Datasheet LTC3560 (Analog Devices) - 9

LTC3560
APPLICATIONS INFORMATION
more than powdered iron core inductors with similar If tantalum capacitors are used, it is critical that the capaci- electrical characteristics. The choice of which style tors are surge tested for use in switching power supplies. inductor to use often depends more on the price versus An excellent choice is the AVX TPS series of surface mount size requirements and any radiated fi eld/EMI requirements tantalum. These are specially constructed and tested for low than on what the LTC3560 requires to operate. Table 1 ESR so they give the lowest ESR for a given volume. Other shows some typical surface mount inductors that work capacitor types include Sanyo POSCAP, Kemet T510 and well in LTC3560 applications. T495 series, and Sprague 593D and 595D series. Consult the manufacturer for other specifi c recommendations.
CIN and COUT Selection Using Ceramic Input and Output Capacitors
In continuous mode, the source current of the top MOSFET is a square wave of duty cycle VOUT/VIN. To prevent large Higher values, lower cost ceramic capacitors are now voltage transients, a low ESR input capacitor sized for the becoming available in smaller case sizes. Their high ripple maximum RMS current must be used. The maximum RMS current, high voltage rating and low ESR make them capacitor current is given by: ideal for switching regulator applications. Because the LTC3560’s control loop does not depend on the output ⎡ 1/ 2 V V ⎣ ( − V )⎤ OUT IN OUT ⎦ capacitor’s ESR for stable operation, ceramic capacitors C required I ≅I IN RMS OMAX V can be used freely to achieve very low output ripple and IN small circuit size. This formula has a maximum at VIN = 2VOUT, where However, care must be taken when ceramic capacitors IRMS = IOUT/2. This simple worst-case condition is are used at the input and the output. When a ceramic commonly used for design because even signifi cant capacitor is used at the input and the power is supplied deviations do not offer much relief. Note that the capacitor by a wall adapter through long wires, a load step at the manufacturer’s ripple current ratings are often based on output can induce ringing at the input, VIN. At best, this 2000 hours of life. This makes it advisable to further ringing can couple to the output and be mistaken as loop derate the capacitor, or choose a capacitor rated at a instability. At worst, a sudden inrush of current through higher temperature than required. Always consult the the long wires can potentially cause a voltage spike at VIN, manufacturer if there is any question. large enough to damage the part. The selection of COUT is driven by the required effective When choosing the input and output ceramic capacitors, series resistance (ESR). Typically, once the ESR requirement choose the X5R or X7R dielectric formulations. These for COUT has been met, the RMS current rating generally dielectrics have the best temperature and voltage charac- far exceeds the IRIPPLE(P-P) requirement. The output ripple teristics of all the ceramics for a given value and size. ΔVOUT is determined by:
Output Voltage Programming
V OUT IL ESR + 1 8fC The output voltage is set by a resistive divider according OUT to the following formula: where f = operating frequency, C OUT = output capacitance V = 0.6V 1+ R2 and ΔI OUT L = ripple current in the inductor. For a fi xed output R1 (2) voltage, the output ripple is highest at maximum input voltage since ΔIL increases with input voltage. The external resistive divider is connected to the output, allowing remote voltage sensing as shown in Figure 2. 3560fb 9