Datasheet LTC3406A (Analog Devices) - 9

LTC3406A
APPLICATIONS INFORMATION
The selection of COUT is driven by the required effective induce ringing at the input, VIN. At best, this ringing can series resistance (ESR). couple to the output and be mistaken as loop instability. At worst, a sudden inrush of current through the long wires Typically, once the ESR requirement for COUT has been can potentially cause a voltage spike at V met, the RMS current rating generally far exceeds the IN, large enough to damage the part. IRIPPLE(P-P) requirement. The output ripple ∆VOUT is determined by: When choosing the input and output ceramic capacitors, choose the X5R or X7R dielectric formulations. These V OUT IL ESR + 1 dielectrics have the best temperature and voltage charac- 8fC OUT teristics of all the ceramics for a given value and size. where f = operating frequency, COUT = output capacitance
Output Voltage Programming
and ∆IL = ripple current in the inductor. For a fi xed output voltage, the output ripple is highest at maximum input In the adjustable version, the output voltage is set by a voltage since ∆I resistive divider according to the following formula: L increases with input voltage. Aluminum electrolytic and dry tantalum capacitors are both V = OUT 0.6V 1+ R2 available in surface mount confi gurations. In the case of R1 (2) tantalum, it is critical that the capacitors are surge tested 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 0.6V ≤ VOUT ≤ 5.5V the lowest ESR for a given volume. Other capacitor types include Sanyo POSCAP, Kemet T510 and T495 series, and R2 Sprague 593D and 595D series. Consult the manufacturer VFB for other specifi c recommendations. LTC3406A R1 GND
Using Ceramic Input and Output Capacitors
3406A F01
Figure 1. Setting the LTC3406A Output Voltage
Higher values, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple
Effi ciency Considerations
current, high voltage rating and low ESR make them ideal for switching regulator applications. Because the The effi ciency of a switching regulator is equal to the output LTC3406A’s control loop does not depend on the output power divided by the input power times 100%. It is often capacitor’s ESR for stable operation, ceramic capacitors useful to analyze individual losses to determine what is can be used freely to achieve very low output ripple and limiting the effi ciency and which change would produce small circuit size. the most improvement. Effi ciency can be expressed as: However, care must be taken when ceramic capacitors are Effi ciency = 100% – (L1 + L2 + L3 + ...) used at the input and the output. When a ceramic capacitor where L1, L2, etc. are the individual losses as a percent- is used at the input and the power is supplied by a wall age of input power. adapter through long wires, a load step at the output can 3406afa 9