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RC5061 数据表(PDF) 14 Page - Fairchild Semiconductor |
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RC5061 数据表(HTML) 14 Page - Fairchild Semiconductor |
14 / 18 page RC5061 PRODUCT SPECIFICATION 14 REV. 1.0.0 7/6/00 the voltage is reduced to 16.4A * 40m Ω = 650mV. The output voltage does not return to its nominal value until the output current is reduced to a value within the safe operating ranges for the DC-DC converter. If any of the linear regulator outputs are loaded heavily enough that their output voltage drops below 80% of nominal for > 30µsec, all RC5061 outputs, including the switcher, are shut off and remain off until power is recycled. Schottky Diode Selection The application circuit 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 MOSFET is turning off and the lower MOSFET is turning on. It is undesirable for this diode to conduct because its high forward 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 forward 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 capacitance 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 5 shows 3 x 1000µF, but the exact number required will vary with the speed and type of the processor. For the top speed Katmai and Coppermine, the capacitors should be rated to take 9A and 6A of ripple current respectively. Capacitor ripple current rating is a function of temperature, and so the manufacturer should be contacted to find out the ripple current rating at the expected operational temperature. For details on the design of an input filter, refer to Applica- tions Bulletin AB-15. Figure 5. Input Filter Active Droop The RC5061 includes active droop; as the ouptut current increases, the output voltage drops. This is done in order to allow maximum headroom for transient response of the con- verter. The current is sensed by measuring the voltage across the high-side MOSFET during its on time. Note that this makes the droop dependent on the temperature of the MOSFET. How- ever, when the formula given for selecting RS (current limit) is used, there is a maximum droop possible (-40mV), and when this value is reached, additional drop across the MOSFET will not cause any increase in droop—until current limit is reached. Additional droop can be added to the active droop using a discrete resistor (typically a PCB trace) outside the control loop, as shown in Figure 2. This is typically only required for the most demanding applications, such as for the next gener- ation Intel processor (tolerance = +40/-70mV), as shown in Figure 2. Remote Sense The RC5061 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 16, 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 RC5061 may be found in Applications Bulletin AB-24. 2.5 µH 5V 0.1 µF 1000 µF, 10V Electrolytic Vin |
类似零件编号 - RC5061 |
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类似说明 - RC5061 |
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