LTC3718EG#PBF Linear Technology, LTC3718EG#PBF Datasheet - Page 12

• Manufacturer Part Number: LTC3718EG#PBF
• Description: IC DC/DC CONTRLR DDR/QDR 24-SSOP
• Manufacturer: Linear Technology

Specifications of LTC3718EG#PBF

• Applications: Controller, DDR, QDR
• Voltage - Input: 1.5 ~ 36 V
• Number Of Outputs: 1
• Voltage - Output: 0.75 ~ 18 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-SSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIO S I FOR ATIO
LTC3718
filter out the I
resistor load on I
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:
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
12
Kool M is a registered trademark of Magnetics, Inc.
L
I
L
f I
V
V
L MAX
OUT
OUT
fL
TH
(
variations at the switching frequency. The
TH
)
V
1
U
OUT
reduces the DC gain of the error amp
1
OUT(MAX)
IN
V
V
OUT
. To guarantee that ripple current
R
R
IN
V
VON1
30k
VON2
100k
U
IN MAX
V
®
OUT
(
cores. A variety of inductors
. The largest ripple current
Figure 3. Adjusting Frequency Shift with Load Current Changes
C
R
C
)
C
(3a)
W
C
0.01 F
VON
V
I
U
TH
LTC3718
ON
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
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
RMS
IN
INTV
I
RMS
V
and C
OUT
= I
CC
OUT(MAX)
2N5087
R
OUT
VON1
10k
3k
I
OUT MAX
Q1
Selection
(
R
10k
VON2
/ 2. This simple worst-case condition is
OUT
(3b)
)
V
is primarily determined by the ESR
V
OUT
IN
IN
C
C
0.01 F
VON
C
R
is required to filter the square
C
V
V
OUT
V
I
IN
TH
LTC3718
ON
3718 F03
– 1
IN
= 2V
OUT
, where
3718fa