ltc3608ewkg Linear Technology Corporation, ltc3608ewkg Datasheet - Page 13

• Manufacturer Part Number: ltc3608ewkg
• Description: 18v, 8a Monolithic Synchronous Step-down Dc/dc Converter
• Manufacturer: Linear Technology Corporation

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
2.0
1.5
1.0
0.5
f L
OUT
OUT
OUT
CC
0
0
2N5087
R
R
R
30k
VON1
10k
VON1
VON2
100k
3k
1
Q1
0.25
DUTY CYCLE (V
V
R
10k
V
VON2
OUT
C
R
IN
C
C
(2a)
(2b)
0.50
OUT
C
0.01μF
C
0.01μF
C
VON
VON
C
R
DROPOUT
REGION
/V
C
IN
0.75
)
V
I
V
I
TH
TH
LTC3608
LTC3608
ON
ON
3608 F03
3608 F02
1.0
ripple. Highest effi ciency operation is obtained at low
frequency with small ripple current. However, achieving
this requires a large inductor. There is a tradeoff between
component size, effi ciency 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 specifi ed maximum, the inductance
should be chosen according to:
Once the value for L is known, the type of inductor must
be selected. High effi ciency 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 signifi cant 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
ESR required to minimize voltage ripple and load step
transients. The output ripple ΔV
bounded by:
RMS
IN
I
L =
RMS
and C
V
= I
OUT
≅I
OUT(MAX)
f I
OUT
OUT(MAX)
V
L(MAX)
OUT
Selection
I
L
/2. This simple worst-case condition is
ESR +
OUT
V
V
OUT(MAX)
OUT
1
IN
IN
IN
is primarily determined by the
8fC
. To guarantee that ripple current
V
is required to fi lter the square
IN(MAX)
V
1
OUT
V
OUT
V
OUT
. The largest ripple current
IN
– 1
OUT
IN
LTC3608
is approximately
= 2V
OUT
, where
13
3608fa