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LM2742 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM2742 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
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11 / 22 page  Application Information (Continued) 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 LM2742 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 LM2742 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 output ripple, efficiency, and response time. The smaller the output inductor, the more quickly the converter can respond to transients in the load current. If the inductor value is increased, the ripple through the output capacitor is reduced and thus the output ripple is reduced. As shown in the efficiency calculations, a smaller inductor requires a higher switching frequency to maintain the same level of output current ripple. An increase in fre- quency can mean increasing loss in the FETs due to the charging and discharging of the gates. Generally the switch- ing frequency is chosen so that conduction loss outweighs switching loss. The equation for output inductor selection is: A good range for ∆I o is 25 to 50% of the output current. In the past, 30% was considered a maximum value for output currents higher than about 2Amps, but as output capacitor technology improves the ripple current can be allowed to increase. Plugging in the values for output current ripple, input voltage, output voltage, switching frequency, and as- suming 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: In this example, in order to maintain a 2% peak-to-peak output voltage ripple and a 40% peak-to-peak inductor cur- rent ripple, the required maximum ESR is 6m Ω. Three Sanyo 10MV5600AX capacitors in parallel will give an equivalent ESR of 6m Ω. The total bulk capacitance of 16.8mF is www.national.com 11 |
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