LTC3406B Linear Technology, LTC3406B Datasheet - Page 9

• Manufacturer Part Number: LTC3406B
• Description: Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
The selection of C
series resistance (ESR).
Typically, once the ESR requirement for C
met, the RMS current rating generally far exceeds the
I
mined by:
where f = operating frequency, C
and I
voltage, the output ripple is highest at maximum input
voltage since I
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
RIPPLE(P-P)
V
OUT
L
= ripple current in the inductor. For a fixed output
requirement. The output ripple V
I ESR
L
L
increases with input voltage.
OUT
U
is driven by the required effective
8
U
fC
1
OUT
OUT
W
= output capacitance
OUT
OUT
U
has been
is deter-
induce ringing at the input, V
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
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:
The external resistive divider is connected to the output,
allowing remote voltage sensing as shown in Figure 3.
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:
where L1, L2, etc. are the individual losses as a percentage
of input power.
Efficiency = 100% – (L1 + L2 + L3 + ...)
V
OUT
Figure 3. Setting the LTC3406 Output Voltage
0 6
.
LTC3406-1.5/LTC3406-1.8
V
LTC3406
1
R
R
GND
2
V
1
FB
IN
. At best, this ringing can
0.6V V
OUT
R2
R1
IN
3406 F03
5.5V
LTC3406
, large enough
sn3406 3406fs
9
(2)