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MAX652CSA Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX652CSA Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 16 page 5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers ______________________________________________________________________________________ 11 Standard wire-wound and metal-film resistors have an inductance high enough to degrade performance. Surface-mount (chip) resistors have very little induc- tance and are well suited for use as current-sense resistors. A wire resistor made by IRC works well in through-hole applications. Because this resistor is a band of metal shaped as a “U”, its inductance is less than 10nH (an order of magnitude less than metal film resistors). Resistance values between 5m Ω and 0.1Ω are available (see Table 1). Inductor Selection Practical inductor values range from 10µH to 50µH or more. The circuit operates in discontinuous-conduction mode if: VOUT x (R + 1) VD V+ ≤ ———————— + —— + VSW RR R, the switch on-time/off-time ratio, equals 6.7. VD is the diode’s drop, and VSW is the voltage drop across the P-channel FET. To get the full output capability in discontinuous-conduction mode, choose an inductor value no larger than: RSENSE x 12µs x (V+ - VSW - VOUT) L(max) = ————————————————— VCS where VCS is the current-sense voltage. In both the continuous and discontinuous modes, the lower limit of the inductor is more important. With a small inductor value, the current rises faster and over- shoots the desired peak current limit because the cur- rent-limit comparator cannot respond fast enough. This reduces efficiency slightly and, more importantly, could cause the current rating of the external components to be exceeded. Calculate the minimum inductor value as follows: (V+(max) - VSW - VOUT) x 0.3µs L(min) = ————————————––—— ∆I x ILIM(min) where ∆I is the percentage of inductor-current over- shoot, where ILIM = VCS/RSENSE and 0.3µs is the time it takes the comparator to switch. An overshoot of 10% is usually not a problem. Inductance values above the minimum work well if the maximum value defined above is not exceeded. Smaller inductance values cause higher output ripple because of overshoot. Larger val- ues tend to produce physically larger coils. For highest efficiency, use a coil with low DC resis- tance; a value smaller than 0.1V/ILIM works best. To minimize radiated noise, use a toroid, pot core, or shielded-bobbin inductor. Inductors with a ferrite core or equivalent are recommended. Make sure the induc- tor’s saturation-current rating is greater than ILIM(max). However, it is generally acceptable to bias the inductor into saturation by about 20% (the point where the inductance is 20% below its nominal value). The peak current of Figure 1 is 2.35A for a 1.5A output. The inductor used in this circuit is specified to drop by 10% at 2.2A (worst case); a curve provided by the manufacturer shows that the inductance typically drops by 20% at 3.1A. Using a slightly underrated inductor can sometimes reduce size and cost, with only a minor impact on efficiency. The MAX649/MAX651/MAX652 current limit prevents any damage from an underrated inductor’s low inductance at high currents. Table 1 lists inductor types and suppliers for various applications. The efficiencies of the listed surface- mount inductors are nearly equivalent to those of the larger size through-hole versions. Diode Selection The MAX649/MAX651/MAX652’s high switching fre- quency demands a high-speed rectifier (commonly called a catch diode when used in switching-regulator circuits). Schottky diodes, such as the 1N5817 through 1N5822 families (and their surface-mount equivalents), are recommended. Choose a diode with an average current rating equal to or greater than ILIM(max) and a voltage rating higher than V+(max). For high-tempera- ture applications, where Schottky diodes can be inadequate because of high leakage currents, use high-speed silicon diodes instead. At heavy loads and high temperatures, the disadvantages of a Schottky diode’s high leakage current may outweigh the benefits of its low forward voltage. Table 1 lists diode types and suppliers for various applications. External Switching Transistor The MAX649/MAX651/MAX652 drive P-channel enhancement-mode MOSFET transistors only. The choice of power transistor is primarily dictated by the input voltage and the peak current. The transistor's on-resistance, gate-source threshold, and gate capacitance must also be appropriately chosen. The drain-to-source and gate-to-source breakdown voltage ratings must be greater than V+. The total gate-charge specification is normally not critical, but values should be less than 100nC for best efficiency. The MOSFET should be capable of handling the peak current and, for maximum efficiency, have a very low on-resistance at that current. Also, the on-resistance must be low for the minimum available VGS, which equals V+(min). Select a transistor with an on-resistance between 50% and 100% of the current-sense resistor. The Si9430 transistor chosen for the Typical Operating Circuit has |
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