Datasheet LT1933 (Analog Devices) - 8

LT1933
APPLICATIONS INFORMATION
saturation current should be about 30% higher. For robust
Catch Diode
operation in fault conditions the saturation current should A 0.5A or 1A Schottky diode is recommended for the catch be ~1A. To keep effi ciency high, the series resistance (DCR) diode, D1. The diode must have a reverse voltage rating should be less than 0.2Ω. Table 1 lists several vendors equal to or greater than the maximum input voltage. The and types that are suitable. ON Semiconductor MBR0540 is a good choice; it is rated Of course, such a simple design guide will not always re- for 0.5A forward current and a maximum reverse voltage sult in the optimum inductor for your application. A larger of 40V. The MBRM140 provides better effi ciency, and will value provides a slightly higher maximum load current, handle extended overload conditions. and will reduce the output voltage ripple. If your load is lower than 500mA, then you can decrease the value of
Input Capacitor
the inductor and operate with higher ripple current. This Bypass the input of the LT1933 circuit with a 2.2μF or allows you to use a physically smaller inductor, or one higher value ceramic capacitor of X7R or X5R type. Y5V with a lower DCR resulting in higher effi ciency. There are types have poor performance over temperature and ap- several graphs in the Typical Performance Characteristics plied voltage, and should not be used. A 2.2μF ceramic section of this data sheet that show the maximum load is adequate to bypass the LT1933 and will easily handle current as a function of input voltage and inductor value the ripple current. However, if the input power source has for several popular output voltages. Low inductance may high impedance, or there is signifi cant inductance due to result in discontinuous mode operation, which is OK, but long wires or cables, additional bulk capacitance may be further reduces maximum load current. For details of necessary. This can be provided with a low performance maximum output current and discontinuous mode opera- electrolytic capacitor. tion, see Linear Technology Application Note 44. Finally, for duty cycles greater than 50% (V Step-down regulators draw current from the input sup- OUT/VIN > 0.5), there is a minimum inductance required to avoid subharmonic ply in pulses with very fast rise and fall times. The input oscillations. Choosing L greater than 3(V capacitor is required to reduce the resulting voltage OUT + VD) μH prevents subharmonic oscillations at all duty cycles. ripple at the LT1933 and to force this very high frequency
Table 1.Inductor Vendors Vendor URL Part Series Inductance Range (μH) Size (mm)
Coilcraft www.coilcraft.com D01608C 10 to 22 2.9 × 4.5 × 6.6 MSS5131 10 to 22 3.1 × 5.1 × 5.1 MSS6122 10 to 33 2.2 × 6.1 × 6.1 Sumida www.sumida.com CR43 10 to 22 3.5 × 4.3 × 4.8 CDRH4D28 10 to 33 3.0 × 5.0 × 5.0 CDRH5D28 22 to 47 3.0 × 5.7 × 5.7 Toko www.toko.com D52LC 10 to 22 2.0 × 5.0 × 5.0 D53LC 22 to 47 3.0 × 5.0 × 5.0 Würth Elektronik www.we-online.com WE-TPC MH 10 to 22 2.8 × 4.8 × 4.8 WE-PD4 S 10 to 22 2.9 × 4.5 × 6.6 WE-PD2 S 10 to 47 3.2 × 4.0 × 4.5 1933fe 8