Datasheet LTC3410 (Analog Devices) - 9

LTC3410
U U W U APPLICATIO S I FOR ATIO
2000 hours of life. This makes it advisable to further derate can potentially cause a voltage spike at VIN, large enough the capacitor, or choose a capacitor rated at a higher to damage the part. temperature than required. Always consult the manufac- When choosing the input and output ceramic capacitors, turer if there is any question. choose the X5R or X7R dielectric formulations. These The selection of C dielectrics have the best temperature and voltage charac- OUT is driven by the required effective series resistance (ESR). Typically, once the ESR require- teristics of all the ceramics for a given value and size. ment for COUT has been met, the RMS current rating The recommended capacitance value to use is 4.7µF for generally far exceeds the IRIPPLE(P-P) requirement. The both input and output capacitor. For applications with output ripple ∆VOUT is determined by: VOUT greater than 2.5V, the recommended value for output capacitance should be increased. See Table 2. ∆ 1 VOUT ≅ ∆ ⎛ I ESR L + ⎞
Table 2. Capacitance Selection
⎝⎜ fC 8 OUT ⎠⎟
OUTPUT OUTPUT INPUT VOLTAGE RANGE CAPACITANCE CAPACITANCE
where f = operating frequency, COUT = output capacitance 0.8V ≤ V and OUT ≤ 2.5V 4.7µF 4.7µF ∆IL = ripple current in the inductor. For a fixed output V voltage, the output ripple is highest at maximum input OUT > 2.5V 10µH or 2x 4.7µF 4.7µF voltage since ∆IL increases with input voltage.
Output Voltage Programming (LTC3410 Only)
If tantalum capacitors are used, it is critical that the capacitors are surge tested for use in switching power The output voltage is set by a resistive divider according supplies. An excellent choice is the AVX TPS series of to the following formula: surface mount tantalum. These are specially constructed ⎛ R2⎞ and tested for low ESR so they give the lowest ESR for a V = 0. V 8 + OUT given volume. Other capacitor types include Sanyo ⎝⎜1 (2) R ⎠⎟ 1 POSCAP, Kemet T510 and T495 series, and Sprague 593D The external resistive divider is connected to the output, and 595D series. Consult the manufacturer for other allowing remote voltage sensing as shown in Figure 2. specific recommendations.
Efficiency Considerations Using Ceramic Input and Output Capacitors
The efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is Higher values, lower cost ceramic capacitors are now often useful to analyze individual losses to determine what becoming available in smaller case sizes. Their high ripple is limiting the efficiency and which change would produce current, high voltage rating and low ESR make them ideal the most improvement. Efficiency can be expressed as: for switching regulator applications. Because the LTC3410’s control loop does not depend on the output Efficiency = 100% – (L1 + L2 + L3 + ...) capacitor’s ESR for stable operation, ceramic capacitors can be used freely to achieve very low output ripple and 0.8V ≤ VOUT ≤ 5.5V small circuit size. R2 However, care must be taken when ceramic capacitors are VFB used at the input and the output. When a ceramic capacitor LTC3410 R1 is used at the input and the power is supplied by a wall GND adapter through long wires, a load step at the output can 3410 F02 induce ringing at the input, VIN. At best, this ringing can couple to the output and be mistaken as loop instability. At
Figure 2. Setting the LTC3410 Output Voltage
worst, a sudden inrush of current through the long wires 3410fb 9