Datasheet

10
LTC3717
sn3717 3717fs
APPLICATIO S I FOR ATIO
WUUU
on-time t
ON
of the top MOSFET switch. The on-time is set
by the current into the I
ON
pin according to:
t
V
I
pF
ON
ION
=
(. )
()
07
10
Tying a resistor R
ON
from V
IN
to the I
ON
pin yields an on-
time inversely proportional to V
IN
. For a step-down con-
verter, this results in approximately constant frequency
operation as the input supply varies:
f
V
VR pF
H
OUT
ON
Z
=
(. ) ( )
[]
07 10
Because the voltage at the I
ON
pin is about 0.7V, the
current into this pin is not exactly inversely proportional to
V
IN
, especially in applications with lower input voltages.
A more exact equation taking in account the 0.7V drop on
the I
ON
pin is:
f
VV V
V R pF V
H
OUT IN
ON IN
Z
=
(–.)
(. ) ( )
[]
07
07 10
To correct for this error, an additional resistor R
ON2
connected from the I
ON
pin to the 5V INTV
CC
supply will
further stabilize the frequency.
R
V
V
R
ON ON2
5
07
=
.
Inductor 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 core 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
LMAX
OUT
IN MAX
=






−






∆
() ()
1
Once the value for L is known, the type of inductor must be
selected. High efficiency converters generally cannot af-
ford 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 manu-
facturers 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
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
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
Kool Mµ is a registered trademark of Magnetics, Inc.