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LM2727 Folha de dados(PDF) 11 Page - National Semiconductor (TI) |
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LM2727 Folha de dados(HTML) 11 Page - National Semiconductor (TI) |
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11 / 22 page  Application Information (Continued) DESIGN CONSIDERATIONS The following is a design procedure for all the components needed to create the circuit shown in Figure 3 in the Ex- ample Circuits section, a 5V in to 1.2V out converter, capable of delivering 10A with an efficiency of 85%. The switching frequency is 300kHz. The same procedures can be followed to create the circuit shown in Figure 3, Figure 4, and to create many other designs with varying input voltages, out- put voltages, and output currents. INPUT CAPACITOR The input capacitors in a Buck switching converter are sub- jected to high stress due to the input current waveform, which is a square wave. Hence input caps are selected for their ripple current capability and their ability to withstand the heat generated as that ripple current runs through their ESR. Input rms ripple current is approximately: The power dissipated by each input capacitor is: Here, n is the number of capacitors, and indicates that power loss in each cap decreases rapidly as the number of input caps increase. The worst-case ripple for a Buck converter occurs during full load, when the duty cycle D = 50%. In the 5V to 1.2V case, D = 1.2/5 = 0.24. With a 10A maximum load the ripple current is 4.3A. The Sanyo 10MV5600AX aluminum electrolytic capacitor has a ripple current rating of 2.35A, up to 105˚C. Two such capacitors make a conservative design that allows for unequal current sharing between individual caps. Each capacitor has a maxi- mum ESR of 18m Ω at 100 kHz. Power loss in each device is then 0.05W, and total loss is 0.1W. Other possibilities for input and output capacitors include MLCC, tantalum, OSCON, SP, and POSCAPS. INPUT INDUCTOR The input inductor serves two basic purposes. First, in high power applications, the input inductor helps insulate the input power supply from switching noise. This is especially important if other switching converters draw current from the same supply. Noise at high frequency, such as that devel- oped by the LM2727 at 1MHz operation, could pass through the input stage of a slower converter, contaminating and possibly interfering with its operation. An input inductor also helps shield the LM2727 from high frequency noise generated by other switching converters. The second purpose of the input inductor is to limit the input current slew rate. During a change from no-load to full-load, the input inductor sees the highest voltage change across it, equal to the full load current times the input capacitor ESR. This value divided by the maximum allowable input current slew rate gives the minimum input inductance: In the case of a desktop computer system, the input current slew rate is the system power supply or "silver box" output current slew rate, which is typically about 0.1A/µs. Total input capacitor ESR is 9m Ω, hence ∆V is 10*0.009 = 90 mV, and the minimum inductance required is 0.9µH. The input induc- tor should be rated to handle the DC input current, which is approximated by: In this case I IN-DC is about 2.8A. One possible choice is the TDK SLF12575T-1R2N8R2, a 1.2µH device that can handle 8.2Arms, and has a DCR of 7m Ω. OUTPUT INDUCTOR The output inductor forms the first half of the power stage in a Buck converter. It is responsible for smoothing the square wave created by the switching action and for controlling the output current ripple. ( ∆I o) The inductance is chosen by selecting between tradeoffs in efficiency and response time. The smaller the output inductor, the more quickly the con- verter can respond to transients in the load current. As shown in the efficiency calculations, however, a smaller in- ductor requires a higher switching frequency to maintain the same level of output current ripple. An increase in frequency can mean increasing loss in the FETs due to the charging and discharging of the gates. Generally the switching fre- quency is chosen so that conduction loss outweighs switch- ing loss. The equation for output inductor selection is: Plugging in the values for output current ripple, input voltage, output voltage, switching frequency, and assuming a 40% peak-to-peak output current ripple yields an inductance of 1.5µH. The output inductor must be rated to handle the peak current (also equal to the peak switch current), which is (Io + 0.5* ∆I o). This is 12A for a 10A design. The Coilcraft D05022- 152HC is 1.5µH, is rated to 15Arms, and has a DCR of 4m Ω. OUTPUT CAPACITOR The output capacitor forms the second half of the power stage of a Buck switching converter. It is used to control the output voltage ripple ( ∆V o) and to supply load current during fast load transients. In this example the output current is 10A and the expected type of capacitor is an aluminum electrolytic, as with the input capacitors. (Other possibilities include ceramic, tanta- lum, and solid electrolyte capacitors, however the ceramic type often do not have the large capacitance needed to supply current for load transients, and tantalums tend to be more expensive than aluminum electrolytic.) Aluminum ca- pacitors tend to have very high capacitance and fairly low ESR, meaning that the ESR zero, which affects system stability, will be much lower than the switching frequency. The large capacitance means that at switching frequency, the ESR is dominant, hence the type and number of output capacitors is selected on the basis of ESR. One simple formula to find the maximum ESR based on the desired output voltage ripple, ∆V o and the designed output current ripple, ∆I o, is: www.national.com 11 |
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