Datasheet

LTC3406AB
9
3406abfa
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 Δ V
OUT
is
determined by:
Δ≅Δ +
⎛
⎝
⎜
⎞
⎠
⎟
V I ESR
fC
OUT L
OUT
1
8
where f = operating frequency, C
OUT
= output capacitance
and
Δ I
L
= ripple current in the inductor. For a fi xed output
voltage, the output ripple is highest at maximum input
voltage since
Δ I
L
increases with input voltage.
Aluminum electrolytic and dry tantalum capacitors are both
available in surface mount confi gurations. 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 specifi c 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
LTC3406AB’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 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
In the adjustable version, the output voltage is set by a
resistive divider according to the following formula:
VV
R
R
OUT
=+
⎛
⎝
⎜
⎞
⎠
⎟
06 1
2
1
.
(2)
The external resistive divider is connected to the output,
allowing remote voltage sensing as shown in Figure 1.
APPLICATIONS INFORMATION
V
FB
GND
LTC3406AB
0.6V ≤ V
OUT
≤ 5.5V
R2
R1
3406AB F03
Figure 1. Setting the LTC3406AB Output Voltage
Effi ciency Considerations
The effi ciency 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 effi ciency and which change would produce
the most improvement. Effi ciency can be expressed as:
Effi ciency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percent-
age of input power.