LTC3411 Linear Technology, LTC3411 Datasheet - Page 8

• Manufacturer Part Number: LTC3411
• Description: 1.25A/ 4MHz/ Synchronous Step-Down DC/DC Converter
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC3411
A general LTC3411 application circuit is shown in
Figure 5. External component selection is driven by the
load requirement, and begins with the selection of the
inductor L1. Once L1 is chosen, C
selected.
Operating Frequency
Selection of the operating frequency is a tradeoff between
efficiency and component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequencies improves efficiency by
reducing internal gate charge losses but requires larger
inductance values and/or capacitance to maintain low
output ripple voltage.
The operating frequency, f
by an external resistor that is connected between the R
pin and ground. The value of the resistor sets the ramp
current that is used to charge and discharge an internal
timing capacitor within the oscillator and can be calculated
by using the following equation:
or can be selected using Figure 2.
The maximum usable operating frequency is limited by the
minimum on-time and the duty cycle. This can be calcu-
lated as:
The minimum frequency is limited by leakage and noise
coupling due to the large resistance of R
Inductor Selection
Although the inductor does not influence the operating
frequency, the inductor value has a direct effect on ripple
current. The inductor ripple current I
higher inductance and increases with higher V
Accepting larger values of I
inductances, but results in higher output voltage ripple,
greater core losses, and lower output current capability.
8
f
R
O(MAX)
T
I
L
9 78 10
V
f
.
OUT
O
•
6.67 • (V
L
•
• 1
11
U
f
O
OUT
V
V
OUT
IN
1 08
O
/ V
U
.
, of the LTC3411 is determined
IN(MAX)
L
allows the use of low
) (MHz)
W
IN
and C
L
T
decreases with
.
OUT
IN
U
or V
can be
OUT
T
:
A reasonable starting point for setting ripple current is
The largest ripple current I
input voltage. To guarantee that the ripple current stays
below a specified maximum, the inductor value should be
chosen according to the following equation:
The inductor value will also have an effect on Burst Mode
operation. The transition from low current operation begins
when the peak inductor current falls below a level set by the
burst clamp. Lower inductor values result in higher ripple
current which causes this to occur at lower load currents.
This causes a dip in efficiency in the upper range of low
current operation. In Burst Mode operation, lower induc-
tance values will cause the burst frequency to increase.
Inductor Core Selection
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. Tor-
oid or shielded pot cores in ferrite or permalloy materials
are small and don’t radiate much energy, but generally cost
more than powdered iron core inductors with similar
electrical characteristics. The choice of which style induc-
tor to use often depends more on the price vs size require-
ments and any radiated field/EMI requirements than on
what the LTC3411 requires to operate. Table 1 shows some
I
L
L
= 0.3 • I
f
V
O
OUT
•
4.5
4.0
3.5
3.0
2.5
2.0
1.0
1.5
0.5
LIM
0
I
0
L
T
, where I
A
•
= 25 C
Figure 2. Frequency vs R
1
V
500
LIM
IN MAX
V
OUT
(
R
is the peak switch current limit.
T
(k )
)
L
1000
occurs at the maximum
T
3411 F02
1500
sn3411 3411fs