Datasheet LTC3405A-1.5, LTC3405A-1.8 (Analog Devices) - 9

LTC3405A-1.5/LTC3405A-1.8
U U W U APPLICATIO S I FOR ATIO
Aluminum electrolytic and dry tantalum capacitors are where L1, L2, etc. are the individual losses as a percentage both available in surface mount configurations. In the case of input power. of tantalum, it is critical that the capacitors are surge tested Although all dissipative elements in the circuit produce for use in switching power supplies. An excellent choice is losses, two main sources usually account for most of the the AVX TPS series of surface mount tantalum. These are losses in LTC3405A series parts circuits: VIN quiescent specially constructed and tested for low ESR so they give current and I2R losses. The VIN quiescent current loss the lowest ESR for a given volume. Other capacitor types dominates the efficiency loss at very low load currents include Sanyo POSCAP, Kemet T510 and T495 series, and whereas the I2R loss dominates the efficiency loss at Sprague 593D and 595D series. Consult the manufacturer medium to high load currents. In a typical efficiency plot, for other specific recommendations. the efficiency curve at very low load currents can be misleading since the actual power lost is of no conse-
Using Ceramic Input and Output Capacitors
quence as illustrated in Figure 3. Higher values, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple 1 VIN = 3.6V current, high voltage rating and low ESR make them ideal for switching regulator applications. Because the LTC3405A 0.1 series’ control loop does not depend on the output capacitor’s ESR for stable operation, ceramic capacitors VOUT = 1.8V can be used freely to achieve very low output ripple and 0.01 small circuit size. POWER LOST (W) VOUT = 1.5V 0.001 Care must be taken when ceramic capacitors are used at the input and the output. When a ceramic capacitor is used at the input and the power is supplied by a wall adapter 0.0001 0.1 1 10 100 1000 through long wires, a load step at the output can induce LOAD CURRENT (mA) ringing at the input, VIN. At best, this ringing can couple to 3405A1518 F03 the output and be mistaken as loop instability. At worst, a
Figure 3. Power Lost vs Load Current
sudden inrush of current through the long wires can potentially cause a voltage spike at VIN, large enough to 1. The VIN quiescent current is due to two components: damage the part. the DC bias current as given in the electrical character- When choosing the input and output ceramic capacitors, istics and the internal main switch and synchronous choose the X5R or X7R dielectric formulations. These switch gate charge currents. The gate charge current dielectrics have the best temperature and voltage charac- results from switching the gate capacitance of the teristics of all the ceramics for a given value and size. internal power MOSFET switches. Each time the gate is switched from high to low to high again, a packet of
Efficiency Considerations
charge, dQ, moves from VIN to ground. The resulting dQ/dt is the current out of V The efficiency of a switching regulator is equal to the IN that is typically larger than the DC bias current. In continuous mode, I output power divided by the input power times 100%. It is GATECHG = f(Q often useful to analyze individual losses to determine what T + QB) where QT and QB are the gate charges of the internal top and bottom switches. Both the DC bias and is limiting the efficiency and which change would produce gate charge losses are proportional to V the most improvement. Efficiency can be expressed as: IN and thus their effects will be more pronounced at higher supply Efficiency = 100% – (L1 + L2 + L3 + ...) voltages. 3405a1518fa 9