Datasheet LTC3405A (Analog Devices) - 9

LTC3405A
U U W U APPLICATIO S I FOR ATIO
The selection of COUT is driven by the required effective couple to the output and be mistaken as loop instability. At series resistance (ESR). Typically, once the ESR require- worst, a sudden inrush of current through the long wires ment for COUT has been met, the RMS current rating can potentially cause a voltage spike at VIN, large enough generally far exceeds the IRIPPLE(P-P) requirement. The to damage the part. output ripple ∆VOUT is determined by: When choosing the input and output ceramic capacitors, choose the X5R or X7R dielectric formulations. These ⎛ 1 ⎞ ∆VOUT ≅ ∆IL⎜ESR + ⎟ dielectrics have the best temperature and voltage charac- ⎝ fC 8 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 fixed output voltage, the output ripple is highest at maximum input The output voltage is set by a resistive divider according voltage since ∆I to the following formula: L increases with input voltage. If tantalum capacitors are used, it is critical that the ⎛ R2⎞ V 0. V 8 1 capacitors are surge tested for use in switching power OUT = ⎜ + ⎟ (2) ⎝ R ⎠ 1 supplies. An excellent choice is the AVX TPS series of The external resistive divider is connected to the output, surface mount tantalum. These are specially constructed allowing remote voltage sensing as shown in Figure 3. 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 0.8V ≤ VOUT ≤ 5.5V and 595D series. Consult the manufacturer for other R2 specific recommendations. VFB LTC3405A R1
Using Ceramic Input and Output Capacitors
GND Higher values, lower cost ceramic capacitors are now 3405A F03 becoming available in smaller case sizes. Their high ripple
Figure 3. Setting the LTC3405A Output Voltage
current, high voltage rating and low ESR make them ideal for switching regulator applications. Because the
Efficiency Considerations
LTC3405A’s control loop does not depend on the output capacitor’s ESR for stable operation, ceramic capacitors The efficiency of a switching regulator is equal to the can be used
freely
to achieve very low output ripple and output power divided by the input power times 100%. It is small circuit size. often useful to analyze individual losses to determine what is limiting the efficiency and which change would produce However, care must be taken when ceramic capacitors are the most improvement. Efficiency can be expressed as: used at the input and the output. When a ceramic capacitor is used at the input and the power is supplied by a wall Efficiency = 100% – (L1 + L2 + L3 + ...) adapter through long wires, a load step at the output can where L1, L2, etc. are the individual losses as a percentage induce ringing at the input, VIN. At best, this ringing can of input power. 3405afa 9