Datasheet LTC3406AB-2 (Analog Devices) - 10

LTC3406AB-2
applicaTions inForMaTion
Typical y, once the ESR requirement for COUT has been met, the long wires can potentially cause a voltage spike at VIN, the RMS current rating generally far exceeds the IRIPPLE(P-P) large enough to damage the part. requirement. The output ripple DVOUT is determined by: When choosing the input and output ceramic capacitors,  1  DV choose the X5R or X7R dielectric formulations. These OUT ≅ DIL ESR + dielectrics have the best temperature and voltage char-  8fCOUT  acteristics of all the ceramics for a given value and size. where f = operating frequency, COUT = output capacitance and DI
Output Voltage Programming
L = ripple current in the inductor. For a fixed output voltage, the output ripple is highest at maximum input In the adjustable version, the output voltage is set by a voltage since DIL increases with input voltage. resistive divider according to the following formula: Aluminum electrolytic and dry tantalum capacitors are both  R2 available in surface mount configurations. In the case of VOUT = 0.6V 1+  R1 tantalum, it is critical that the capacitors are surge tested (2) for use in switching power supplies. An excellent choice is The external resistive divider is connected to the output, the AVX TPS series of surface mount tantalum. These are allowing remote voltage sensing as shown in Figure 1. specially constructed and tested for low ESR so they give the lowest ESR for a given volume. Other capacitor types include Sanyo POSCAP, Kemet T510 and T495 series, and Sprague 593D and 595D series. Consult the manufacturer for other specific recommendations. 0.6V ≤ VOUT ≤ 5.5V R2
Using Ceramic Input and Output Capacitors
VFB Higher values, lower cost ceramic capacitors are now LTC3406AB-2 R1 becoming available in smaller case sizes. Their high GND ripple current, high voltage rating and low ESR make 3406AB2 F01 them ideal for switching regulator applications. Because
Figure 1. Setting the LTC3406AB-2 Output Voltage
the LTC3406AB-2’s control loop does not depend on the output capacitor’s ESR for stable operation, ceramic
Efficiency Considerations
capacitors can be used freely to achieve very low output The efficiency of a switching regulator is equal to the output ripple and small circuit size. power divided by the input power times 100%. It is often However, care must be taken when ceramic capacitors useful to analyze individual losses to determine what is are used at the input and the output. When a ceramic limiting the efficiency and which change would produce capacitor is used at the input and the power is supplied the most improvement. Efficiency can be expressed as: by a wall adapter through long wires, a load step at the Efficiency = 100% – (L1 + L2 + L3 + ...) output can induce ringing at the input, VIN. At best, this ringing can couple to the output and be mistaken as loop where L1, L2, etc. are the individual losses as a percent- instability. At worst, a sudden inrush of current through age of input power. 3406ab2fa 10 For more information www.linear.com/LTC3406AB-2