LTC3561A
9
3561afa
For more information www.linear.com/LTC3561A
APPLICATIONS INFORMATION
A general LTC3561A application circuit is shown in
Figure
4. External component selection is driven by the load
requirement, and begins with the selection of the inductor
L1. Once L1 is chosen, C
IN
and C
OUT
can be selected.
Operating Frequency
Selection of the operating frequency is a trade-off 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
O
, of the LTC3561A is determined
by an external resistor that is connected between the R
T
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:
R
T
≈ 5 × 10
7
(f
O
)
–1.6508
(kΩ)
where f
O
is in kHz, or can be selected using Figure 1.
The maximum usable operating frequency is limited by
the minimum on-time and the duty cycle. This can be
calculated as:
f
O(MAX)
≈ 6.67 •
OUT
V
IN(MAX)
(MHz
The minimum frequency is internally set at around
200kHz
Inductor Selection
The operating frequency, f
O
, has a direct effect on the
inductor value, which in turn influences the inductor ripple
current, ΔI
L
:
ΔI
L
=
V
OUT
f
O
• L
• 1−
V
OUT
V
IN
The inductor ripple current decreases with larger induc-
tance or frequency, and increases with higher V
IN
or V
OUT
.
Accepting larger values of ΔI
L
allows the use of lower
inductances, but results in higher output ripple voltage,
greater core loss and lower output capability.
A reasonable starting point for setting ripple current is
ΔI
L
= 0.4 • I
OUT(MAX)
, where I
OUT(MAX)
is 1A. The largest
ripple current ΔI
L
occurs at the maximum input voltage.
To guarantee that the ripple current stays below a specified
maximum, the inductor value should be chosen according
to the following equation:
L =
V
OUT
f
O
• ΔI
L
• 1−
V
OUT
V
IN(MAX)
Inductor Core Selection
Different core materials and shapes will change the
size/current and price/current relationship of an induc
-
tor. Toroid 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 inductor to use often depends more on the price vs
Figure 1. Frequency vs R
T
R
T
(kΩ)
0
0
500
1500
2000
2500
5000
4500
3561A F01
1000
400 800 1200 1600
3000
3500
4000
T
A
= 25°C
