0.5A Step-Down Switching Regulator
TC2574
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/orhigh input
voltages.
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 TC2574 regulator. Standard 50/60Hz rectifier diodes,
such as the 1N4001 series or 1N5400 series are NOT
suitable.
To simplify the inductor selection process, an inductor
selection guide for the TC2574 regulator was added to this
datasheet(Figures39through41).Thisguideassumesthat
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
asdifferentdesignloadcurrentsareselected.Forlightloads
(less than approximately 0.2A) it may be desirable to oper-
ate the regulator in the discontinuous mode, because the
inductor value and size can be kept relatively low. Conse-
quently, the percentage of inductor peak-to-peak current
increases. Thisdiscontinuousmodeofoperationisperfectly
acceptable for this type of switching converter. Any buck
regulator will be forced to enter discontinuous mode if the
load current is light enough.
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 have 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 transi-
tions 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 semicon-
ductors 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.
Selecting the Right Inductor Style
Continuous and Discontinuous Mode of Operation.
The TC2574 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 condi-
tions, the circuit will be forced to the discontinuousmode
when inductor current falls to zero for certain period of time
(see Figure 5 and Figure 6). Each mode has distinctively
different operating characteristics, which can affect the
regulatorperformanceandrequirements.Inmanycasesthe
preferredmodeofoperationisthecontinuousmode.Itoffers
Some important considerations when selecting a
coretype are core material, cost, the output power of the
powersupply, the physical volume the inductor must fit
within, and the amount of EMI (Electro–Magnetic Interfer-
ence) shielding that the core must provide. There are many
different styles of inductors available, 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
Continuous Mode Switching Current
Waveforms
Continuous Mode Switching Current
Waveforms
Inductor
Current
Waveform
0.1A
0A
0.5A
Inductor
Current
Waveform
0A
0.5A
Power
Switch
Current
Power
Switch
Current
0.1A
0A
Waveform
Waveform
0A
HORIZONTAL TIME BASE: 5.0µsec/DIV
Figure 6. Continuous Mode Switching Current Waveforms
HORIZONTAL TIME BASE: 5.0µsec/DIV
Figure 5. Continuous Mode Switching Current Waveforms
13
TC2574-1 1/6/00
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