Datasheet LTC3564 (Analog Devices) - 10

LTC3564
U U W U APPLICATIO S I FOR ATIO CIN and COUT Selection
Aluminum electrolytic and dry tantalum capacitors are both available in surface mount configurations. In the case In continuous mode, the source current of the top MOSFET is a square wave of duty cycle V of tantalum, it is critical that the capacitors are surge tested OUT/VIN. To prevent large for use in switching power supplies. An excellent choice is voltage transients, a low ESR input capacitor sized for the the AVX TPS series of surface mount tantalum capacitors. maximum RMS current must be used. The maximum RMS capacitor current is given by: These are specially constructed and tested for low ESR so they give the lowest ESR for a given volume. Other / V [ (V −V )]1 2 capacitor types include Sanyo POSCAP, Kemet T510 and OUT IN OUT C required I I IN RMS ≅ OMAX T495 series, and Sprague 593D and 595D series. Consult VIN the manufacturer for other specific recommendations. This formula has a maximum at VIN = 2VOUT, where
Using Ceramic Input and Output Capacitors
IRMS = IOUT/2. This simple worst-case condition is com- monly used for design because even significant deviations Higher values, lower cost ceramic capacitors are now do not offer much relief. Note that the capacitor becoming available in smaller case sizes. Their high ripple manufacturer’s ripple current ratings are often based on current, high voltage rating and low ESR make them ideal 2000 hours of life. This makes it advisable to further derate for switching regulator applications. Because the the capacitor, or choose a capacitor rated at a higher LTC3564’s control loop does not depend on the output temperature than required. Always consult the manufac- capacitor’s ESR for stable operation, ceramic capacitors turer if there is any question. can be used freely to achieve very low output ripple and small circuit size. The selection of COUT is driven by the required effective series resistance (ESR). However, care must be taken when ceramic capacitors are used at the input and the output. When a ceramic capacitor Typically, once the ESR requirement for COUT has been is used at the input and the power is supplied by a wall met, the RMS current rating generally far exceeds the adapter through long wires, a load step at the output can IRIPPLE(P-P) requirement. The output ripple ΔVOUT is deter- induce ringing at the input, V mined by: IN. At best, this ringing can couple to the output and be mistaken as loop instability. At worst, a sudden inrush of current through the long wires Δ 1 VOUT ≅ Δ ⎛ I ESR L + ⎞ ⎝⎜ fC 8 can potentially cause a voltage spike at VIN, large enough OUT ⎠ ⎟ to damage the part. where f = operating frequency, COUT = output capacitance When choosing the input and output ceramic capacitors, and ΔIL = ripple current in the inductor. For a fixed output choose the X5R or X7R dielectric formulations. These voltage, the output ripple is highest at maximum input dielectrics have the best temperature and voltage charac- voltage since ΔIL increases with input voltage. teristics of all the ceramics for a given value and size. 3564f 10