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UC3548N 데이터시트(PDF) 6 Page - Texas Instruments |
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UC3548N 데이터시트(HTML) 6 Page - Texas Instruments |
6 / 10 page UC1548 UC2548 UC3548 There are several methods to program IOFF. If accurate maximum current control is required, IOFF must track out- put voltage. The method shown in Figure 4 derives a voltage proportional to VIN • D (where D = duty cycle). In a buck converter, output voltage is proportional to VIN • D. A resistively loaded diode connection to the bootstrap winding yields a square wave whose amplitude is propor- tional to VIN and is duty cycle modulated by the control circuit. Averaging this waveform with a filter generates a primary side replica of secondary regulated Vo. A single pole filter is shown, but in practice a two or three pole filter provides better transient response. Filtered voltage is con- verted by ROFF to a current to the IOFF pin to control CI downslope. If accurate system maximum current is not a critical re- quirement, Figure 5 shows the simplest method of downslope generation: a single resistor (ROFF = 40k) from IOFF to VREF. The discharge current is then 100 µA. The disadvantage to this approach is that the synthesizer continues to generate a down slope when the switch is off even during short circuit conditions. Actual inductor downslope is closer to zero during a short circuit. The penalty is that the average current is understated by an amount approximately equal to the nominal inductor rip- ple current. Output short circuit is therefore higher than the designed maximum output current. A third method of generating IOFF is to add a second winding to the output inductor core (Figure 6). When the power switch is off and inductor current flows in the free wheeling diode, the voltage across the inductor is equal to the output voltage plus the diode drop. This voltage is then transformed by the second winding to the primary side of the converter. The advantages to this approach are its inherent accuracy and bandwidth. Winding the sec- ond coil on the output inductor core while maintaining the required isolation makes this a more costly solution. In the example, ROFF = Vo / 100 µA. The 4 • ROFF resistor is added to compensate the one volt input level of the IOFF pin. Without this compensation, a minor current foldback behavior will be observed. INDUCTOR CURRENT WAVEFORM SYNTHESIZER (cont.) Figure 4: Inductor Current Waveform Synthesizer UDG-95041 Figure 5: Fixed IOFF Figure 6: Second Inductor Winding Generation of IOFF UDG-95042 UDG-95043 6 |
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