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UC1584 Scheda tecnica(PDF) 6 Page - Texas Instruments |
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UC1584 Scheda tecnica(HTML) 6 Page - Texas Instruments |
6 / 8 page 6 UC1584 UC2584 UNITRODE CORPORATION 7 CONTINENTAL BLVD. • MERRIMACK, NH 03054 TEL. (603) 424-2410 FAX (603) 424-3460 the driver during the on-time of the switch. Charge is be- ing replenished to the bootstrap capacitor during the off-time of the switch through the bootstrap diode con- nected between VCC and VFLT as shown in the typical application diagram. Soft Start The UC3584 Soft Start circuitry is designed to implement closed loop startup of the power supply output. During Soft Start, the reference to the noninverting input of the error amplifier is controlled by the voltage across the soft start capacitor on SS. As this voltage rises, it provides an increasing reference to the error amplifier. Once the soft start capacitor charges above the 1.5V precision refer- ence of the error amplifier, SS gets disconnected from the noninverting input of the error amplifier. This tech- nique allows the error amplifier to stay in its linear mode and to regulate the output voltage of the power supply according to the gradually increasing reference voltage on its noninverting input. Further advantage of the closed loop start up scheme is the absence of output voltage overshoot during power up of the power supply output. Fault Detection Fault Detection feature is implemented to detect exces- sive overload conditions. Under these conditions the er- ror amplifier output goes high to command the maximum duty cycle. As soon as the error amplifier’s output ex- ceeds 5V, the fault delay capacitor connected to the CDLY pin starts charging. If CDLY capacitor voltage reaches 2V before the error amplifier output falls back below 5V, a fault condition is declared, the PWM output is disabled and soft start cycle is initiated. Under persis- tent fault conditions the UC3584 will continuously cycle through soft start sequence, attempting to bring the out- put to its regulated, nominal voltage. The value of CDLY capacitor should be chosen large enough to delay the activation of the fault sequence in case of load transients which can also cause the error amplifier output to go high temporarily. Error Amplifier The Error Amplifier of the UC3584 is used to regulate the voltage of an auxiliary output in a power supply. The noninverting input of the error amplifier is connected to an internal, 1.5V reference. The inverting input (FB pin) is tied to an output voltage divider. The compensation network of the negative feedback loop is connected be- tween the amplifier’s output (COMP pin) and FB. The noninverting input of the error amplifier is also connected to the SS node through a diode. This arrangement allows closed loop soft start for the output of a power supply regulated by the UC3584. Closed loop soft start assures that the error amplifier is kept in active mode and the out- put voltage of the converter follows the reference voltage on its noninverting input as it ramps up (following the SS node). If a fault condition is detected, SS node gets pulled to ground, forcing the error amplifier’s reference low. Consequently, the error amplifier’s output voltage goes low and duty cycle is reduced. APPLICATION INFORMATION (cont.) |
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