TC2575 Datasheet, PDF (14/25 Pages) TelCom Semiconductor, Inc – 1.0A Step-Down Switching Regulator

1.0A Step-Down Switching Regulator
TC2575
Table 4 provides a list of suitable diodes for the TC2575
regulator. Standard 50/60Hz rectifier diodes, such as the
1N4001 series or 1N5400 series are NOT suitable.
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.
Continuous and Discontinuous Mode of Operation.
The TC2575 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 discontinuous mode
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
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/orhigh input
voltages.
To simplify the inductor selection process, an inductor
selection guide for the TC2575 regulator was added to this
data sheet (Figures 32 through 36). 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 different design load currents are selected. For light loads
(less than approximately 200mA) 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.
Selecting the Right Inductor Style
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. 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 prob-
lems in sensitive circuits. The least expensive is the bobbin
core type, which consists of wire wound on a ferrite rod core.
1.0
0.1
0
0
1.0
0
HORIZONTAL TIME BASE: 5.0µsec/DIV
0.1
0
HORIZONTAL TIME BASE: 5.0µsec/DIV
Figure 5. Continuous Mode Switching Current Waveforms
TC2575-1 3/13/00
Figure 6. Discontinuous Mode Switching Current Waveforms
14