LM2576HVT-5.0 데이터 시트보기 (PDF) - Unspecified
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LM2576HVT-5.0 Datasheet PDF : 23 Pages
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Application Hints(Continued)
INDUCTOR SELECTION
All switching regulators have two basic modes of operation:
continuous and discontinuous. The difference between the two
types relates to the inductor current, whether it is flowing
continuously, or if it drops to zero for a period of time in the
normal switching cycle. Each mode has distinctively different
operating characteristics, which can affect the regulator
performance and requirements.
The LM2576 (or any of the SIMPLE SWITCHER family) can be
used for both continuous and discontinuous modes of
operation.
The inductor value selection guides in Figure 3 through Figure 7
were designed for buck regulator designs of the continuous
inductor current type. When using inductor values shown in the
inductor selection guide, the peak-to-peak inductor ripple
current will be approximately 20% to 30% of the maximum DC
current. With relatively heavy load currents, the circuit operates
in the continuous mode (inductor current always flowing), but
under light load conditions, the circuit will be forced to the
discontinuous mode (inductor current falls to zero for a period of
time). This discontinuous mode of operation is perfectly
acceptable. For light loads (less than approximately 300mA) it
may be desirable to operate the regulator in the discontinuous
mode, primarily because of the lower inductor values required
for the discontinuous mode.
The selection guide chooses inductor values suitable for
continuous mode operation, but if the inductor value chosen is
prohibitively high, the designer should investigate the possibility
of discontinuous operation. The computer design software
Switchers Made Simple will provide all component values for
discontinuous (as well as continuous) mode of operation.
Inductors are available in different styles such as pot core,
toriod, E-frame, bobbin core, etc., as well as different core
materials, such as ferrites and powdered iron. The least
expensive, the bobbin core type, consists of wire wrapped on a
ferrite rod core. This type of construction makes for an
inexpensive inductor, but since the magnetic flux is not
completely contained within the core, it generates more
electromagnetic interference (EMI). This EMI can cause
problems in sensitive circuits, or can give incorrect scope
readings because of induced voltages in the scope probe.
The inductors listed in the selection chart include ferrite pot core
construction for AIE, powdered iron toroid for Pulse Engineering,
and ferrite bobbin core for Renco.
An inductor should not be operated beyond its maximum rated
current because it may saturate. When an inductor begins to
saturate, the inductance decreases rapidly and the inductor
begins to look mainly resistive ( the DC resistance of winding).
LM2576/LM2576HV
This will cause the switch current to rise very rapidly. Different
inductor types have different saturation characteristics, and this
should be kept in mind when selecting an inductor.
The inductor manufacturer’s data sheets include current and
energy limits to avoid inductor saturation.
INDUCTOR RIPPLE CURRENT
When the switcher is operating in the continuous mode, the
inductor current waveform ranges from a triangular to a
sawtooth type of waveform (depending on the input voltage).
For a given input voltage and output voltage, the peak-to-peak
amplitude of this inductor current waveform remains constant.
As the load current rises or falls, the entire sawtooth current
waveform also rises or falls. The average DC value of this
waveform is equal to the DC load current (in the buck regulator
configuration).
If the load current drops to a low enough level, the bottom of the
sawtooth current waveform will reach zero, and the swithcher
will change to a discontinuous mode of operation. This is a
perfectly acceptable mode of operation. Any buck switching
regulator (no matter how large the inductor value is) will be
forced to run discontinuous if the load current is light enough.
OUTPUT CAPACITOR
An output capacitor is required to filter the output voltage and is
needed for loop stability. The capacitor should be located near
the LM2576 using short pc board traces. Standard aluminum
electrolytics are usually adequate, but low ESR types are
recommended for low output ripple voltage and good stability.
The ESR of a capacitor depends on many factors, some which
are: the value, the voltage rating, physical size and the type of
construction. In general, low value or low voltage (less than 12V)
electrolytic capacitors usually have higher ESR numbers.
The amount of output ripple voltage is primarily a function of the
ESR (Equivalent Series Resistance) of the output capacitor and
the amplitude of the inductor ripple current (△I IND ). See the
section on inductor ripple current in Application Hints.
The lower capacitor values (220 μF - 1000 μF) will allow
typically 50mV to 150mV of output ripple voltage, while
larger-value capacitors will reduce the ripple to approximately
20 mV to 50Mv.
Output Ripple Voltage =(△I IND ) (ESR of COUT)
To further reduce the output ripple voltage, several standard
electrolytic capacitors may be paralleled, or a higher-grade
capacitor may be used. Such capacitors are often called
“high-frequency,” ”low-inductance,” or “low-ESR.” These will
reduce the output ripple to 10mV or 20Mv. However, when
operating in the continuous mode, reducing the ESR below 0.03
Ω can cause instability in the regulator.
Tantalum capacitors can have a very low ESR, and should be
carefully evaluated if it is the only output capacitor. Because of
their good low temperature characteristics, a tantalum can be
used in parallel with aluminum electrolytics, with the tantalum
making up 10% or 20% of the total capacitance.
The capacitor’s ripple current rating at 52kHz should be at least
50% higher than the peak-to-peak inductor ripple current.
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