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LM2576T-5.0 Datasheet(PDF) 16 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. LM2576T-5.0
Description  SIMPLE SWITCHER 3A Step-Down Voltage Regulator
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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 16 page
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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 per-
formance 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 val-
ues 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 cur-
rents, 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 300 mA) it may be desirable to
operate the regulator in the discontinuous mode, primarily
because of the lower inductor values required for the discon-
tinuous 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 com-
pletely contained within the core, it generates more electro-
magnetic 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
the winding). 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 select-
ing 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 saw-
tooth 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
switcher 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, physi-
cal 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 ca-
pacitor and the amplitude of the inductor ripple current
(
∆I
IND). See the section on inductor ripple current in Applica-
tion Hints.
The lower capacitor values (220 µF–1000 µF) will allow
typically 50 mV to 150 mV of output ripple voltage, while
larger-value capacitors will reduce the ripple to approxi-
mately 20 mV to 50 mV.
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 10 mV or 20 mV. 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. Be-
cause of their good low temperature characteristics, a tan-
talum can be used in parallel with aluminum electrolytics,
with the tantalum making up 10% or 20% of the total capaci-
tance.
The capacitor’s ripple current rating at 52 kHz should be at
least 50% higher than the peak-to-peak inductor ripple cur-
rent.
www.national.com
16




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