Datenblatt-Suchmaschine für elektronische Bauteile |
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UC3825B Datenblatt(PDF) 5 Page - Texas Instruments |
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UC3825B Datenblatt(HTML) 5 Page - Texas Instruments |
5 / 9 page 5 UC1823A,B/1825A,B UC2823A,B/2825A,B UC3823A,B/3825A,B OSCILLATOR (cont.) APPLICATIONS INFORMATION (cont.) Oscillator Frequency vs. RT and CT Curve Maximum Duty Cycle vs RT Curve UDG-95104 UDG-95103 LEADING EDGE BLANKING The UC3823A,B/3825A,B performs fixed frequency pulse width modulation control. The UC3823A,B out- puts operate together at the switching frequency and can vary from 0 to some value less than 100%. The UC3825A,B outputs are alternately controlled. During every other cycle, one output will be off. Each output then, switches at one-half the oscillator frequency, vary- ing in duty cycle from 0 to less than 50%. To limit maximum duty cycle, the internal clock pulse blanks both outputs low during the discharge time of the oscillator. On the falling edge of the clock, the ap- propriate output(s) is driven high. The end of the pulse is controlled by the PWM comparator, current limit com- parator, or the overcurrent comparator. Normally the PWM comparator will sense a ramp crossing a control voltage (error amp output) and termi- nate the pulse. Leading edge blanking (LEB) causes the PWM comparator to be ignored for a fixed amount of time after the start of the pulse. This allows noise in- herent with switched mode power conversion to be re- jected. The PWM ramp input may not require any filtering as result of leading edge blanking. To program a Leading Edge Blanking period, connect a capacitor, C, to CLK/LEB. The discharge time set by C and the internal 10k resistor will determine the blanked interval. The 10k resistor has a 10% tolerance. For more accuracy, an external 2k 1% resistor, R, can be added, resulting in an equivalent resistance of 1.66k with a tolerance of 2.4%. The design equation is: tLEB = 0.5 • (R | | 10k) • C. Values of R less than 2k should not be used Leading edge blanking is also applied to the current limit comparator. After LEB, if the ILIM pin exceeds the one volt threshold, the pulse is terminated. The over current comparator, however, is not blanked. It will catch catastrophic over current faults without a blanking delay. Any time the ILIM pin exceeds 1.2V, the fault latch will be set and the outputs driven low. For this rea- son, some noise filtering may be required on the ILIM pin. LEB Operational Waveforms UDG-95105 |
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Ähnliche Beschreibung - UC3825B |
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