LTC3406
LTC3406-1.5/LTC3406-1.8
APPLICATIO S I FOR ATIO
The selection of C
OUT
is driven by the required effective
series resistance (ESR).
Typically, once the ESR requirement for C
OUT
has been
met, the RMS current rating generally far exceeds the
I
RIPPLE(P-P)
requirement. The output ripple
∆V
OUT
is deter-
mined by:
⎛
1
⎞
∆
V
OUT
≅ ∆
I
L
⎜
ESR
+
⎟
⎝
8fC
OUT
⎠
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 values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. Because the
LTC3406’s control loop does not depend on the output
capacitor’s ESR for stable operation, ceramic capacitors
can be used
freely
to achieve very low output ripple and
small circuit size.
However, care must be taken when ceramic 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
U
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 (LTC3406 Only)
In the adjustable version, the output voltage is set by a
resistive divider according to the following formula:
⎛
R2
⎞
V
OUT
=
0.6 V
⎜
1
+
⎟
⎝
R1
⎠
(2)
W
U U
The external resistive divider is connected to the output,
allowing remote voltage sensing as shown in Figure 3.
0.6V
≤
V
OUT
≤
5.5V
R2
V
FB
LTC3406
GND
3406 F03
R1
Figure 3. Setting the LTC3406 Output Voltage
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.
3406fa
9
LINER [ LINEAR TECHNOLOGY ]
