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UCC1581 Datasheet(PDF) 7 Page - Texas Instruments |
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UCC1581 Datasheet(HTML) 7 Page - Texas Instruments |
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7 / 16 page  7 UCC1581 UCC2581 UCC3581 A rather large soft start capacitor was chosen to give a startup time of several hundred milliseconds, reducing the input surge current while the output is coming up. Note that for stability, the UCC3581 VREF bypass capaci- tor needs to be at least 1 µF. The VDD supply also needs some capacitance to hold it up between pulses at light load and high line, where the frequency may drop to less than 1kHz due to pulse skipping. Otherwise it may drop low enough for the startup MOSFET to be biased on, lowering efficiency. If the sync input is used, it should not be left in a high im- pedance state where noise could cause false triggering. If unused, it should be grounded. The transformer was designed with a standard Magnetics RM8 ferrite core using P material, gapped for an AL of 1600mH/1000Turn 2. The primary consists of 44 turns, while the 5V secondary has 10 turns and the bootstrap winding 18 turns. For simplicity, all the windings can be #28 AWG. A two section bobbin was used to provide high primary to secondary isolation. A much smaller design, with reduced isolation, could have been done for this low power level. APPLICATION INFORMATION (cont.) 3.90 3.92 3.94 3.96 3.98 4.00 4.02 4.04 4.06 4.08 4.10 -75 -50 -25 0 25 50 75 100 125 TEMPERATURE [°C] Figure 5. Reference voltage vs. temperature. TYPICAL CHARACTERISTIC CURVES 0 20 40 60 80 100 120 140 100 1000 10000 CT [pF] 1M/10k 470k/47k 680k/12k 220k/10k Figure 6. Frequency vs. CT vs. RT1 and RT2. 0 200 400 600 800 1000 1200 1400 1600 0 20 40 60 80 100 FREQUENCY [kHz] 1nF LOAD NO LOAD Figure 8. IDD vs. frequency RT1 = 680k, RT2 = 12k. 30 40 50 60 70 80 90 100 0 2040 6080 100 FREQUENCY [kHz] 220K/10K 680K/12K 470K/47K 1M/10K Figure 7. Duty cycle vs. frequency vs. RT1 / RT2. |
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Similar Description - UCC1581 |
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