Datasheet

LTC3412
9
3412fb
OPERATIO
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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 LTC3412, 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 current
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 4.8A, the top
power MOSFET will be held off and switching cycles will be
skipped until the inductor current falls to a safe level.
APPLICATIO S I FOR ATIO
WUU
U
The basic LTC3412 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
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 LTC3412 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
fHz
k
OSC
= Ω− Ω
323 10
10
11
.•
()
()
Although frequencies as high as 4MHz are possible, the
minimum on-time of the LTC3412 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
and decreases
with higher inductance.
Δ =
⎡
⎣
⎢
⎤
⎦
⎥
−
⎡
⎣
⎢
⎤
⎦
⎥
I
V
fL
V
V
L
OUT OUT
IN
1
Having a lower ripple current reduces the ESR losses in
the output capacitors and the output voltage ripple. High-
est 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
fI
V
V
OUT
LMAX
OUT
IN MAX
=
Δ
⎛
⎝
⎜
⎞
⎠
⎟
−
⎛
⎝
⎜
⎞
⎠
⎟
() ()
1
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.