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TPS72518 Datasheet(PDF) 10 Page - Texas Instruments |
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TPS72518 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 18 page www.ti.com Regulator Protection THERMAL INFORMATION P D max + VI(avg) * VO(avg) I O(avg) ) V I(avg) x I (Q) (3) TPS72501 TPS72515, TPS72516 TPS72518, TPS72525 SLVS341D – MAY 2002 – REVISED MARCH 2004 APPLICATION INFORMATION (continued) The TPS725xx pass element has a built-in back diode that safely conducts reverse current when the input voltage drops below the output voltage (e.g., during power down). Current is conducted from the output to the input and is not internally limited. If extended reverse voltage is anticipated, external limiting might be appropriate. The TPS725xx also features internal current limiting and thermal protection. During normal operation, the TPS725xx limits output current to approximately 1.6 A. When current limiting engages, the output voltage scales back linearly until the overcurrent condition ends. While current limiting is designed to prevent gross device failure, care should be taken not to exceed the power dissipation ratings of the package. If the temperature of the device exceeds 165 °C, thermal-protection circuitry shuts it down. Once the device has cooled down to below 145 °C, regulator operation resumes. The amount of heat that an LDO linear regulator generates is directly proportional to the amount of power it dissipates during operation. All integrated circuits have a maximum allowable junction temperature (TJmax) above which normal operation is not assured. A system designer must design the operating environment so that the operating junction temperature (TJ) does not exceed the maximum junction temperature (TJmax). The two main environmental variables that a designer can use to improve thermal performance are air flow and external heatsinks. The purpose of this information is to aid the designer in determining the proper operating environment for a linear regulator that is operating at a specific power level. In general, the maximum expected power (PD(max)) consumed by a linear regulator is computed as: Where: • VI(avg) is the average input voltage. • VO(avg) is the average output voltage. • IO(avg) is the average output current. • I(Q) is the quiescent current. For most TI LDO regulators, the quiescent current is insignificant compared to the average output current; therefore, the term VI(avg) x I(Q) can be neglected. The operating junction temperature is computed by adding the ambient temperature (TA) and the increase in temperature due to the regulator's power dissipation. The temperature rise is computed by multiplying the maximum expected power dissipation by the sum of the thermal resistances between the junction and the case (RθJC), the case to heatsink (RθCS), and the heatsink to ambient (RθSA). Thermal resistances are measures of how effectively an object dissipates heat. Typically, the larger the device, the more surface area available for power dissipation and the lower the object's thermal resistance. Figure 20 illustrates these thermal resistances for (a) a SOT223 package mounted in a JEDEC low-K board, and (b) a DDPAK package mounted on a JEDEC high-K board. 10 |
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