Datasheet LT3493 (Analog Devices) - 10

LT3493
APPLICATIONS INFORMATION Table 1. Inductor Values VENDOR URL PART SERIES INDUCTANCE RANGE (μH) 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
Input Capacitor
determines the output ripple so low impedance at the switching frequency is important. The second function Bypass the input of the LT3493 circuit with a 1μF or is to store energy in order to satisfy transient loads and higher value ceramic capacitor of X7R or X5R type. Y5V stabilize the LT3493’s control loop. types have poor performance over temperature and ap- plied voltage and should not be used. A 1μF ceramic is Ceramic capacitors have very low equivalent series re- adequate to bypass the LT3493 and will easily handle the sistance (ESR) and provide the best ripple performance. ripple current. However, if the input power source has A good value is: high impedance, or there is signifi cant inductance due to COUT = 65/VOUT long wires or cables, additional bulk capacitance may be necessary. This can be provided with a low performance where COUT is in μF. Use X5R or X7R types and keep in electrolytic capacitor. mind that a ceramic capacitor biased with VOUT will have less than its nominal capacitance. This choice will provide Step-down regulators draw current from the input sup- low output ripple and good transient response. Transient ply in pulses with very fast rise and fall times. The input performance can be improved with a high value capacitor, capacitor is required to reduce the resulting voltage but a phase lead capacitor across the feedback resistor ripple at the LT3493 and to force this very high frequency R1 may be required to get the full benefi t (see the Com- switching current into a tight local loop, minimizing EMI. pensation section). A 1μF capacitor is capable of this task, but only if it is placed close to the LT3493 and the catch diode; see the For small size, the output capacitor can be chosen ac- PCB Layout section. A second precaution regarding the cording to: ceramic input capacitor concerns the maximum input COUT = 25/VOUT voltage rating of the LT3493. A ceramic input capacitor combined with trace or cable inductance forms a high where COUT is in μF. However, using an output capacitor quality (underdamped) tank circuit. If the LT3493 circuit this small results in an increased loop crossover frequency is plugged into a live supply, the input voltage can ring to and increased sensitivity to noise. A 22pF capacitor con- twice its nominal value, possibly exceeding the LT3493’s nected between VOUT and the FB pin is required to fi lter voltage rating. This situation is easily avoided; see the Hot noise at the FB pin and ensure stability. Plugging Safely section. High performance electrolytic capacitors can be used for the output capacitor. Low ESR is important, so choose one
Output Capacitor
that is intended for use in switching regulators. The ESR The output capacitor has two essential functions. Along should be specifi ed by the supplier and should be 0.1Ω with the inductor, it fi lters the square wave generated or less. Such a capacitor will be larger than a ceramic by the LT3493 to produce the DC output. In this role it capacitor and will have a larger capacitance, because the 3493fb 10