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UCC1809-2 Datasheet(PDF) 7 Page - Texas Instruments |
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UCC1809-2 Datasheet(HTML) 7 Page - Texas Instruments |
7 / 16 page 7 UCC1809-1/-2 UCC2809-1/-2 UCC3809-1/-2 3 4 UCC3809 OSCILLATOR RT2 CT RT1 1k 2N2222A 424 SYNC PULSE OPTION I 3 4 UCC3809 OSCILLATOR RT2 CT RT1 424 SYNC PULSE 0.1µF 2N2222A 24 +5V OPTION II Figure 5. UCC3809 synchronization options. UDG-99006 Synchronization Both of the synchronization schemes shown in Figure 5 can be successfully implemented with the internal oscilla- tor of the UCC3809. Both schemes allow access to the timing ramp needed for slope compensation and have minimal impact on the programmed maximum duty cycle. In the absence of a sync pulse, the PWM controller will run independently at the frequency set by RT1, RT2, and CT. This free running frequency must be approximately 15 to 20% lower than the sync pulse frequency to insure the free running oscillator does not cross the comparator threshold before the desired sync pulse. Option I uses the synchronization pulse to pull pin 3 low, triggering the internal 1.67V comparator to reset the RS latch and initiate a charging cycle. The valley voltage of the CT waveform is higher when synchronized using this configuration, decreasing the ramp charge and discharge times, thereby increasing the operating frequency; other- wise the overall shape of the CT voltage waveform is un- changed. Option II uses the synchronization pulse to superimpose the sync voltage onto the peak of the CT waveform. This triggers the internal 3.33V comparator, initiating a dis- charge cycle. The sync pulse is summed with the free running oscillator waveform at the CT node, resulting in a spike on top of the CT peak voltage. ADDITIONAL INFORMATION Please refer to the following Unitrode application topics for additional information. [1] Application Note U-165, Design Review: Isolated 50W Flyback Converter with the UCC3809 Primary Side Con- troller by Lisa Dinwoodie. [2] Design Note DN-89, Comparing the UC3842, UCC3802, and UCC3809 Primary Side PWM Controllers by Lisa Dinwoodie. APPLICATION INFORMATION (cont.) |
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Similar Description - UCC1809-2 |
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