TC2575 datasheet(15/25 Pages) TELCOM

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TC2575

TC2575-1 3/13/00

1.0A Step-Down Switching Regulator

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.

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.

Do Not Operate an Inductor Beyond its

Maximum Rated Current

Exceeding an inductor’s maximum current rating may

cause the inductor to overheat because of the copper wire

losses, or the core may saturate. Core saturation occurs

when the flux density is too high and consequently the cross

sectional area of the core can no longer support additional

lines of magnetic flux.

This causes the permeability of the core to drop, the

inductance value decreases rapidly and the inductor begins

to look mainly resistive. It has only the DC resistance of the

winding. This can cause the switch current to rise very

rapidly and force the TC2575 internal switch into cycle-by-

cycle current limit, thus reducing the DC output load current.

This can also result in overheating of the inductor and/or the

TC2575. Different inductor types have different saturation

characteristics, and this should be kept in mind when select-

ing an inductor.

GENERAL RECOMMENDATIONS

Output Voltage Ripple and Transients

Source of the Output Ripple

Since the TC2575 is a switch mode power supply

regulator, its output voltage, if left unfiltered, will contain a

sawtooth ripple voltage at the switching frequency. The

output ripple voltage value ranges from 0.5% to 3% of the

output voltage. It is caused mainly by the inductor sawtooth

ripple current multiplied by the ESR of the output capacitor.

Short Voltage Spikes and How to

Reduce Them

The regulator output voltage may also contain short

voltage spikes at the peaks of the sawtooth waveform (see

Figure 7). These voltage spikes are present because of the

fast switching action of the output switch, and the parasitic

inductance of the output filter capacitor. There are some

other important factors such as wiring inductance, stray

capacitance, as well as the scope probe used to evaluate

these transients, all these contribute to the amplitude of

these spikes. To minimize these voltage spikes, low induc-

tance capacitors should be used, and their lead lengths

must be kept short. The importance of quality printed circuit

board layout design should also be highlighted.

Minimizing the Output Ripple

In order to minimize the output ripple voltage it is

possible to enlarge the inductance value of the inductor L1

and/or to use a larger value output capacitor. There is also

another way to smooth the output by means of an additional

LC filter (20

µH, 100µF), that can be added to the output

(see Figure 16) to further reduce the amount of output ripple

and transients.

With such a filter it is possible to reduce the output ripple

voltage transients 10 times or more. Figure 7 shows the

difference between filtered and unfiltered output waveforms

of the regulator shown in Figure 16.

The upper waveform is from the normal unfiltered output

of the converter, while the lower waveform shows the output

ripple voltage filtered by an additional LC filter.

Heatsinking and Thermal Considerations

The Through-Hole-Package TO-220

The TC2575 is available in a 5-Pin TO-220 package.

There are many applications that require no heatsink to

keep the TC2575 junction temperature within the allowed

operating range. The TO-220 package can be used without

a heatsink for ambient temperatures up to approximately

°C (depending on the output voltage and load current).

Higher ambient temperatures require some heat sinking,

either to the printed circuit (PC) board or an external heatsink.

Figure 7. Output Ripple Voltage Waveforms

UNFILITERED

OUTPUT

VOLTAGE

VERTICAL

RESOLUTION:

20mV/DIV

FILITERED

OUTPUT

VOLTAGE

Voltage spikes caused by switching action of the output

switch and the parasitic inductance of the output capacitor

HORIZONTAL TIME BASE: 10

µsec/DIV

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