TC2575 datasheet(14/25 Pages) TELCOM

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TC2575

1.0A Step-Down Switching Regulator

TC2575-1 3/13/00

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.

Figure 5. Continuous Mode Switching Current Waveforms

1.0

HORIZONTAL TIME BASE: 5.0

µsec/DIV

1.0

Figure 6. Discontinuous Mode Switching Current Waveforms

0.1

HORIZONTAL TIME BASE: 5.0

µsec/DIV

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