LTC3717EGN Linear Technology, LTC3717EGN Datasheet - Page 10

• Manufacturer Part Number: LTC3717EGN
• Description: IC STEP-DWN CONTRLR SYNC 16-SSOP
• Manufacturer: Linear Technology

Specifications of LTC3717EGN

• Applications: Controller, DDR, QDR
• Voltage - Input: 4 ~ 36 V
• Number Of Outputs: 1
• Voltage - Output: 2 ~ 18 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
LTC3717
on-time t
by the current into the I
Tying a resistor R
time inversely proportional to V
verter, this results in approximately constant frequency
operation as the input supply varies:
Because the voltage at the I
current into this pin is not exactly inversely proportional to
V
A more exact equation taking in account the 0.7V drop on
the I
To correct for this error, an additional resistor R
connected from the I
further stabilize the frequency.
Inductor Selection
Given the desired input and output voltages, the inductor
value and operating frequency determine the ripple
current:
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
10
IN
R
t
f
f
, especially in applications with lower input voltages.
ON
ON
ON
I
L
( .
( .
2
0 7
pin is:
0 7
V
ON
( .
OUT IN
0 7
I
V R
V R
V
ION
0 7 .
of the top MOSFET switch. The on-time is set
OUT
f L
)
)
5
V
V
V
(
OUT
V
V
)
ON
ON
(
R
10
ON
(
(
ON
– .
10
10
1
U
pF
0 7
from V
ON
pF
pF V
)
V
V
ON
V
OUT
)
)
pin to the 5V INTV
IN
)
[ ]
U
IN
H
pin according to:
IN
Z
[ ]
H
ON
to the I
Z
IN
pin is about 0.7V, the
. For a step-down con-
W
ON
pin yields an on-
CC
supply will
U
ON2
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 af-
ford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
or Kool M
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
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
Kool M is a registered trademark of Magnetics, Inc.
RMS
IN
I
L
RMS
and C
= I
OUT(MAX)
f I
OUT
®
I
V
OUT MAX
cores. A variety of inductors designed for high
L MAX
OUT
(
Selection
(
/ 2. This simple worst-case condition is
)
)
V
OUT(MAX)
1
V
OUT
IN
IN
IN
. To guarantee that ripple current
V
IN MAX
is required to filter the square
V
OUT
(
V
V
OUT
. The largest ripple current
IN
)
– 1
IN
= 2V
OUT
sn3717 3717fs
, where