LTC3410-1.875 UUWUAPPLICATIO S I FOR ATIOCUsing Ceramic Input and Output CapacitorsIN and COUT Selection In continuous mode, the source current of the top MOSFET Higher values, lower cost ceramic capacitors are now is a square wave of duty cycle V becoming available in smaller case sizes. Their high ripple OUT/VIN. To prevent large voltage transients, a low ESR input capacitor sized for the current, high voltage rating and low ESR make them ideal maximum RMS current must be used. The maximum for switching regulator applications. Because the LTC3410- RMS capacitor current is given by: 1.875’s control loop does not depend on the output / capacitor’s ESR for stable operation, ceramic capacitors V [ (V −V )]1 2 OUT IN OUT can be used freely to achieve very low output ripple and C required I ≅I IN RMS OMAX VIN small circuit size. This formula has a maximum at VIN = 2VOUT, where However, care must be taken when ceramic capacitors are IRMS = IOUT/2. This simple worst-case condition is com- used at the input and the output. When a ceramic capacitor monly used for design because even significant deviations is used at the input and the power is supplied by a wall do not offer much relief. Note that the capacitor adapter through long wires, a load step at the output can manufacturer’s ripple current ratings are often based on induce ringing at the input, VIN. At best, this ringing can 2000 hours of life. This makes it advisable to further derate couple to the output and be mistaken as loop instability. At the capacitor, or choose a capacitor rated at a higher worst, a sudden inrush of current through the long wires temperature than required. Always consult the manufac- can potentially cause a voltage spike at VIN, large enough turer if there is any question. 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 require- choose the X5R or X7R dielectric formulations. These ment for COUT has been met, the RMS current rating dielectrics have the best temperature and voltage charac- generally far exceeds the IRIPPLE(P-P) requirement. The teristics of all the ceramics for a given value and size. output ripple ∆VOUT is determined by: The recommended capacitance value to use is 4.7µF for both the input and output capacitors. ∆ 1 VOUT ≅ ∆ ⎛ I ESR L + ⎞ ⎝⎜ fC 8 OUT ⎠⎟ where f = operating frequency, COUT = output capacitance and ∆IL = ripple current in the inductor. For a fixed output voltage, the output ripple is highest at maximum input voltage since ∆IL increases with input voltage. If tantalum capacitors are used, it is critical that the capacitors are surge tested for use in switching power supplies. An excellent choice is the AVX TPS series of surface mount tantalum. These are specially constructed and tested for low ESR so they give the lowest ESR for a given volume. Other capacitor types include Sanyo POSCAP, Kemet T510 and T495 series, and Sprague 593D and 595D series. Consult the manufacturer for other specific recommendations. 34101875f 9
