Datasheet LTC3406AB (Analog Devices) - 10

LTC3406AB
applicaTions inForMaTion
Typical y, once the ESR requirement for COUT has been met, instability. At worst, a sudden inrush of current through the RMS current rating generally far exceeds the IRIPPLE(P-P) the long wires can potentially cause a voltage spike at VIN, requirement. The output ripple DVOUT is determined by: large enough to damage the part.  1  DV When choosing the input and output ceramic capacitors, OUT @ DIL ESR + choose the X5R or X7R dielectric formulations. These  8fCOUT  dielectrics have the best temperature and voltage char- where f = operating frequency, COUT = output capacitance acteristics of all the ceramics for a given value and size. and DIL = ripple current in the inductor. For a fixed output voltage, the output ripple is highest at maximum input
Output Voltage Programming
voltage since DIL increases with input voltage. In the adjustable version, the output voltage is set by a Aluminum electrolytic and dry tantalum capacitors are both resistive divider according to the following formula: available in surface mount configurations. In the case of  R2 tantalum, it is critical that the capacitors are surge tested V  OUT = 0.6V 1+ for use in switching power supplies. An excellent choice is  R1 (2) the AVX TPS series of surface mount tantalum. These are specially constructed and tested for low ESR so they give The external resistive divider is connected to the output, the lowest ESR for a given volume. Other capacitor types allowing remote voltage sensing as shown in Figure 1. include Sanyo POSCAP, Kemet T510 and T495 series, and Sprague 593D and 595D series. Consult the manufacturer 0.6V ≤ VOUT ≤ 5.5V for other specific recommendations. R2
Using Ceramic Input and Output Capacitors
VFB LTC3406AB Higher values, lower cost ceramic capacitors are now R1 GND becoming available in smaller case sizes. Their high ripple 3406AB F03 current, high voltage rating and low ESR make them
Figure 1. Setting the LTC3406AB Output Voltage
ideal for switching regulator applications. Because the LTC3406AB’s control loop does not depend on the output
Efficiency Considerations
capacitor’s ESR for stable operation, ceramic capacitors The efficiency of a switching regulator is equal to the output can be used freely to achieve very low output ripple and power divided by the input power times 100%. It is often small circuit size. useful to analyze individual losses to determine what is However, care must be taken when ceramic capacitors limiting the efficiency and which change would produce are used at the input and the output. When a ceramic the most improvement. Efficiency can be expressed as: capacitor is used at the input and the power is supplied Efficiency = 100% – (L1 + L2 + L3 + ...) by a wall adapter through long wires, a load step at the output can induce ringing at the input, VIN. At best, this where L1, L2, etc. are the individual losses as a percent- ringing can couple to the output and be mistaken as loop age of input power. 3406abfb 10 For more information www.linear.com/LTC3406AB