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UC2825AMDWREPG4 Scheda tecnica(PDF) 8 Page - Texas Instruments |
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UC2825AMDWREPG4 Scheda tecnica(HTML) 8 Page - Texas Instruments |
8 / 21 page t LEB + 0.5 R ø 10 kW C (2) UDG−95105 CT CLK/LEB LEB RAMP Input Blanked RAMP to PWM UC2825A-EP SGLS305D – JULY 2005 – REVISED SEPTEMBER 2010 www.ti.com Leading-Edge Blanking (LEB) The UC2825A-EP performs fixed-frequency PWM control. The UC2825A-EP outputs are alternately controlled. During every other cycle, one output is 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 senses a ramp crossing a control voltage (error-amplifier output) and terminates the pulse. 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 a result of LEB. To program an LEB period, connect a capacitor, C, to CLK/LEB. The discharge time set by C and the internal 10-k Ω resistor determines the blanked interval. The 10-kΩ resistor has a 10% tolerance. For more accuracy, an external 2-k Ω 1% resistor (R) can be added, resulting in an equivalent resistance of 1.66 kΩ with a tolerance of 2.4%. The design equation is: Values of R less than 2 k Ω should not be used. LEB also is applied to the current-limit comparator. After LEB, if the ILIM pin exceeds the 1-V threshold, the pulse is terminated. The overcurrent comparator, however, is not blanked. It catches catastrophic overcurrent faults without a blanking delay. Any time the ILIM pin exceeds 1.2 V, the fault latch is set, and the outputs are driven low. For this reason, some noise filtering may be required on the ILIM pin. Figure 4. LEB Operational Waveforms 8 Submit Documentation Feedback Copyright © 2005–2010, Texas Instruments Incorporated Product Folder Link(s): UC2825A-EP |
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