LM2576
Table 3. Example of Several Inductor Manufacturers Phone/Fax Numbers
Phone
Fax
+ 1–619–674–8100
+ 1–619–674–8262
Pulse Engineering, Inc.
Pulse Engineering, Inc. Europe
Renco Electronics, Inc.
Tech 39
Phone
Fax
+ 353–9324–107
+ 353–9324–459
Phone
Fax
+ 1–516–645–5828
+ 1–516–586–5562
Phone
Fax
+ 33–1–4115–1681
+ 33–1–4709–5051
Phone
Fax
+ 1–612–475–1173
+ 1–612–475–1786
Schott Corporation
EXTERNAL COMPONENTS
An aluminium electrolytic capacitor’s ESR value is related
Input Capacitor (C
The Input Capacitor Should Have a Low ESR
)
in
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.
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
capacitor 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
0.05 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.
RMS Current Rating of C
in
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 consequence of operating an electrolytic
capacitor beyond the RMS current rating is a shortened
operating life. In order to assure maximum capacitor
operating lifetime, the capacitor’s RMS ripple current rating
should be:
At Low Temperatures, Put in Parallel Aluminium
Electrolytic Capacitors with Tantalum Capacitors
Electrolytic capacitors are not recommended for
temperatures 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
recommended 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 52 kHz than the
peak–to–peak inductor ripple current.
I
> 1.2 x d x I
rms
Load
where d is the duty cycle, for a buck regulator
V
t
Catch Diode
on
T
out
d
Locate the Catch Diode Close to the LM2576
The LM2576 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 LM2576 using short leads and short printed circuit traces
to avoid EMI problems.
V
in
|V
|
t
on
T
out
and d
for a buck boost regulator.
|V
|
V
out
in
Output Capacitor (C
out
)
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.
Use a Schottky or a Soft Switching
Ultra–Fast Recovery Diode
Since the rectifier diodes are very significant sources of
losses within switching power supplies, choosing the rectifier
that best fits into the converter design is an important
process. Schottky diodes provide the best performance
15
MOTOROLA ANALOG IC DEVICE DATA
MOTOROLA [ MOTOROLA ]
