LTC3409 Datasheet, PDF (11/16 Pages) Linear Technology – 600mA Low Vin Buck Regulator in 3mm x 3mm DFN

LTC3409
APPLICATIO S I FOR ATIO
where f = operating frequency, C_OUT = output capacitance
and ∆I_L = ripple current in the inductor. For a fixed output
voltage, the output ripple is highest at maximum input
voltage since ∆I_L increases with input voltage. Aluminum
electrolytic and dry tantalum capacitors are both available
in surface mount configurations. In the case of tantalum,
it is critical that the capacitors are surge tested for use in
switching power supplies. An excellent choice is the AVX
TPS series of surface mount tantalum. These are 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.
Using Ceramic Input and Output Capacitors
Higher value, lower cost ceramic capacitors are now
available in smaller case sizes. Their high ripple current,
high voltage rating and low ESR make them ideal for
switching regulator applications. Because the LTC3409’s
control loop does not depend on the output capacitor’s
ESR for stable operation, ceramic capacitors can be used
to achieve very low output ripple and small circuit size.
However, care must be taken when these capacitors are
used at the input and the output. When a ceramic capacitor
is used at the input and the power is supplied by a wall
adapter through long wires, a load step at the output can
induce ringing at the input, V_IN. At best, this ringing can
couple to the output and be mistaken as loop instability. At
worst, a sudden inrush of current through the long wires
can potentially cause a voltage spike at V_IN, large enough
to damage the part.
When choosing the input and output ceramic capacitors,
choose the X5R or X7R dielectric formulations. These
dielectrics have the best temperature and voltage charac-
teristics of all the ceramics for a given value and size.
Output Voltage Programming
The output voltage is set by a resistive divider according
to the following formula:
V_OUT = 0.613V^{⎛⎝⎜}1+ _RR21^{⎞⎠⎟}
The external resistive divider is connected to the output,
allowing remote voltage sensing as shown in Figure 1.
V_FB
LTC3409
GND
V_OUT
R1
R2
3409 F01
Figure 1
Efficiency Considerations
The efficiency of a switching regulator is equal to the
output power divided by the input power times 100%. It is
often useful to analyze individual losses to determine what
is limiting the efficiency and which change would produce
the most improvement. Efficiency can be expressed as:
Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percentage
of input power.
Although all dissipative elements in the circuit produce
losses, two main sources usually account for most of the
losses in LTC3409 circuits: V_IN quiescent current and I²R
losses. The V_IN quiescent current loss dominates the
efficiency loss at very low load currents whereas the I²R
loss dominates the efficiency loss at medium to high load
currents. In a typical efficiency plot, the efficiency curve at
very low load currents can be misleading since the actual
power lost is of no consequence as illustrated in Figure 2.
3409f
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