1.0A Step-Down Switching Regulator
TC2575
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.
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.
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 0.05Ω), 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
usedinparallelwithaluminiumelectrolytics.Thevalueofthe
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.
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
currentratings. 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:
Catch Diode
IRMS > 1.2 x d x ILOAD
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.
where d is the duty cycle, for a buck regulator
tON VOUT
d =
=
T
VIN
tON
T
IVOUT I
IVOUTI + VIN
and d =
=
for a buck-boost regulator.
Use a Schottky or a Soft Switching
Ultra–Fast Recovery Diode
Output Capacitor (COUT
)
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
voltageapplications(5.0Vandlower). Anotherchoicecould
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.
Forlowoutputripplevoltageandgoodstability,lowESR
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
thepeak–to–peakvalueoftheinductorripplecurrentarethe
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
voltagerating, thephysicalsizeandthetypeofconstruction.
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TC2575-1 3/13/00
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