Datasheet LTC3406, LTC3406-1.5, LTC3406-1.8 (Analog Devices) - 9

LTC3406 LTC3406-1.5/LTC3406-1.8
U U W U APPLICATIO S I FOR ATIO
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 Typically, once the ESR requirement for C worst, a sudden inrush of current through the long wires OUT 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 deter- mined by: When choosing the input and output ceramic capacitors, choose the X5R or X7R dielectric formulations. These ⎛ 1 ⎞ ∆V dielectrics have the best temperature and voltage charac- OUT ≅ ∆IL ⎜ESR + ⎟ ⎝ fC 8 OUT ⎠ teristics of all the ceramics for a given value and size. where f = operating frequency, COUT = output capacitance
Output Voltage Programming (LTC3406 Only)
and ∆IL = 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 ∆I resistive divider according to the following formula: L increases with input voltage. Aluminum electrolytic and dry tantalum capacitors are ⎛ R2⎞ V 0 6 . V OUT = ⎜1+ ⎟ (2) both available in surface mount configurations. In the case ⎝ R ⎠ 1 of tantalum, it is critical that the capacitors are surge tested The external resistive divider is connected to the output, for use in switching power supplies. An excellent choice is allowing remote voltage sensing as shown in Figure 3. the AVX TPS series of surface mount tantalum. These are 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 specific recommendations. LTC3406 R1 GND
Using Ceramic Input and Output Capacitors
3406 F03 Higher values, lower cost ceramic capacitors are now
Figure 3. Setting the LTC3406 Output Voltage
becoming available in smaller case sizes. Their high ripple current, high voltage rating and low ESR make them ideal
Efficiency Considerations
for switching regulator applications. Because the The efficiency of a switching regulator is equal to the LTC3406’s control loop does not depend on the output output power divided by the input power times 100%. It is capacitor’s ESR for stable operation, ceramic capacitors often useful to analyze individual losses to determine what can be used
freely
to achieve very low output ripple and is limiting the efficiency and which change would produce small circuit size. the most improvement. Efficiency can be expressed as: However, care must be taken when ceramic capacitors are Efficiency = 100% – (L1 + L2 + L3 + ...) used at the input and the output. When a ceramic capacitor is used at the input and the power is supplied by a wall where L1, L2, etc. are the individual losses as a percentage adapter through long wires, a load step at the output can of input power. 3406fa 9