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MAX724CCK Datasheet(PDF) 7 Page - Maxim Integrated Products |
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MAX724CCK Datasheet(HTML) 7 Page - Maxim Integrated Products |
7 / 12 page 5A/2A Step-Down, PWM, Switch-Mode DC-DC Regulators _______________________________________________________________________________________ 7 Inductor Selection Although most MAX724 designs perform satisfactorily with 50 µH inductors (100µH for the MAX726), the MAX724/MAX726 are able to operate with values rang- ing from 5 µH to 200µH. In some cases, inductors other than 50 µH may be desired to minimize size (lower inductance), or reduce ripple (higher inductance). In any case, inductor current must at least be rated for the desired output current. In high-current applications, pay particular attention to both the RMS and peak inductor ratings. The induc- tor's peak current is limited by core saturation. Exceeding the saturation limit actually reduces the coil's inductance and energy storage ability, and increases power loss. Inductor RMS current ratings depend on heating effects in the coil windings. The following equation calculates maximum output cur- rent as a function of inductance and input conditions: IOUT = ISW - VOUT (VIN - VOUT) 2 fOSC VINL where ISW is the maximum switch current (5.5A for MAX724), VIN is the maximum input voltage, VOUT is the output voltage, and fOSC is the switching frequency. For the MAX724 example in Figure 2, with L = 50 µH and VIN = 25V, IOUT = 5.5A - 5V (25V - 5V) = 5.1A 2 (105Hz) 25V (50 x 10-6H) Note that increasing or decreasing inductor value pro- vides only small changes in maximum output current (100 µH = 5.3A, 20µH = 4.5A). The equation shows that output current is mostly a function of the MAX724/MAX726 current-limit value. Again, a 50 µH inductor works well in most applications and provides 5A with a wide range of input voltages. Catch Diode D1 provides a path for inductor current when VSW turns off. Under normal load conditions, the average diode current may only be a fraction of load current; but dur- ing short-circuit or current-limit, diode current is higher. Conservative design dictates that the diode average current rating be 2 times the desired output current. If operation with extended short-circuit or overload time is expected, then the diode current rating must exceed the current limit (6.5A = MAX724, 2.6A = MAX726), and heat sinking may be necessary. Under normal operating conditions (not shorted), power dissipated in the diode PD is calculated by: PD = IOUT (VIN - VOUT) VD VIN where VD is forward drop of the diode at a current equal to IOUT. In nearly all circuits, Schottky diodes provide the best performance and are recommended due to their fast switching times and low forward voltage drop. Standard power rectifiers such as the 1N4000 series are too slow for DC-DC conversion circuits and are not recommended. Output Filter Capacitor For most MAX724/MAX726 applications, a high-quality, low-ESR, 470 µF or 500µF output filter capacitor will suf- fice. To reduce ripple, minimize capacitor lead length and connect the capacitor directly to the GND pin. Capacitor suppliers are listed in Table 1. Output ripple is a function of inductor value and output capacitor effective series resistance (ESR). In continuous-con- duction mode: VCR(p-p) = ESR (VOUT) (1 - VOUT/VIN) L fOSC It is interesting to note that input voltage (VIN), and not load current, affects output ripple in CCM. This is because only the DC, and not the peak-to-peak, induc- tor current changes with load (see Figure 3). In discontinuous-conduction mode, the equation is dif- ferent because the peak-to-peak inductor current does depend on load: VDR(p-p) = ESR √2IOUTVOUT(VIN-VOUT) L fOSC VIN where output ripple is proportional to the square root of load current. Refer to the earlier equation for IDCM to determine where DCM occurs and hence when the DCM ripple equation should be used. Input Bypass Capacitor An input capacitor (200 µF or 220µF) is required for step- down converters because the input current, rather than being continuous (like output current), is a square wave. For this reason the capacitor must have low ESR and a ripple-current rating sufficiently large so that its ESR and the AC input current do not conspire to overheat the capacitor. In CCM, the capacitor's RMS ripple current is: IR(RMS) = IOUT √VOUT(VIN-VOUT) VIN2 The power dissipated in the input capacitor is then PC: PC = IR(RMS)2 (ESR) |
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