Datasheet

12
LTC3718
3718fa
filter out the I
TH
variations at the switching frequency. The
resistor load on I
TH
reduces the DC gain of the error amp
and degrades load regulation, which can be avoided by
using the PNP emitter follower of Figure 3b.
Inductor L1 Selection
Given the desired input and output voltages, the inductor
value and operating frequency determine the ripple
current:
∆=






−






I
V
fL
V
V
L
OUT OUT
IN
1
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
OUT(MAX)
. The largest ripple current
occurs at the highest V
IN
. To guarantee that ripple current
does not exceed a specified maximum, the inductance
should be chosen according to:
L
V
fI
V
V
OUT
L MAX
OUT
IN MAX
=
∆






−






() ()
1
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 Mµ
®
cores. A variety of inductors
designed for high current, low voltage applications are
available from manufacturers such as Sumida, Pana-
sonic, Coiltronics, Coilcraft and Toko.
Schottky Diode D1, D2 Selection
The Schottky diodes, D1 and D2, shown in Figure 1
conduct during the dead time between the conduction of
the power MOSFET switches. It is intended to prevent the
body diodes of the top and bottom MOSFETs from turning
on and storing charge during the dead time, which can
cause a modest (about 1%) efficiency loss. The diodes can
be rated for about one half to one fifth of the full load current
since they are 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 diodes can be omitted if the efficiency loss is tolerable.
C
IN
and C
OUT
Selection
The input capacitance C
IN
is required to filter the square
wave current at the drain of the top MOSFET. Use a low
ESR capacitor sized to handle the maximum RMS current.
II
V
V
V
V
RMS OUT MAX
OUT
IN
IN
OUT
≅
()
–1
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT(MAX)
/2. This simple worst-case condition is
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
OUT
is primarily determined by the ESR
required to minimize voltage ripple and load step
APPLICATIO S I FOR ATIO
WUUU
C
VON
0.01µF
R
VON2
100k
R
VON1
30k
C
C
V
OUT
R
C
V
ON
I
TH
LTC3718
C
VON
0.01µF
R
VON2
10k
Q1
2N5087
R
VON1
3k
10k
C
C
3718 F03
V
OUT
INTV
CC
R
C
V
ON
I
TH
LTC3718
(3a) (3b)
Figure 3. Adjusting Frequency Shift with Load Current Changes
Kool Mµ is a registered trademark of Magnetics, Inc.