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UC1725 Scheda tecnica(PDF) 4 Page - Texas Instruments |
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UC1725 Scheda tecnica(HTML) 4 Page - Texas Instruments |
4 / 5 page UC1725 UC2725 UC3725 CURRENT LIMIT AND TIMING: Current sensing and shutdown can be implemented directly at the output us- ing the scheme shown in Figure 4. Alternatively, a current transformer can be used in place of RSENSE. A small RC filter in series with the input (pin 4) is generally needed to eliminate the leading edge current spike caused by parasitic circuit capacitances being charged during turn on. Due to the speed of the current sense circuit, it is very important to ground CF directly to Gnd as shown to eliminate false triggering of the one shot caused by ground drops. One shot timing is easily programmed using an external capacitor and resistor as shown in Figure 4. This, in turn, controls the output off time according to the formula: TOFF= 1.28 • RC. If current limit feature is not required, simply ground pin 4 and leave pin 5 open. OUTPUT: Gate drive to the power FET is provided by a totem pole output stage capable of sourcing and sinking currents in excess of 1 amp. The undervoltage lockout circuit guarantees that the high level output will never be less than 9 volts. In addition, during undervoltage lock- out, the output stage will actively sink current to eliminate the need for an external gate to source resistor. High level output is also clamped to 15 volts. Under high ca- pacitive loading however, the output may overshoot 2 to 3 volts, due to the drivers’ inabitlity to switch from full to zero output current instantaneously. In a practical circuit this is not normally a concern. A few ohms of series gate resistance is normally required to prevent parasitic oscil- lations, and will also eliminate overshoot at the gate. ENABLE: An enable pin is provided as a fast, digital in- put that can be used in a number of applications to di- rectly switch the output. Figure 6 shows a simple means of providing a fast, high voltage translation by using a small signal, high voltage transistor in a cascode configu- ration. Note that the UC1725 is still used to provide power, drive and protection circuitry for the power FET. FIGURE 4 - Current Limit UDG-92050 FIGURE 5 - Output Circuit FIGURE 6 - Using Enable Pin as a High Speed Input Path UDG-92052 UDG-92053 UNITRODE INTEGRATED CIRCUITS 7 CONTINENTAL BLVD. • MERRIMACK, NH 03054 TEL. (603) 424-2410 • FAX (603) 424-3460 4 |
Codice articolo simile - UC1725_12 |
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Descrizione simile - UC1725_12 |
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