LM2576, LM2576HV
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SNVS107C –JUNE 1999–REVISED APRIL 2013
APPLICATION HINTS
INPUT CAPACITOR (CIN)
To maintain stability, the regulator input pin must be bypassed with at least a 100 μF electrolytic capacitor. The
capacitor's leads must be kept short, and located near the regulator.
If the operating temperature range includes temperatures below −25°C, the input capacitor value may need to be
larger. With most electrolytic capacitors, the capacitance value decreases and the ESR increases with lower
temperatures and age. Paralleling a ceramic or solid tantalum capacitor will increase the regulator stability at cold
temperatures. For maximum capacitor operating lifetime, the capacitor's RMS ripple current rating should be
greater than
(1)
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 23 through Figure 27 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 300 mA) 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, and so on, 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 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 selecting an inductor.
The inductor manufacturer's data sheets include current and energy limits to avoid inductor saturation.
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