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FAN5071 Arkusz danych(PDF) 11 Page - Fairchild Semiconductor |
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FAN5071 Arkusz danych(HTML) 11 Page - Fairchild Semiconductor |
11 / 15 page PRODUCT SPECIFICATION FAN5071 REV. 1.0.4 1/29/02 11 controller to reduce the converter’s duty cycle to approxi- mately 20%. This causes a drastic reduction in the output voltage as the load regulation collapses into the short circuit control mode. With a 4m Ω output short, the voltage is reduced to 29A * 4m Ω = 116mV. The output voltage does not return to its nominal value until the output current is reduced to a value within the safe operating range for the DC-DC converter. Schottky Diode Selection The application circuits of Figure 1 shows a Schottky diode, D1, which is used as a free-wheeling diode to assure that the body-diode in Q2 does not conduct when the upper MOS- FET is turning off and the lower MOSFET is turning on. It is undesirable for this diode to conduct because its high for- ward voltage drop and long reverse recovery time degrades efficiency, and so the Schottky provides a shunt path for the current. Since this time duration is very short, the selection criterion for the diode is that the forward voltage of the Schottky at the output current should be less than the for- ward voltage of the MOSFET’s body diode. Output Filter Capacitors The output bulk capacitors of a converter help determine its output ripple voltage and its transient response. It has already been seen in the section on selecting an inductor that the ESR helps set the minimum inductance, and the capaci- tance value helps set the maximum inductance. For most converters, however, the number of capacitors required is determined by the transient response and the output ripple voltage, and these are determined by the ESR and not the capacitance value. That is, in order to achieve the necessary ESR to meet the transient and ripple requirements, the capacitance value required is already very large. The most commonly used choice for output bulk capacitors is aluminum electrolytics, because of their low cost and low ESR. The only type of aluminum capacitor used should be those that have an ESR rated at 100kHz. Consult Application Bulletin AB-14 for detailed information on output capacitor selection. The output capacitance should also include a number of small value ceramic capacitors placed as close as possible to the processor; 0.1µF and 0.01µF are recommended values. Input Filter The DC-DC converter design may include an input inductor between the system +5V supply and the converter input as shown in Figure 5. This inductor serves to isolate the +5V supply from the noise in the switching portion of the DC-DC converter, and to limit the inrush current into the input capac- itors during power up. A value of 2.5µH is recommended. It is necessary to have some low ESR aluminum electrolytic capacitors at the input to the converter. These capacitors deliver current when the high side MOSFET switches on. Figure 4 shows 3 x 1000µF, but the exact number required will vary with the speed and type of the processor. Capacitor ripple current rating is a function of temperature, and so the manufacturer should be contacted to find out the ripple cur- rent rating at the expected operational temperature. For details on the design of an input filter, refer to Applications Bulletin AB-15. Figure 4. Input Filter Programmable Active Droop™ The FAN5071 includes Programmable Active Droop™: as the output current increases, the output voltage drops, and the amount of this drop is user adjustable. This is done in order to allow maximum headroom for transient response of the converter. The current is typically sensed by measuring the voltage across the RDS,on of the high-side MOSFET during its on time, as shown in Figures 1 and 2. To program the amount of droop, use the formula where Imax is the current at which the droop occurs, and Rsense is the resistance of the current sensor, either the source resistor or the high-side MOSFET’s on-resistance. For exam- ple, to get 120mV of droop with a maximum output current of 30A and a 10m Ω sense resistor, use R D = 14.4KΩ * 30A * 10m Ω/(120mV *3) = 12KΩ. The value of the product Imax*Rsense must be < 600mV for proper functioning of the droop circuit. If this product is exceeded, a lower resis- tance MOSFET must be used. Further details on use of the Programmable Active Droop™ may be found in Applications Bulletin AB-24. Remote Sense The FAN5071 offers remote sense of the output voltage to minimize the output capacitor requirements of the converter. It is highly recommended that the remote sense pin, Pin 20, be tied directly to the processor power pins, so that the effects of power plane impedance are eliminated. Further details on use of the remote sense feature of the FAN5071 may be found in Applications Bulletin AB-24. 2.5 µH 5V 0.1 µF 1000 µF, 10V Electrolytic Vin RD 14.4K Ω *I max *Rsense VDroop *3 |
Podobny numer części - FAN5071 |
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Podobny opis - FAN5071 |
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