ltc3405a-1.375 Linear Technology Corporation, ltc3405a-1.375 Datasheet - Page 8

• Manufacturer Part Number: ltc3405a-1.375
• Description: 1.375v, 1.5mhz, 300ma Synchronous Step-down Regulators In Thinsot
• Manufacturer: Linear Technology Corporation

LTC3405A-1.375
APPLICATIO S I FOR ATIO
Table 1 shows some typical surface mount inductors that
work well in LTC3405A-1.375 applications.
C
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle V
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum
RMS capacitor current is given by:
This formula has a maximum at V
I
monly used for design because even significant deviations
do not offer much relief. Note that the capacitor
manufacturer’s ripple current ratings are often based on
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
series resistance (ESR). Typically, once the ESR require-
ment for C
generally far exceeds the I
output ripple ∆V
8
Table 1. Representative Surface Mount Inductors
MANUFACTURER PART NUMBER
Taiyo Yuden
Panasonic
Murata
Taiyo Yuden
Panasonic
Panasonic
Sumida
RMS
IN
C
and C
IN
= I
required I
OUT
OUT
/2. This simple worst-case condition is com-
OUT
Selection
LB2016T2R2M
LB2012T2R2M
LB2016T3R3M
ELT5KT4R7M
LQH32CN2R2M33 4.7µH
LB2016T4R7M
ELT5KT6R8M
ELT5KT100M
CMD4D116R8MC 6.8µH
has been met, the RMS current rating
OUT
RMS
OUT
U
is determined by:
≅
is driven by the required effective
I
OMAX
U
RIPPLE(P-P)
VALUE CURRENT DCR HEIGHT
2.2µH
2.2µH
3.3µH
4.7µH
4.7µH
6.8µH
10µH
[
V
OUT
OUT
W
/V
MAX DC
315mA
240mA
280mA
950mA
450mA
210mA
760mA
680mA
620mA
(
IN
IN
V
IN
requirement. The
. To prevent large
= 2V
V
−
IN
V
0.13Ω 1.6mm
0.23Ω 1.25mm
0.25Ω 1.6mm
0.36Ω 1.2mm
0.23Ω 1.2mm
OUT
0.2Ω 1.6mm
0.2Ω 1.2mm
0.2Ω
0.3Ω 1.2mm
OUT
U
)
, where
]
1 2 /
2mm
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
LTC3405A-1.375’s control loop does not depend on the
output capacitor’s ESR for stable operation, ceramic ca-
pacitors can be used freely to achieve very low output
ripple and small circuit size.
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
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
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.
∆
V
OUT
L
= ripple current in the inductor. For a fixed output
≅ ∆
I ESR
L
L
⎛
⎜
⎝
increases with input voltage.
IN
+
. At best, this ringing can couple to
8
fC
1
OUT
OUT
⎞
⎟
⎠
= output capacitance
IN
, large enough to
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