LTC3409_15 Datasheet, PDF (11/16 Pages) Linear Technology – 600mA Low VIN Buck Regulator in 3mm 3mm DFN

LTC3409
APPLICATIONS INFORMATION
where f = operating frequency, C_OUT = output capacitance
and ΔI_L = ripple current in the inductor. For a fixed output
voltage, the output ripple is highest at maximum input
voltage since ΔI_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 avail-
able 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+
R1
R2 
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 percent-
age 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.
3409fc
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