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TPS71219 Datasheet(PDF) 11 Page - Texas Instruments |
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TPS71219 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 15 page www.ti.com TRANSIENT RESPONSE SHUTDOWN POWER DISSIPATION INTERNAL CURRENT LIMIT THERMAL PROTECTION P D + (VIN * VOUT) I OUT (4) TPS71202, TPS71219 TPS71229, TPS71247 TPS71256, TPS71257 SBVS049B – MAY 2004 – REVISED MARCH 2005 Depending on power dissipation, thermal resistance, and ambient temperature, the thermal protection As with any regulator, increasing the size of the circuit may cycle on and off. This limits the dissipation output capacitor will reduce over/undershoot magni- of the regulator, protecting it from damage due to tude but increase duration of the transient response. overheating. In the adjustable version, the addition of a capacitor, CFB, from the output to the feedback pin will also Any tendency to activate the thermal protection circuit improve stability and transient response. The transi- indicates excessive power dissipation or an inad- ent response of the TPS712xx is enhanced with an equate heatsink. For reliable operation, junction tem- active pull-down that engages when the output is perature should be limited to +125°C maximum. To over-voltaged. The active pull-down decreases the estimate the margin of safety in a complete design output recovery time when the load is removed. (including heatsink), increase the ambient tempera- Figure 13 in the Typical Characteristics section shows ture until the thermal protection is triggered; use the output transient response. worst-case loads and signal conditions. For good reliability, thermal protection should trigger at least +35°C above the maximum expected ambient con- dition of your application. This produces a worst-case Both enable pins are active high and are compatible junction temperature of +125°C at the highest ex- with standard TTL-CMOS levels. The device is only pected ambient temperature and worst-case load. completely disabled when both EN1 and EN2 are The internal protection circuitry of the TPS712xx was logic low. In this state, the LDO is completely off and designed to protect against overload conditions. It the ground pin current drops to approximately 100 was not intended to replace proper heatsinking. nA. With one output disabled, the ground pin current Continuously running the TPS712xx into thermal is slightly greater than half the nominal value. When shutdown will degrade device reliability. shutdown capability is not required, the enable pins should be connected to the input supply. The ability to remove heat from the die is different for each package type, presenting different consider- The TPS712xx internal current limit helps protect the ations in the PCB layout. The PCB area around the regulator during fault conditions. During current limit, device that is free of other components moves the the output will source a fixed amount of current that is heat from the device to the ambient air. Performance largely independent of the output voltage. data for a JEDEC high-K board is shown in the The TPS712xx PMOS-pass transistors have a built-in Dissipation Ratings table. Using heavier copper will back diode that conducts reverse current when the increase the effectiveness in removing heat from the input voltage drops below the output voltage (that is, device. The addition of plated through-holes to during power-down). Current is conducted from the heat-dissipating layers will also improve the heat-sink output to the input and is not internally limited. If effectiveness. extended reverse voltage operation is anticipated, Power dissipation depends on input voltage and load external limiting may be appropriate. conditions. Power dissipation is equal to the product of the output current times the voltage drop across the output pass element (VIN to VOUT): Thermal protection disables both outputs when the junction temperature of either channel rises to ap- proximately +160°C, allowing the device to cool. Power dissipation can be minimized by using the When the junction temperature cools to approxi- lowest possible input voltage necessary to assure the mately +140°C, the output circuitry is again enabled. required output voltage. 11 |
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