LM2576
because of their fast switching speed and low forward
voltage drop.
different design load currents are selected. For light loads
(less than approximately 300 mA) it may be desirable to
operate the regulator in the discontinuous mode, because
the inductor value and size can be kept relatively low.
Consequently, the percentage of inductor peak–to–peak
current increases. This discontinuous mode of operation is
perfectly acceptable for this type of switching converter. Any
buck regulator will be forced to enter discontinuous mode if
the load current is light enough.
They provide the best efficiency especially in low output
voltage applications (5.0 V and lower). Another choice could
be Fast–Recovery, or Ultra–Fast Recovery diodes. It has to
be noted, that some types of these diodes with an abrupt
turnoff characteristic may cause instability or EMI troubles.
A fast–recovery diode with soft recovery characteristics
can better fulfill some quality, low noise design requirements.
Table 1 provides a list of suitable diodes for the LM2576
regulator. Standard 50/60 Hz rectifier diodes, such as the
1N4001 series or 1N5400 series are NOT suitable.
Figure 23. Continuous Mode Switching Current
Waveforms
Inductor
The magnetic components are the cornerstone of all
switching power supply designs. The style of the core and
the winding technique used in the magnetic component’s
design has a great influence on the reliability of the overall
power supply.
Using an improper or poorly designed inductor can cause
high voltage spikes generated by the rate of transitions in
current within the switching power supply, and the possibility
of core saturation can arise during an abnormal operational
mode. Voltage spikes can cause the semiconductors to enter
avalanche breakdown and the part can instantly fail if enough
energy is applied. It can also cause significant RFI (Radio
Frequency Interference) and EMI (Electro–Magnetic
Interference) problems.
2.0 A
Inductor
Current
Waveform
0 A
2.0 A
Power
Switch
Current
Waveform
0 A
HORIZONTAL TIME BASE: 5.0 µs/DIV
Continuous and Discontinuous Mode of Operation
The LM2576 step–down converter can operate in both the
continuous and the discontinuous modes of operation. The
regulator works in the continuous mode when loads are
relatively heavy, the current flows through the inductor
continuously and never falls to zero. Under light load
conditions, the circuit will be forced to the discontinuous
mode when inductor current falls to zero for certain period of
time (see Figure 23 and Figure 24). Each mode has
distinctively different operating characteristics, which can
affect the regulator performance and requirements. In many
cases the preferred mode of operation is the continuous
mode. It offers greater output power, lower peak currents in
the switch, inductor and diode, and can have a lower output
ripple voltage. On the other hand it does require larger
inductor values to keep the inductor current flowing
continuously, especially at low output load currents and/or
high input voltages.
Selecting the Right Inductor Style
Some important considerations when selecting a core
type are core material, cost, the output power of the power
supply, the physical volume the inductor must fit within, and
the amount of EMI (Electro–Magnetic Interference) shielding
that the core must provide. The inductor selection guide
covers different styles of inductors, such as pot core, E–core,
toroid and bobbin core, as well as different core materials
such as ferrites and powdered iron from different
manufacturers.
For high quality design regulators the toroid core seems to
be the best choice. Since the magnetic flux is contained
within the core, it generates less EMI, reducing noise
problems in sensitive circuits. The least expensive is the
bobbin core type, which consists of wire wound on a ferrite
rod core. This type of inductor generates more EMI due to the
fact that its core is open, and the magnetic flux is not
contained within the core.
To simplify the inductor selection process, an inductor
selection guide for the LM2576 regulator was added to this
data sheet (Figures 18 through 22). This guide assumes that
the regulator is operating in the continuous mode, and
selects an inductor that will allow a peak–to–peak inductor
ripple current to be a certain percentage of the maximum
design load current. This percentage is allowed to change as
When multiple switching regulators are located on the
same printed circuit board, open core magnetics can cause
interference between two or more of the regulator circuits,
especially at high currents due to mutual coupling. A toroid,
pot core or E–core (closed magnetic structure) should be
used in such applications.
16
MOTOROLA ANALOG IC DEVICE DATA
MOTOROLA [ MOTOROLA ]
