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參數(shù)資料
| 型號： | LT3412AEUF#TRPBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 3 A SWITCHING REGULATOR, 4 kHz SWITCHING FREQ-MAX, PQCC16 |
| 封裝： | 4 X 4 MM, LEAD FREE, PLASTIC, QFN-16 |
| 文件頁數(shù)： | 2/20頁 |
| 文件大?。?/td> | 267K |
| 代理商： | LT3412AEUF#TRPBF |
LTC3412A
10
3412afb
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 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 4.4A, 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 Figure 1. External component selection is determined
by the maximum load current and begins with the selec-
tion of the operating frequency and inductor value fol-
lowed by CIN and COUT.
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 RT
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
f
k
OSC
=Ω
()
Ω
308 10
10
11
.
–
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
ΔIL increases with higher VIN or VOUT and
decreases with higher inductance.
Δ=
I
V
fL
V
L
OUT
IN
1–
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
ΔIL = 0.4(IMAX). The largest ripple current occurs at the
highest VIN. 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
OUT
LMAX
OUT
IN MAX
=
Δ
()
–
1
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 in-
crease.
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.
APPLICATIO S I FOR ATIO
WU
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