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参数资料
| 型号： | LM2574DW-ADJR2 |
| 厂商： | MOTOROLA INC |
| 元件分类： | 稳压器 |
| 英文描述： | 1.8 A SWITCHING REGULATOR, 63 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封装： | PLASTIC, SO-16 |
| 文件页数： | 7/24页 |
| 文件大小： | 396K |
| 代理商： | LM2574DW-ADJR2 |
LM2574
15
MOTOROLA ANALOG IC DEVICE DATA
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
capacitor 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 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:
Irms u 1.2 x d x ILoad
where d is the duty cycle, for a continuous mode buck
regulator
d
+
ton
T
+
Vout
V
in
and d
+
ton
T
+
|Vout|
|Vout| ) Vin
for a buck–boost regulator.
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 related
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
0.03
), 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
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.
Catch Diode
Locate the Catch Diode Close to the LM2574
The LM2574 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 LM2574 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 rectifier
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 some quality, low noise design requirements.
Table 1 provides a list of suitable diodes for the LM2574
regulator. Standard 50/60 Hz 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 transitions 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 semiconductors 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.
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