LTC3547 UUWUAPPLICATIO S I FOR ATIOInput Capacitor (CIN) Selection where f = operating frequency, COUT = output capacitance and In continuous mode, the input current of the converter ΔIL = ripple current in the inductor. For a fi xed output voltage, the output ripple is highest at maximum input is a square wave with a duty cycle of approximately voltage since V ΔIL increases with input voltage. OUT/VIN. To prevent large voltage transients, a low equiv- alent series resistance (ESR) input capacitor sized for If tantalum capacitors are used, it is critical that the capaci- the maximum RMS current must be used. The max- tors are surge tested for use in switching power supplies. imum RMS capacitor current is given by: 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 V (V − V ) I ≈ I OUT IN OUT capacitor types include Sanyo POSCAP, Kemet T510 and RMS MAX V (2) IN T495 series, and Sprague 593D and 595D series. Consult the manufacturer for other specifi c recommendations. Where the maximum average output current IMAX equals the peak current minus half the peak-to-peak ripple cur- Using Ceramic Input and Output Capacitors rent, IMAX = ILIM – ΔIL/2. Higher values, lower cost ceramic capacitors are now This formula has a maximum at VIN = 2VOUT, where IRMS becoming available in smaller case sizes. Their high = IOUT/2. This simple worst-case is commonly used to ripple current, high voltage rating and low ESR make design because even signifi cant deviations do not offer them ideal for switching regulator applications. Because much relief. Note that capacitor manufacturer’s ripple cur- the LTC3547 control loop does not depend on the output rent ratings are often based on only 2000 hours lifetime. capacitor’s ESR for stable operation, ceramic capacitors This makes it advisable to further derate the capacitor, can be used freely to achieve very low output ripple and or choose a capacitor rated at a higher temperature than small circuit size. required. Several capacitors may also be paralleled to meet However, care must be taken when ceramic capacitors are the size or height requirements of the design. An addi- used at the input. When a ceramic capacitor is used at the tional 0.1µF to 1µF ceramic capacitor is also recommended input and the power is supplied by a wall adapter through on VIN for high frequency decoupling when not using an long wires, a load step at the output can induce ringing at all-ceramic capacitor solution. the input, VIN. At best, this ringing can couple to the output Output Capacitor (C and be mistaken as loop instability. At worst, a sudden OUT) Selection inrush of current through the long wires can potentially The selection of COUT is driven by the required effective cause a voltage spike at VIN, large enough to damage the series resistance (ESR). Typically, once the ESR require- part. For more information, see Application Note 88. ment for COUT has been met, the RMS current rating generally far exceeds the I When choosing the input and output ceramic capacitors, RIPPLE(P-P) requirement. The output ripple ΔV choose the X5R or X7R dielectric formulations. These OUT is determined by: dielectrics have the best temperature and voltage charac- ⎛ 1 ⎞ teristics of all the ceramics for a given value and size. ∆VOUT ≅ ∆I ESR L + ⎝⎜ 8fCOUT ⎠⎟ (3) 3547fa 9
