LTC3564 LINER [Linear Technology], LTC3564 Datasheet - Page 10

• Manufacturer Part Number: LTC3564
• Description: 2.25MHz, 1.25A Synchronous Step-Down Regulator
• Manufacturer: LINER [Linear Technology]

LTC3564
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 requirement for C
met, the RMS current rating generally far exceeds the
I
mined by:
where f = operating frequency, C
and ΔI
voltage, the output ripple is highest at maximum input
voltage since ΔI
10
APPLICATIO S I FOR ATIO
RMS
RIPPLE(P-P)
IN
C
Δ
and C
IN
V
= I
OUT
L
required I
OUT
= ripple current in the inductor. For a fixed output
OUT
≅ Δ
requirement. The output ripple ΔV
/2. This simple worst-case condition is com-
Selection
I ESR
L
L
⎛
⎜
⎝
RMS
increases with input voltage.
OUT
U
≅
is driven by the required effective
+
I
OMAX
8
U
fC
1
OUT
[
OUT
V
OUT
OUT
⎞
⎟
⎠
/V
W
= output capacitance
(
IN
IN
V
. To prevent large
IN
= 2V
V
−
IN
OUT
V
OUT
OUT
OUT
U
has been
is deter-
, where
)
]
1 2 /
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 capacitors.
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
LTC3564’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.
IN
. At best, this ringing can
IN
, large enough
3564f