ltc3405a Linear Technology Corporation, ltc3405a Datasheet - Page 9

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ltc3405a

Manufacturer Part Number
ltc3405a
Description
1.5mhz, 300ma Synchronous Step-down Regulator In Thinsot
Manufacturer
Linear Technology Corporation
Datasheet

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APPLICATIO S I FOR ATIO
The selection of C
series resistance (ESR). Typically, once the ESR require-
ment for C
generally far exceeds the I
output ripple ∆V
where f = operating frequency, C
and ∆I
voltage, the output ripple is highest at maximum input
voltage since ∆I
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
LTC3405A’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
V
OUT
L
= ripple current in the inductor. For a fixed output
≅ ∆
OUT
I ESR
L
has been met, the RMS current rating
L
OUT
increases with input voltage.
OUT
U
is determined by:
is driven by the required effective
+
8
U
fC
RIPPLE(P-P)
1
OUT
IN
. At best, this ringing can
OUT
W
= output capacitance
requirement. The
U
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
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 LTC3405A Output Voltage
=
0 8 1
.
V
LTC3405A
+
R
R
GND
V
2
1
FB
0.8V ≤ V
OUT
LTC3405A
R1
R2
≤ 5.5V
3405A F03
IN
, large enough
3405afa
9
(2)

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