielectric copper clad PC board. PDMAX = (TJMAX – TAMAX) JA Where all terms are previously defined. Equation 2. Equation 1 can be used in conjunction with Equation 2 to ensure regulator thermal operation is within limits. For example: GIVEN: VINMAX= 3.0V ± 10% VOUTMIN= 2.7V ± 0.5% ILOADMAX = 250mA TJMAX = 125°C TAMAX = 55°C JA = 200°C/W 8-Pin MSOP Package FIND: 1. Actual power dissipation 2. Maximum allowable dissipation Actual power dissipation: PD ≈ (VINMAX - VOUTMIN)ILOADMAX = [(3.0 x 1.1) - (2.7 x .995)]250 x 10-3 = 155mW Maximum allowable power dissipation: PD ≈ (TJMAX– TAMAX) JA = (125 – 55) 200 = 350mW In this example, the TC1173 dissipates a maximum of only 155mW; far below the allowable limit of 350mW. In a similar manner, Equation 1 and Equation 2 can be used to calculate maximum current and/or input voltage limits. For example, the maximum allowable VIN is found by substitut- ing the maximum allowable power dissipation of 350mW into Equation 1, from which VINMAX = 4.1V. 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 JA and, therefore, increase the maximum allowable power dissipation limit. |
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