LTC3405A-1.375_15 Datasheet, PDF (8/12 Pages) Linear Technology – 1.375V, 1.5MHz, 300mA Synchronous Step-Down Regulators in ThinSOT

LTC3405A-1.375
APPLICATIO S I FOR ATIO
Table 1 shows some typical surface mount inductors that
work well in LTC3405A-1.375 applications.
Table 1. Representative Surface Mount Inductors
MAX DC
MANUFACTURER PART NUMBER VALUE CURRENT DCR HEIGHT
Taiyo Yuden
LB2016T2R2M
LB2012T2R2M
LB2016T3R3M
2.2µH 315mA 0.13Ω 1.6mm
2.2µH 240mA 0.23Ω 1.25mm
3.3µH 280mA 0.2Ω 1.6mm
Panasonic
ELT5KT4R7M 4.7µH 950mA 0.2Ω 1.2mm
Murata
LQH32CN2R2M33 4.7µH 450mA 0.2Ω 2mm
Taiyo Yuden
LB2016T4R7M 4.7µH 210mA 0.25Ω 1.6mm
Panasonic
ELT5KT6R8M 6.8µH 760mA 0.3Ω 1.2mm
Panasonic
ELT5KT100M 10µH 680mA 0.36Ω 1.2mm
Sumida
CMD4D116R8MC 6.8µH 620mA 0.23Ω 1.2mm
C_IN and C_OUT Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle V_OUT/V_IN. To prevent large
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum
RMS capacitor current is given by:
[ ^{( )}]^1/2
V_OUT V_IN − V_OUT
C_IN required I_RMS ≅ I_OMAX
V_IN
This formula has a maximum at V_IN = 2V_OUT, where
I_RMS = I_OUT/2. This simple worst-case condition is com-
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_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:
∆V_OUT
≅
⎛
∆I_{L ⎝⎜}ESR
+
1
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.
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_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.
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