Datasheet LTC3406-1.2 (Analog Devices) - 8

LTC3406-1.2
U U W U APPLICATIO S I FOR ATIO
Typically, once the ESR requirement for COUT has been When choosing the input and output ceramic capacitors, met, the RMS current rating generally far exceeds the choose the X5R or X7R dielectric formulations. These IRIPPLE(P-P) requirement. The output ripple ∆VOUT is deter- dielectrics have the best temperature and voltage charac- mined by: teristics of all the ceramics for a given value and size. ∆ 1 V
Efficiency Considerations
OUT ≅ ∆ ⎛ I ESR L + ⎞ ⎝⎜ fC 8 OUT ⎠⎟ The efficiency of a switching regulator is equal to the where f = operating frequency, C output power divided by the input power times 100%. It is OUT = output capacitance and ∆I often useful to analyze individual losses to determine what L = ripple current in the inductor. For a fixed output voltage, the output ripple is highest at maximum input is limiting the efficiency and which change would produce voltage since ∆I the most improvement. Efficiency can be expressed as: L increases with input voltage. Aluminum electrolytic and dry tantalum capacitors are Efficiency = 100% – (L1 + L2 + L3 + ...) both available in surface mount configurations. In the case where L1, L2, etc. are the individual losses as a percentage of tantalum, it is critical that the capacitors are surge tested of input power. for use in switching power supplies. An excellent choice is Although all dissipative elements in the circuit produce the AVX TPS series of surface mount tantalum. These are losses, two main sources usually account for most of the specially constructed and tested for low ESR so they give losses in LTC3406-1.2 circuits: V the lowest ESR for a given volume. Other capacitor types IN quiescent current and I2R losses. The V include Sanyo POSCAP, Kemet T510 and T495 series, and IN quiescent current loss dominates the efficiency loss at very low load currents whereas the I2R Sprague 593D and 595D series. Consult the manufacturer loss dominates the efficiency loss at medium to high load for other specific recommendations. currents. In a typical efficiency plot, the efficiency curve at
Using Ceramic Input and Output Capacitors
very low load currents can be misleading since the actual power lost is of no consequence as illustrated in Figure 2. Higher values, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple 1 current, high voltage rating and low ESR make them ideal for switching regulator applications. Because the LTC3406- 0.1 1.2’s control loop does not depend on the output capacitor’s ESR for stable operation, ceramic capacitors can be used 0.01
freely
to achieve very low output ripple and small circuit size. 0.001 POWER LOSS (W) However, care must be taken when ceramic capacitors are 0.0001 used at the input and the output. When a ceramic capacitor VIN = 2.7V VIN = 3.6V is used at the input and the power is supplied by a wall VIN = 4.2V 0.00001 adapter through long wires, a load step at the output can 0.1 1 10 100 1000 LOAD CURRENT (mA) induce ringing at the input, VIN. At best, this ringing can 340612 F02 couple to the output and be mistaken as loop instability. At
Figure 2. Power Loss vs Load Current
worst, a sudden inrush of current through the long wires can potentially cause a voltage spike at VIN, large enough to damage the part. 340612fa 8