Datasheet LT3493-3 (Analog Devices) - 10

LT3493-3
U U W U APPLICATIO S I FOR ATIO Table 1. Inductor Vendors Vendor URL Part Series Inductance Range (
µ
H) Size (mm)
Sumida www.sumida.com CDRH4D28 1.2 to 4.7 4.5 × 4.5 CDRH5D28 2.5 to 10 5.5 × 5.5 CDRH8D28 2.5 to 33 8.3 × 8.3 Toko www.toko.com A916CY 2 to 12 6.3 × 6.2 D585LC 1.1 to 39 8.1 × 8.0 Würth Elektronik www.we-online.com WE-TPC(M) 1 to 10 4.8 × 4.8 WE-PD2(M) 2.2 to 22 5.2 × 5.8 WE-PD(S) 1 to 27 7.3 × 7.3 larger value provides a higher maximum load current and necessary. This can be provided with a low performance reduces output voltage ripple at the expense of slower electrolytic capacitor. transient response. If your load is lower than 1.2A, then Step-down regulators draw current from the input supply you can decrease the value of the inductor and operate in pulses with very fast rise and fall times. The input with higher ripple current. This allows you to use a capacitor is required to reduce the resulting voltage ripple physically smaller inductor, or one with a lower DCR at the LT3493-3 and to force this very high frequency resulting in higher efficiency. There are several graphs in switching current into a tight local loop, minimizing EMI. the Typical Performance Characteristics section of this A 1µF capacitor is capable of this task, but only if it is data sheet that show the maximum load current as a placed close to the LT3493-3 and the catch diode; see the function of input voltage and inductor value for several PCB Layout section. A second precaution regarding the popular output voltages. Low inductance may result in ceramic input capacitor concerns the maximum input discontinuous mode operation, which is okay, but further voltage rating of the LT3493-3. A ceramic input capacitor reduces maximum load current. For details of the maxi- combined with trace or cable inductance forms a high mum output current and discontinuous mode operation, quality (underdamped) tank circuit. If the LT3493-3 circuit see Linear Technology Application Note 44. is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3493-
Catch Diode
3’s voltage rating. This situation is easily avoided; see the Depending on load current, a 1A to 2A Schottky diode is Hot Plugging Safely section. recommended for the catch diode, D1. The diode must have a reverse voltage rating equal to or greater than the
Output Capacitor
maximum input voltage. The ON Semiconductor MBRM140 The output capacitor has two essential functions. Along is a good choice; it is rated for 1A continuous forward with the inductor, it filters the square wave generated by current and a maximum reverse voltage of 40V. the LT3493-3 to produce the DC output. In this role it determines the output ripple so low impedance at the
Input Capacitor
switching frequency is important. The second function is Bypass the input of the LT3493-3 circuit with a 1µF or to store energy in order to satisfy transient loads and higher value ceramic capacitor of X7R or X5R type. Y5V stabilize the LT3493-3’s control loop. types have poor performance over temperature and ap- Ceramic capacitors have very low equivalent series resis- plied voltage and should not be used. A 1µF ceramic is tance (ESR) and provide the best ripple performance. A adequate to bypass the LT3493-3 and will easily handle good value is: the ripple current. However, if the input power source has high impedance, or there is significant inductance due to COUT = 65/VOUT long wires or cables, additional bulk capacitance may be 3493-3f 10