LTC3602_15 Datasheet, PDF (9/20 Pages) Linear Technology – 2.5A, 10V, Monolithic Synchronous Step-Down Regulator

LTC3602
APPLICATIONS INFORMATION
The basic LTC3602 application circuit is shown on the front
page of this data sheet. External component selection is
determined by the maximum load current and begins with
the selection of the inductor value and operating frequency
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 and switching losses but
requires larger inductance values and/or capacitance to
maintain low output ripple voltage. The operating frequency
of the LTC3602 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
=
1.15 • 10¹¹
f(Hz)
–
10k
Although frequencies as high as 3MHz are possible, the
minimum on-time of the LTC3602 imposes a minimum
limit on the operating duty cycle. The minimum on-time
is typically 90ns. Therefore, the minimum duty cycle is
equal to 100 • 90ns • f(Hz).
Inductor Selection
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current ΔI_L increases with higher V_IN and decreases
with higher inductance.
ΔI_L
=
⎛
⎝⎜
V_OUT
fL
⎞
⎠⎟
•
⎛
⎝⎜
1–
V_OUT
V_IN
⎞
⎠⎟
Having a lower ripple current reduces 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 ΔI_L = 0.4(I_MAX), where I_MAX is the maximum output
current. 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
fΔI_{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. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of the more expensive ferrite cores. 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. Un-
fortunately, increased inductance requires more turns of
wire and therefore copper losses will increase.
Ferrite designs have very low core losses and are pre-
ferred at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
inductance 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!
Different core materials and shapes will change the
size/current and price/current relationship of an inductor.
Toroid or shielded pot cores in ferrite or permalloy ma-
terials are small and do not radiate energy but generally
cost more than powdered iron core inductors with similar
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