TC2575 datasheet(13/25 Pages) TELCOM

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TC2575

TC2575-1 3/13/00

1.0A Step-Down Switching Regulator

EXTERNAL COMPONENTS

Input Capacitor (CIN)

The Input Capacitor Should Have a Low ESR

For stable operation of the switch mode converter a low

ESR (Equivalent Series Resistance) aluminium or solid

tantalum bypass capacitor is needed between the input pin

and the ground pin, to prevent large voltage transients from

appearing at the input. It must be located near the regulator

and use short leads. With most electrolytic capacitors, the

capacitance value decreases and the ESR increases with

lower temperatures. For reliable operation in temperatures

below –25

°C larger values of the input capacitor may be

needed. Also paralleling a ceramic or solid tantalum capaci-

tor will increase the regulator stability at cold temperatures.

RMS Current Rating of CIN

The important parameter of the input capacitor is the

RMS current rating. Capacitors that are physically large and

have large surface area will typically have higher RMS

current ratings. For a given capacitor value, a higher voltage

electrolytic capacitor will be physically larger than a lower

voltage capacitor, and thus be able to dissipate more heat to

the surrounding air, and therefore will have a higher RMS

current rating. The consequences of operating an electro-

lytic capacitor beyond the RMS current rating is a shortened

operating life. In order to assure maximum capacitor oper-

ating lifetime, the capacitor’s RMS ripple current rating

should be:

IRMS > 1.2 x d x ILOAD

where d is the duty cycle, for a buck regulator

d =

tON

VOUT

VIN

and d =

tON

IVOUT I

for a buck-boost regulator.

IVOUTI + VIN

Output Capacitor (COUT)

For low output ripple voltage and good stability, low ESR

output capacitors are recommended. An output capacitor

has two main functions: it filters the output and provides

regulator loop stability. The ESR of the output capacitor and

the peak–to–peak value of the inductor ripple current are the

main factors contributing to the output ripple voltage value.

Standard aluminium electrolytics could be adequate for

some applications but for quality design, low ESR types are

recommended.

An aluminium electrolytic capacitor’s ESR value is re-

lated to many factors, such as the capacitance value, the

voltage rating, the physical size and the type of construction.

In most cases, the higher voltage electrolytic capacitors

have lower ESR value. Often capacitors with much higher

voltage ratings may be needed to provide low ESR values,

that are required for low output ripple voltage.

The Output Capacitor Requires an ESR Value that has

an Upper and Lower Limit

As mentioned above, a low ESR value is needed for

low output ripple voltage, typically 1% to 2% of the output

voltage. But if the selected capacitor’s ESR is extremely low

(below 0.05

Ω), there is a possibility of an unstable feedback

loop, resulting in oscillation at the output. This situation can

occur when a tantalum capacitor, that can have a very low

ESR, is used as the only output capacitor.

At Low Temperatures, Put in Parallel Aluminium

Electrolytic Capacitors with Tantalum Capacitors

Electrolytic capacitors are not recommended for tem-

peratures below –25

°C. The ESR rises dramatically at cold

temperatures and typically rises 3 times at –25

°C and as

much as 10 times at –40

°C. Solid tantalum capacitors have

much better ESR spec at cold temperatures and are recom-

mended for temperatures below –25

°C. They can be also

used in parallel with aluminium electrolytics. The value of the

tantalum capacitor should be about 10% or 20% of the total

capacitance. The output capacitor should have at least

50% higher RMS ripple current rating at 52kHz than the

peak–to–peak inductor ripple current.

Catch Diode

Locate the Catch Diode Close to the TC2575

The TC2575 is a step–down buck converter, it requires

a fast diode to provide a return path for the inductor current

when the switch turns off. This diode must be located close

to the TC2575 using short leads and short printed circuit

traces to avoid EMI problems.

Use a Schottky or a Soft Switching

Ultra–Fast Recovery Diode

Since the rectifier diodes are very significant source of

losses within switching power supplies, choosing the recti-

fier that best fits into the converter design is an important

process. Schottky diodes provide the best performance

because of their fast switching speed and low forward

voltage drop.

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 a quality, low noise design requirements.

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