Datasheet

LTC3610
12
3610ff
applications inForMation
Inductor Selection
Given the desired input and output voltages, the induc-
tor value and operating frequency determine the ripple
current:
ΔI
L
=
V
OUT
f L
⎛
⎝
⎜
⎞
⎠
⎟
1−
V
OUT
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
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 trade-off between
component size, efficiency and operating frequency.
A reasonable starting point is to choose a ripple current
that is about 40% of I
OUT(MAX)
. The largest ripple current
occurs at the highest V
IN
. To guarantee that ripple current
does not exceed a specified maximum, the inductance
should be chosen according to:
L =
V
OUT
f ΔI
L(MAX)
⎛
⎝
⎜
⎞
⎠
⎟
1−
V
OUT
V
IN(MAX)
⎛
⎝
⎜
⎞
⎠
⎟
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
IN
and C
OUT
Selection
The input capacitance C
IN
is required to filter the square
wave current at the drain of the top MOSFET. Use a low ESR
capacitor sized to handle the maximum RMS current.
I
RMS
≅I
OUT(MAX)
V
OUT
V
IN
V
IN
V
OUT
– 1
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT(MAX)
/2. This simple worst-case condition is
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
OUT
is primarily determined by the
ESR required to minimize voltage ripple and load step
transients. The output ripple ΔV
OUT
is approximately
bounded by:
ΔV
OUT
≤ ΔI
L
ESR +
1
8fC
OUT
⎛
⎝
⎜
⎞
⎠
⎟
Figure 3. Maximum Switching Frequency vs Duty Cycle
2.0
1.5
1.0
0.5
0
0 0.25 0.50 0.75
3610 F03
1.0
DROPOUT
REGION
DUTY CYCLE (V
OUT
/V
IN
)
SWITCHING FREQUENCY (MHz)
Figure 2. Correcting Frequency Shift with Load Current Changes
C
VON
0.01µF
R
VON2
100k
R
VON1
30k
C
C
V
OUT
R
C
(2a)
(2b)
V
ON
I
TH
LTC3610
C
VON
0.01µF
R
VON2
10k
Q1
2N5087
R
VON1
3k
10k
C
C
3610 F02
V
OUT
INTV
CC
R
C
V
ON
I
TH
LTC3610