ltc3713 Linear Technology Corporation, ltc3713 Datasheet - Page 13

• Manufacturer Part Number: ltc3713
• Description: Low Input Voltage, High Power, No Rsense Synchronous Buck Dc/dc Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
specific application. A good starting point is to feed about
25% of the voltage change at the I
shown in Figure 4a. Place capacitance on the V
filter out the I
resistor load on I
and degrades load regulation, which can be avoided by
using the PNP emitter follower of Figure 4b.
Inductor L1 Selection
Given the desired input and output voltages, the inductor
value and operating frequency determine the ripple
current:
Lower ripple current reduces cores losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Highest efficiency operation is obtained at low
frequency with small ripple current. However, achieving
this requires a large inductor. There is a tradeoff between
component size, efficiency and operating frequency.
A reasonable starting point is to choose a ripple current
that is about 40% of I
occurs at the highest V
does not exceed a specified maximum, the inductance
should be chosen according to:
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron
cores, forcing the use of more expensive ferrite,
molypermalloy or Kool M
designed for high current, low voltage applications are
available from manufacturers such as Sumida, Panasonic,
Coiltronics, Coilcraft and Toko.
Schottky Diode D1 Selection
The Schottky diode D1 shown in Figure 1 conducts during
the dead time between the conduction of the power
MOSFET switches. It is intended to prevent the body diode
L
I
L
f I
V
V
L MAX
OUT
OUT
fL
TH
(
variations at the switching frequency. The
TH
)
1
U
reduces the DC gain of the error amp
1
OUT(MAX)
V
IN
V
OUT
. To guarantee that ripple current
IN
V
U
IN MAX
V
®
OUT
(
cores. A variety of inductors
. The largest ripple current
)
TH
W
pin to the V
U
ON
ON
pin as
pin to
Kool M is a registered trademark of Magnetics, Inc.
of the bottom MOSFET from turning on and storing charge
during the dead time, which can cause a modest (about
1%) efficiency loss. The diode can be rated for about one
half to one fifth of the full load current since it is on for only
a fraction of the duty cycle. In order for the diode to be
effective, the inductance between it and the bottom MOSFET
must be as small as possible, mandating that these
components be placed adjacently. The diode can be omit-
ted if the efficiency loss is tolerable.
C
The input capacitance C
wave current at the drain of the top MOSFET. Use a low
ESR capacitor sized to handle the maximum RMS current.
This formula has a maximum at V
I
commonly used for design because even significant
deviations do not offer much relief. Note that ripple
current ratings from capacitor manufacturers are often
based on only 2000 hours of life which makes it advisable
to derate the capacitor.
The selection of C
required to minimize voltage ripple and load step
transients. The output ripple V
bounded by:
Since I
highest at maximum input voltage. Typically, once the ESR
requirement is satisfied, the capacitance is adequate for
filtering and has the necessary RMS current rating.
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, special polymer, aluminum electrolytic and
ceramic capacitors are all available in surface mount
packages. Special polymer capacitors offer very low ESR
but have lower capacitance density than other types.
RMS
IN
I
RMS
and C
V
= I
OUT
OUT(MAX)
L
OUT
increases with input voltage, the output ripple is
I
OUT MAX
Selection
I ESR
L
(
/ 2. This simple worst-case condition is
OUT
)
V
is primarily determined by the ESR
V
OUT
IN
IN
8
fC
is required to filter the square
1
OUT
V
V
OUT
IN
– 1
OUT
IN
is approximately
= 2V
LTC3713
OUT
, where
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