LTC3410-1.875 LINER [Linear Technology], LTC3410-1.875 Datasheet - Page 9

• Manufacturer Part Number: LTC3410-1.875
• Description: 2.25MHz, 300mA Synchronous Step-Down Regulator in SC70
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
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
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.
RMS
IN
C required I
∆
and C
IN
V
= I
OUT
L
OUT
= ripple current in the inductor. For a fixed output
OUT
≅ ∆
/2. This simple worst-case condition is com-
OUT
Selection
I ESR
L
has been met, the RMS current rating
L
OUT
⎛
⎜
⎝
RMS
increases with input voltage.
OUT
U
is determined by:
≅
is driven by the required effective
+
I
OMAX
8
U
fC
RIPPLE(P-P)
1
OUT
[
V
OUT
OUT
OUT
⎞
⎟
⎠
W
/V
(
= output capacitance
V
IN
IN
IN
requirement. The
. To prevent large
V
= 2V
−
IN
V
OUT
OUT
U
)
]
, where
1 2 /
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 LTC3410-
1.875’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
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.
The recommended capacitance value to use is 4.7µF for
both the input and output capacitors.
IN
LTC3410-1.875
. At best, this ringing can
IN
, large enough
34101875f
9