LTC3404IMS8 Datasheet, PDF (9/16 Pages) Linear Technology – 1.4MHz High Efficiency Monolithic Synchronous Step-Down Regulator

U
OPERATIO
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant fre-
quency architectures by preventing subharmonic oscilla-
tions at high duty cycles. It is accomplished internally by
adding a compensating ramp to the inductor current
signal at duty cycles in excess of 40%. As a result, the
maximum inductor peak current is reduced for duty cycles
> 40%. This is shown in the decrease of the inductor peak
current as a function of duty cycle graph in Figure 2.
LTC3404
1100
V_IN = 3.3V
1000
900
800
700
600
0
20
40
60
80 100
DUTY CYCLE (%)
3404 F02
Figure 2. Maximum Inductor Peak Current vs Duty Cycle
APPLICATIO S I FOR ATIO
The basic LTC3404 application circuit is shown on the first
page. External component selection is driven by the load
requirement and begins with the selection of L followed by
C_IN and C_OUT.
Inductor Value Calculation
The inductor selection will depend on the operating fre-
quency of the LTC3404. The internal nominal frequency is
1.4MHz, but can be externally synchronized from 1MHz to
1.7MHz.
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. However, oper-
ating at a higher frequency generally results in lower
efficiency because of increased internal gate charge losses.
The inductor value has a direct effect on ripple current. The
ripple current ΔI_L decreases with higher inductance or
frequency and increases with higher V_IN or V_OUT.
ΔI_L
=
1
(^f)(^L)
V_OUT
⎛
⎝⎜
1−
V_OUT
V_IN
⎞
⎠⎟
(1)
Accepting larger values of ΔI_L allows the use of smaller
inductors, but results in higher output voltage ripple and
greater core losses. A reasonable starting point for setting
ripple current is ΔI_L = 0.4(I_MAX).
The inductor value also has an effect on Burst Mode
operation. The transition to low current operation begins
when the inductor current peaks fall to approximately
250mA. Lower inductor values (higher ΔI_L) will cause this
to occur at lower load currents, which can cause 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 more expensive ferrite, molypermalloy,
or Kool Mμ^® cores. Actual core loss is independent of core
size for a fixed inductor value, but it is very dependent on
inductance selected. As inductance increases, core losses
go down. Unfortunately, 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 cur-
rent is exceeded. This results in an abrupt increase in
3404fb
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