LTC3611EWP LINER [Linear Technology], LTC3611EWP Datasheet - Page 13

• Manufacturer Part Number: LTC3611EWP
• Description: 10A, 32V Monolithic Synchronous Step-Down DC/DC Converter
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Inductor Selection
Given the desired input and output voltages, the induc-
tor 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
Figure 2. Correcting Frequency Shift with Load Current Changes
Figure 3. Maximum Switching Frequency vs Duty Cycle
I
L
=
INTV
V
V
V
f L
OUT
OUT
OUT
2.0
1.5
1.0
0.5
CC
0
0
2N5087
R
R
R
10k
VON1
30k
VON2
100k
VON1
3k
1
Q1
0.25
DUTY CYCLE (V
V
R
10k
V
VON2
OUT
C
IN
R
C
C
(2a)
(2b)
0.50
C
0.01μF
C
0.01μF
C
OUT
VON
VON
C
R
DROPOUT
C
REGION
/V
IN
0.75
)
V
I
V
I
TH
TH
ON
LTC3611
ON
LTC3611
3611 F03
3611 F02
1.0
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.
A variety of inductors designed for high current, low volt-
age applications are available from manufacturers such as
Sumida, Panasonic, Coiltronics, Coilcraft and Toko.
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 de-
viations 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 transients.
The output ripple ΔV
RMS
IN
L =
I
RMS
and C
V
= I
OUT
OUT(MAX)
f I
OUT
I
OUT(MAX)
V
L(MAX)
OUT
I
Selection
L
/2. This simple worst-case condition is
ESR +
OUT
V
OUT
V
OUT(MAX)
OUT
1
is primarily determined by the ESR
IN
IN
IN
8fC
. To guarantee that ripple current
is approximately bounded by:
V
IN(MAX)
is required to filter the square
V
1
OUT
V
OUT
V
OUT
. The largest ripple current
IN
– 1
IN
LTC3611
= 2V
OUT
, where
13
3611fb