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UC2849DW Fiches technique(PDF) 9 Page - Texas Instruments |
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UC2849DW Fiches technique(HTML) 9 Page - Texas Instruments |
9 / 21 page www.ti.com VAOCURRENTCOMMAND 3.4V 1.2V 3.6V 1.4V (THRESHOLD) 8.5V 0V OSC CLKSYN OUT MINIMUMOUTPUTLOWTIME 75 100 50 0 25 1 10 100 1k 10k 100k 1M R - T W C =345pF T READ=51 READ=5000 READ=511 1 10 100 1k 10k 100k 1M R - T W CT =345pF 1nF 2.2nF 4.7nF READ=500 W 1000 10k 100 10 1 UC2849 UC3849 SLUS360C – JULY 1995 – REVISED AUGUST 2007 The timing capacitor (CT) is linearly charged with the charge current forcing the OSC pin to charge to a 3.4-V threshold. After exceeding this threshold, the RS flip-flop is set driving CLKSYN high and RDEAD low which discharges CT. This discharge time with the RC time delay of 2 • CT • RDEAD is the minimum output low time. OSC continues to discharge until it reaches a 1.2 -V threshold and resets the RS flip-flop which repeats the charging sequence as shown in Figure 2. Equations to approximate frequency and maximum duty cycle are listed under the OSC pin description. Figure 3 and Figure 4 graphs show measured variation of frequency and maximum duty cycle with varying RT, CT, and RDEAD component values. As shown in Figure 5, several oscillators are synchronized to the highest free running frequency by connecting 100-pF capacitors in series with each CLKSYN pin and connecting the other side of the capacitors together forming the CLKSYN bus. The CLKSYN bus is then pulled down to ground with a resistance of approximately 10 k Ω. Referring to Figure 1, the synchronization threshold is 1.4 V. The oscillator blanks any synchronization pulse that occurs when OSC is below 2.5 V. This allows units, once they discharge below 2.5 V, to continue through the current discharge and subsequent charge cycles whether or not other units on the CLKSYN bus are still synchronizing. This requires the frequency of all free running oscillators to be within 40% of each other to assure synchronization. Figure 2. Oscillator and PWM Output Waveform Figure 3. Output Frequency Figure 4. Maximum Duty Cycle 9 Submit Documentation Feedback |
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Description similaire - UC2849DW |
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