LTC3418 Datasheet, PDF (9/20 Pages) Linear Technology – 8A, 4MHz, Monolithic Synchronous Step-Down Regulator

LTC3418
APPLICATIO S I FOR ATIO
The basic LTC3418 application circuit is shown on the
front page of this data sheet. External component selec-
tion is determined by the maximum load current and
begins with the selection of the operating frequency and
inductor value followed by C_IN and C_OUT.
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 of the LTC3418 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_OSC
=
7.3
• 10¹⁰
f
[^Ω]
–
2.5kΩ
Although frequencies as high as 4MHz are possible, the
minimum on-time of the LTC3418 imposes a minimum
limit on the operating duty cycle. The minimum on-time is
typically 80ns. Therefore, the minimum duty cycle is equal
to:
100 • 80ns • f(Hz)
Inductor Selection
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current ∆I_L increases with higher V_IN or V_OUT and
decreases with higher inductance:
∆I_L
=
⎛
⎝⎜
V_OUT
fL
⎞
⎠⎟
⎛⎝⎜_1–
V_OUT
V_IN
⎞
⎠⎟
Having a lower ripple current reduces the core losses in
the inductor, the ESR losses in the output capacitors and
the output voltage ripple. Highest efficiency operation is
achieved at low frequency with small ripple current. This,
however, requires a large inductor.
A reasonable starting point for selecting the ripple current
is ∆I_L = 0.4(I_MAX). The largest ripple current occurs at the
highest V_IN. 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∆I_L(MAX)
⎞
⎠⎟
⎛
⎝⎜
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 inductance values will cause the burst frequency to
increase.
Inductor Core Selection
Once the value for L is known, the type of inductor must be
selected. Actual core loss is independent of core size for a
fixed inductor value, but it is very dependent on the
inductance selected. As the inductance increases, core
losses decrease. Unfortunately, increased inductance re-
quires more turns of wire and therefore copper losses will
increase.
Ferrite designs have very low core losses and are preferred
at high switching frequencies, so design goals can con-
centrate on copper loss and preventing saturation. Ferrite
core material saturates “hard,” which means that induc-
tance collapses abruptly when the peak design current is
exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
3418f
9