Datasheet LT3493 (Analog Devices) - 9

LT3493
APPLICATIONS INFORMATION
Now the required on-time has decreased below the
Inductor Selection and Maximum Output Current
minimum on time of 120ns. Instead of the switch pulse A good fi rst choice for the inductor value is: width becoming narrower to accommodate the lower duty cycle requirement, the switch pulse width remains fi xed L = 1.6 (VOUT + VD) at 120ns. In Figure 2 the inductor current ramps up to a where VD is the voltage drop of the catch diode (~0.4V) and value exceeding the load current and the output ripple L is in μH. With this value there will be no subharmonic increases to ~200mV. The part then remains off until the oscillation for applications with 50% or greater duty cycle. output voltage dips below 100% of the programmed value The inductor’s RMS current rating must be greater than before it begins switching again. your maximum load current and its saturation current Provided that the load can tolerate the increased output should be about 30% higher. For robust operation in fault voltage ripple and that the components have been properly conditions, the saturation current should be above 2.2A. selected, operation above V To keep effi ciency high, the series resistance (DCR) should IN(MAX) is safe and will not damage the part. Figure 3 illustrates the switching wave- be less than 0.1Ω. Table 1 lists several vendors and types forms when the input voltage is increased to its absolute that are suitable. maximum rating of 40V. Of course, such a simple design guide will not always As the input voltage increases, the inductor current ramps result in the optimum inductor for your application. A up quicker, the number of skipped pulses increases and larger value provides a higher maximum load current and the output voltage ripple increases. For operation above reduces output voltage ripple at the expense of slower V transient response. If your load is lower than 1.2A, then IN(MAX) the only component requirement is that the com- ponents be adequately rated for operation at the intended you can decrease the value of the inductor and operate voltage levels. with higher ripple current. This allows you to use a physi- cally smaller inductor, or one with a lower DCR resulting in The part is robust enough to survive prolonged operation higher effi ciency. There are several graphs in the Typical under these conditions as long as the peak inductor current Performance Characteristics section of this data sheet that does not exceed 2.2A. Inductor current saturation may show the maximum load current as a function of input further limit performance in this operating regime. voltage and inductor value for several popular output volt- ages. Low inductance may result in discontinuous mode operation, which is okay, but further reduces maximum VSW 20V/DIV load current. For details of the maximum output current and discontinuous mode operation, see Linear Technology Application Note 44. IL 0.5A/DIV
Catch Diode
VOUT Depending on load current, a 1A to 2A Schottky diode is 200mV/DIV AC COUPLED recommended for the catch diode, D1. The diode must COUT = 10μF 2μs/DIV 3493 F03 have a reverse voltage rating equal to or greater than the VOUT = 3V VIN = 40V maximum input voltage. The ON Semiconductor MBRM140 ILOAD = 0.75A L = 10μH is a good choice; it is rated for 1A continuous forward
Figure 3
current and a maximum reverse voltage of 40V. 3493fb 9