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PT5405 Datasheet(PDF) 6 Page - Texas Instruments |
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PT5405 Datasheet(HTML) 6 Page - Texas Instruments |
6 / 11 page For technical support and further information, visit http://power.ti.com PT5400 Series Operating Features of the PT5400 SWIFT™ Series of Power Modules Under-Voltage Lockout (UVLO) The PT5400 SWIFT series of power modules incor- porate an under-voltage lockout (UVLO) function. The UVLO function provides a clean transition during power- up and power-down, allowing the regulator to tolerate a slowly rising input voltage. The UVLO prevents opera- tion of the module until the input voltage has risen above 2.95 V. Below this threshold the status of the inhibit control pin is overriden, and the module will not produce an output. When the input voltage rises above this thresh- old, the output status of the module is determined by the inhibit control pin. If the inhibit control is open-circuit (not grounded), the module will automatically power up. The UVLO circuit has approximately 0.16 V of hysteresis, and will completely turn off the module when a falling input voltage drops below about 2.8 V. (Note: Even though the applied input voltage may be above the UVLO threshold, operation to the published specifications requires that the input voltage be at or above the minimum specified for each model in the series. This ensures that the output voltage of the module is in regulation.) Soft-Start Power Up Following either the application of a valid input source voltage, or the removal of a ground signal to the inhibit control pin (with input power applied), the module will initiate a soft-start power up. The soft start has two effects on the start-up characteristic. It introduces a short time delay prior to the start-up of the output voltage, and also slows the rate at which the output voltage rises. Figure 1-1 shows the power-up characteristic of a PT5404 (1.8 V). In this example the delay time, td, is measured from the point at which the input voltage rises above 2.95 V (the UVLO threshold), to the point that the output voltage starts to rise. The time period t(SS) is the rise time of the output voltage ramp. The value of td and t(SS) are are approximately 10 ms and 7.5 ms respectively. Vin (1V/Div) Vout (1V/Div) t (SS) d t Vin =2.95V HORIZ SCALE: 5ms/Div Figure 1-1; Soft-Start Characteristic and Timing If desired, both time periods can be lengthened with the addition of a low value capacitor between the Inhibit control (pin 1) and the COM (pin 3). For a given value of external capacitance, Cinh, the formulas for calculating the approximate effect on td and t(SS) are given below. td ≈ (Cinh + 0.047 µF) × 1.2 V 5 µA t(SS) ≈ (Cinh + 0.047 µF) × 0.7 V 5 µA Note: The capacitor should be placed as close to the regulator as possible. Adding 0.047 µF of external capacitance to the Inhibit pin approximately doubles the value of td and t(SS). Current Limit Protection The output current limit feature is one of two fault protection mechanisms built into the PT5400 modules. Its purpose is to protect both the module and input source against the occurance of a load fault, thereby isolating the fault and preventing it from propagating to other parts of the power system. The PT5400 regulators sense the current switched by the series (high-side) power MOSFET. The circuit implements a continuous current limit char- acteristic. Upon the removal of the fault the output voltage will promptly recover, and the module will return to nor- mal operation. A current limit condition will also increase the module’s power dissipation, which may cause the temperature of the internal components to significantly rise. If the con- dition persists, the module may begin to cycle in and out of thermal shutdown. Thermal Shutdown Thermal shutdown is the second fault protection mecha- nism and protects the module’s internal circuitry against excessively high temperatures. A rise in the temperature of the internal circuitry may be the result of a drop in airflow, a high ambient temperature, or a sustained over- current load fault. If the junction temperature of the internal components exceed 150 °C, the module will shutdown. Once in thermal shutdown, the regulator is disabled and the output voltage is reduced to zero. The recovery is automatic and begins with a soft-start power up. Recovery occurs when the the sensed temperature decreases 10 °C below the trip point. Application Notes |
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