LTC3714 Linear, LTC3714 Datasheet - Page 14

• Manufacturer Part Number: LTC3714
• Description: Wide Operating Range / Step-Down Controller with Internal Op Amp
• Manufacturer: Linear

APPLICATIO S I FOR ATIO
LTC3714
Inductor 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. A variety of inductors designed for high current,
low voltage applications are available from manufacturers
such as Sumida and Panasonic.
14
L
I
L
f I
V
V
V
L MAX
OUT
OUT
OUT
fL
(
R
R
)
VON1
30k
VON2
100k
1
U
1
OUT(MAX)
IN
V
V
OUT
C
. To guarantee that ripple current
R
IN
V
C
U
C
IN MAX
V
(4a)
OUT
(
. The largest ripple current
Figure 4. Correcting Frequency Shift with Load Current Changes
C
0.01 F
)
VON
W
V
I
TH
LTC3714
ON
3714 F04a
U
Schottky Diode D1 Selection
The Schottky diode D1 shown in Figure 1 conducts during
the dead time between the conduction of the power
MOSFET switches. It is intended to prevent the body diode
of the bottom MOSFET from turning on and storing charge
during the dead time, causing a modest (about 1%)
efficiency loss. The diode can be rated for about one half
to one fifth of the full load current since it is 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 diode can be omit-
ted 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.
RMS
IN
I
RMS
and C
= I
OUT(MAX)
OUT
INTV
I
OUT MAX
V
OUT
CC
Selection
(
2N5087
/ 2. This simple worst-case condition is
R
10k
VON1
3k
)
Q1
V
V
OUT
IN
R
10k
IN
VON2
(4b)
is required to filter the square
V
V
OUT
IN
C
0.01 F
C
VON
C
R
– 1
C
IN
V
I
TH
LTC3714
ON
= 2V
3714 F04b
OUT
, where
3714f