LTC3561
6
3561f
APPLICATIO S I FOR ATIO
WUU
U
A general LTC3561 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 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
O
, of the LTC3561 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:
Rf
TO
=
()
Ω
()
−
978 10
11
108
.•
.
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 calcu-
lated as:
f
O(MAX)
≈ 6.67 • (V
OUT
/ V
IN(MAX)
) (MHz)
The minimum frequency is limited by leakage and noise
coupling due to the large resistance of R
T
.
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
L
decreases with
higher inductance and increases with higher V
IN
or V
OUT
:
∆ = −
⎛
⎝
⎜
⎞
⎠
⎟
I
V
fL
V
V
L
OUT
O
OUT
IN
•
•1
Accepting larger values of ∆I
L
allows the use of low
inductances, but results in higher output voltage ripple,
greater core losses, and lower output current capability.
A reasonable starting point for setting ripple current is
∆I
L
= 0.4 × I
OUT(MAX)
, where I
OUT(MAX)
is 1A. The largest
ripple current ∆I
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
fI
V
V
OUT
OL
OUT
IN MAX
=
∆
−
⎛
⎝
⎜
⎞
⎠
⎟
•
•
()
1
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 do not 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 LTC3561 requires to operate. Table 1 shows some
typical surface mount inductors that work well in LTC3561
applications.
R
T
(kΩ)
0
0
FREQUENCY (MHz)
0.5
1.5
2.0
2.5
1000
4.5
T
A
= 25°C
3561 F02
1.0
500 1500
3.0
3.5
4.0
Figure 1. Frequency vs R
T
