LTC3410
APPLICATIO S I FOR ATIO
2000 hours of life. This makes it advisable to further derate
the capacitor, or choose a capacitor rated at a higher
temperature than required. Always consult the manufac-
turer if there is any question.
The selection of C
OUT
is driven by the required effective
series resistance (ESR). Typically, once the ESR require-
ment 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 determined 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.
If tantalum capacitors are used, 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
LTC3410’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
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
U
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.
The recommended capacitance value to use is 4.7µF for
both input and output capacitor. For applications with
V
OUT
greater than 2.5V, the recommended value for output
capacitance should be increased. See Table 2.
Table 2. Capacitance Selection
OUTPUT
VOLTAGE RANGE
0.8V
≤
V
OUT
≤
2.5V
V
OUT
> 2.5V
OUTPUT
CAPACITANCE
4.7µF
10µH or 2x 4.7µF
INPUT
CAPACITANCE
4.7µF
4.7µF
W
U U
Output Voltage Programming (LTC3410 Only)
The output voltage is set by a resistive divider according
to the following formula:
⎛
R2
⎞
V
OUT
=
0.8V
⎜
1
+
⎟
( 2)
⎝
R1
⎠
The external resistive divider is connected to the output,
allowing remote voltage sensing as shown in Figure 2.
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 + ...)
0.8V
≤
V
OUT
≤
5.5V
R2
V
FB
LTC3410
GND
3410 F02
R1
Figure 2. Setting the LTC3410 Output Voltage
3410fb
9
LINER [ LINEAR TECHNOLOGY ]
