LTC3411 Datasheet, PDF (8/20 Pages) Linear Technology – 1.25A, 4MHz, Synchronous Step-Down DC/DC Converter

LTC3411
APPLICATIO S I FOR ATIO
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_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 LTC3411 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 = 9.78 •10¹¹₍f_{O )}^−1.08₍Ω₎
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:
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_L
=
V_OUT
f_O• L
• ^{}1−
V_OUT
V _IN


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.
8
A reasonable starting point for setting ripple current is
∆I_L␣ =␣ 0.3 • I_LIM, where I_LIM is the peak switch current limit.
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_OUT
f _O• ∆I_L

• _1−
V_OUT ^
V_{IN(MAX) }
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.
4.5 _{TA = 25°C}
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0
500
1000
R_T (kΩ)
1500
3411 F02
Figure 2. Frequency vs R_T
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
sn3411 3411fs