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UC3823A датащи(PDF) 5 Page - Texas Instruments |
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UC3823A датащи(HTML) 5 Page - Texas Instruments |
5 / 9 page 5 UC1823A,B/1825A,B UC2823A,B/2825A,B UC3823A,B/3825A,B APPLICATIONS INFORMATION (cont.) Maximum Duty Cycle vs RT Curve Oscillator Frequency vs. RT and CT Curve OSCILLATOR (cont.) LEADING EDGE BLANKING The UC3823A,B/3825A,B performs fixed frequency pulse width modulation control. The UC3823A,B outputs oper- ate 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, varying 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 appropriate output(s) is driven high. The end of the pulse is controlled by the PWM comparator, current limit comparator, or the overcurrent comparator. Normally the PWM comparator will sense a ramp cross- ing a control voltage (error amp output) and terminate 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 inherent with switched mode power conversion to be rejected. 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 inter- val. The 10k resistor has a 10% tolerance. For more ac- curacy, an external 2k 1% resistor, R, can be added, re- sulting 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 catas- trophic 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 reason, some noise filtering may be required on the ILIM pin. LEB Operational Waveforms UDG-95103 UDG-95104 UDG-95105 |
Аналогичный номер детали - UC3823A |
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Аналогичное описание - UC3823A |
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