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参数资料
| 型号： | LT3724MPFE#TR |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 225 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封装： | 4.40 MM, PLASTIC, TSSOP-16 |
| 文件页数： | 4/26页 |
| 文件大小： | 309K |
| 代理商： | LT3724MPFE#TR |
LT3724
12
3724fd
The basic LT3724 step-down (buck) application, shown
in the Typical Application on the front page, converts a
larger positive input voltage to a lower positive or negative
outputvoltage.ThisApplicationInformationsectionassists
selection of external components for the requirements of
the power supply.
RSENSE Selection
The current sense resistor, RSENSE, monitors the inductor
current of the supply (See Typical Application on front
page). Its value is chosen based on the maximum required
output load current. The LT3724 current sense amplifier
has a maximum voltage threshold of, typically, 150mV.
Therefore, the peak inductor current is 150mV/RSENSE.
The maximum output load current, IOUT(MAX), is the peak
inductor current minus half the peak-to-peak ripple cur-
rent, IL.
Allowing adequate margin for ripple current and external
component tolerances, RSENSE can be calculated as fol-
lows:
RSENSE =
100mV
IOUT(MAX)
Typical values for RSENSE are in the range of 0.005Ω
to 0.05Ω.
Inductor Selection
The critical parameters for selection of an inductor are
minimum inductance value, volt-second product, satura-
tion current and/or RMS current.
The minimum inductance value is calculated as follows:
L
≥ VOUT
VIN(MAX) – VOUT
fSW VIN(MAX) IL
fSW is the switch frequency (200kHz).
The typical range of values for IL is (0.2 IOUT(MAX)) to
(0.5 IOUT(MAX)), where IOUT(MAX) is the maximum load
current of the supply. Using IL = 0.3 IOUT(MAX) yields a
good design compromise between inductor performance
versus inductor size and cost. Higher values of IL will
increase the peak currents, requiring more filtering on
the input and output of the supply. If IL is too high,
the slope compensation circuit is ineffective and current
mode instability may occur at duty cycles greater than
50%. Lower values of IL require larger and more costly
magnetics. A value of IL = 0.3 IOUT(MAX) produces a
±15% of IOUT(MAX) ripple current around the DC output
current of the supply.
Some magnetics vendors specify a volt-second product
in their datasheet. If they do not, consult the magnetics
vendortomakesurethespecificationisnotbeingexceeded
by your design. The volt-second product is calculated as
follows:
Volt-second (sec)
=
(VIN(MAX) – VOUT) VOUT
VIN(MAX) fSW
The magnetics vendors specify either the saturation cur-
rent, the RMS current or both. When selecting an inductor
based on inductor saturation current, use the peak cur-
rent through the inductor, IOUT(MAX) + IL/2. The inductor
saturation current specification is the current at which
the inductance, measured at zero current, decreases by
a specified amount, typically 30%.
When selecting an inductor based on RMS current rating,
use the average current through the inductor, IOUT(MAX).
The RMS current specification is the RMS current at which
the part has a specific temperature rise, typically 40°C,
above 25°C ambient.
After calculating the minimum inductance value, the volt-
secondproduct,thesaturationcurrentandtheRMScurrent
for your design, select an off-the-shelf inductor. A list of
magnetics vendors can be found at www.linear.com, or
contact the Linear Technology Application Department.
For more detailed information on selecting an inductor,
please see the “Inductor Selection” section of Linear
Technology Application Note 44.
Step-Down Converter: MOSFET Selection
The selection criteria of the external N-channel standard
level power MOSFET include on resistance(RDS(ON)), re-
verse transfer capacitance (CRSS), maximum drain source
voltage (VDSS), total gate charge (QG), and maximum
continuous drain current.
APPLICATIONS INFORMATION
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