Datasheet LTC3406B-1.2 (Analog Devices) - 8

LTC3406B-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 ≅ ∆IL ⎜ ESR + ⎟ ⎝ 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 LTC3406B-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 Sprague 593D and 595D series. Consult the manufacturer I2R loss dominates the efficiency loss at medium to high for other specific recommendations. load currents. In a typical efficiency plot, the efficiency
Using Ceramic Input and Output Capacitors
curve at very low load currents can be misleading since the actual power lost is of no consequence as illustrated in Higher values, lower cost ceramic capacitors are now Figure 2. becoming available in smaller case sizes. Their high ripple current, high voltage rating and low ESR make them ideal 1 for switching regulator applications. Because the LTC3406B-1.2’s control loop does not depend on the 0.1 output capacitor’s ESR for stable operation, ceramic ca- pacitors can be used
freely
to achieve very low output 0.01 ripple and small circuit size. However, care must be taken when ceramic capacitors are POWER LOSS (W) 0.001 used at the input and the output. When a ceramic capacitor VIN = 2.7V is used at the input and the power is supplied by a wall VIN = 3.6V VIN = 4.2V adapter through long wires, a load step at the output can 0.0001 0.1 1 10 100 1000 induce ringing at the input, VIN. At best, this ringing can LOAD CURRENT (mA) couple to the output and be mistaken as loop instability. At 3406B12 F02 worst, a sudden inrush of current through the long wires
Figure 2. Power Loss vs Load Current
can potentially cause a voltage spike at VIN, large enough to damage the part. sn3406b12 3406b12fs 8