ltc3717-1 Linear Technology Corporation, ltc3717-1 Datasheet - Page 11

• Manufacturer Part Number: ltc3717-1
• Description: Wide Operating Range, No Rsense Step-down Controller For Ddr/qdr Memory Termination
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
25% of the voltage change at the I
shown in Figure 3a. 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 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 af-
ford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
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
(
. The largest ripple current
Figure 3. Adjusting Frequency Shift with Load Current Changes
C
R
C
)
C
TH
(3a)
W
pin to the V
C
0.01 F
VON
LTC3717-1
V
I
TH
U
ON
ON
ON
pin as
pin to
or Kool M
current, low voltage applications are available from manu-
facturers such as Sumida, Panasonic, Coiltronics, Coil-
craft 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
Kool M is a registered trademark of Magnetics, Inc.
RMS
IN
INTV
I
RMS
V
and C
OUT
= I
CC
OUT(MAX)
2N5087
R
OUT
VON1
10k
3k
®
I
OUT MAX
Q1
cores. A variety of inductors designed for high
Selection
(
R
10k
VON2
/ 2. This simple worst-case condition is
(3b)
)
V
V
OUT
IN
IN
C
C
0.01 F
VON
C
R
is required to filter the square
C
V
V
OUT
LTC3717-1
V
I
IN
TH
ON
37171 F03
– 1
IN
LTC3717-1
= 2V
OUT
sn37171 37171fs
, where
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