Datasheet LTC3547B (Analog Devices) - 9

LTC3547B
U U W U APPLICATIO S I FOR ATIO Input Capacitor (CIN) Selection
where f = operating frequency, COUT = output capacitance and ΔI In continuous mode, the input current of the converter L = 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 ΔI V L 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 V I OUT (VIN − VOUT ) ESR so they give the lowest ESR for a given volume. Other RMS ≈ IMAX (2) V IN capacitor types include Sanyo POSCAP, Kemet T510 and T495 series, and Sprague 593D and 595D series. Consult Where the maximum average output current IMAX equals the manufacturer for other specifi c recommendations. the peak current minus half the peak-to-peak ripple cur- rent, IMAX = ILIM – ΔIL/2.
Using Ceramic Input and Output Capacitors
This formula has a maximum at VIN = 2VOUT, where IRMS Higher values, lower cost ceramic capacitors are now = IOUT/2. This simple worst-case is commonly used to becoming available in smaller case sizes. Their high ripple design because even signifi cant deviations do not offer current, high voltage rating and low ESR make them much relief. Note that capacitor manufacturer’s ripple cur- ideal for switching regulator applications. Because the rent ratings are often based on only 2000 hours lifetime. LTC3547B control loop does not depend on the output This makes it advisable to further derate the capacitor, capacitor’s ESR for stable operation, ceramic capacitors or choose a capacitor rated at a higher temperature than can be used freely to achieve very low output ripple and required. Several capacitors may also be paralleled to meet small circuit size. the size or height requirements of the design. An addi- However, care must be taken when ceramic capacitors are tional 0.1μF to 1μF ceramic capacitor is also recommended used at the input. When a ceramic capacitor is used at the on VIN for high frequency decoupling when not using an input and the power is supplied by a wall adapter through all-ceramic capacitor solution. long wires, a load step at the output can induce ringing at
Output Capacitor (C
the input, V
OUT) Selection
IN. At best, this ringing can couple to the output and be mistaken as loop instability. At worst, a sudden The selection of COUT is driven by the required effective inrush of current through the long wires can potentially series resistance (ESR). Typically, once the ESR require- cause a voltage spike at VIN, large enough to damage the ment for COUT has been met, the RMS current rating part. For more information, see Application Note 88. generally far exceeds the IRIPPLE(P-P) requirement. The output ripple ΔV When choosing the input and output ceramic capacitors, OUT is determined by: choose the X5R or X7R dielectric formulations. These V dielectrics have the best temperature and voltage charac- OUT IL ESR + 1 8fC (3) teristics of all the ceramics for a given value and size. OUT 3547bfb 9