LTC3412A Datasheet, PDF (10/20 Pages) Linear Technology – 3A, 4MHz, Monolithic Synchronous Step-Down Regulator

LTC3412A
APPLICATIO S I FOR ATIO
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant fre-
quency architectures by preventing subharmonic oscilla-
tions at duty cycles greater than 50%. It is accomplished
internally by adding a compensating ramp to the inductor
current signal at duty cycles in excess of 40%. Normally,
the maximum inductor peak current is reduced when
slope compensation is added. In the LTC3412A, however,
slope compensation recovery is implemented to keep the
maximum inductor peak current constant throughout the
range of duty cycles. This keeps the maximum output
current relatively constant regardless of duty cycle.
Short-Circuit Protection
When the output is shorted to ground, the inductor cur-
rent decays very slowly during a single switching cycle.
To prevent current runaway from occurring, a secondary
current limit is imposed on the inductor current. If the
inductor valley current increases larger than 4.4A, the top
power MOSFET will be held off and switching cycles will
be skipped until the inductor current is reduced.
The basic LTC3412A application circuit is shown in Fig-
ure 1. External component selection 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 LTC3412A is determined
by an external resistor that is connected between pin R_T
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
=
3.08
•
f
10¹¹
(Ω)
–
10kΩ
10
Although frequencies as high as 4MHz are possible, the
minimum on-time of the LTC3412A imposes a minimum
limit on the operating duty cycle. The minimum on-time
is typically 110ns; therefore, the minimum duty cycle is
equal to 100 • 110ns • 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 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 ΔI_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 to 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
requires more turns of wire and therefore copper losses
will increase.
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