LM2674
SNVS007F –SEPTEMBER 1998–REVISED APRIL 2013
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PROCEDURE (Fixed Output Voltage Version)
EXAMPLE (Fixed Output Voltage Version)
Renco: ferrite stick core inductors; benefits are typically lowest cost
inductors and can withstand E•T and transient peak currents above
rated value. Be aware that these inductors have an external
magnetic field which may generate more EMI than other types of
inductors.
Pulse: powered iron toroid core inductors; these can also be low cost
and can withstand larger than normal E•T and transient peak
currents. Toroid inductors have low EMI.
Coilcraft: ferrite drum core inductors; these are the smallest physical
size inductors, available only as SMT components. Be aware that
these inductors also generate EMI—but less than stick inductors.
Complete specifications for these inductors are available from the
respective manufacturers. A listing of the manufacturers' phone
numbers is located in Table 2.
2. Output Capacitor Selection (COUT
)
2. Output Capacitor Selection (COUT)
A. Select an output capacitor from the output capacitor Table 3.
Using the output voltage and the inductance value found in the
inductor selection guide, step 1, locate the appropriate capacitor
value and voltage rating.
A. Use the 5.0V section in the output capacitor Table 3. Choose a
capacitor value and voltage rating from the line that contains the
inductance value of 47 μH. The capacitance and voltage rating
values corresponding to the 47 μH inductor are the:
The capacitor list contains through-hole electrolytic capacitors from
four different capacitor manufacturers and surface mount tantalum
capacitors from two different capacitor manufacturers. It is
recommended that both the manufacturers and the manufacturer's
series that are listed in the table be used. A listing of the
manufacturers' phone numbers is located in Table 4.
Surface Mount:
68 μF/10V Sprague 594D Series.
100 μF/10V AVX TPS Series.
Through Hole:
68 μF/10V Sanyo OS-CON SA Series.
150 μF/35V Sanyo MV-GX Series.
150 μF/35V Nichicon PL Series.
150 μF/35V Panasonic HFQ Series.
3. Catch Diode Selection (D1)
3. Catch Diode Selection (D1)
A. In normal operation, the average current of the catch diode is the A. Refer to Table 5. In this example, a 1A, 20V Schottky diode will
load current times the catch diode duty cycle, 1-D (D is the switch
duty cycle, which is approximately the output voltage divided by the
input voltage). The largest value of the catch diode average current
occurs at the maximum load current and maximum input voltage
(minimum D). For normal operation, the catch diode current rating
must be at least 1.3 times greater than its maximum average
current. However, if the power supply design must withstand a
continuous output short, the diode should have a current rating equal
to the maximum current limit of the LM2674. The most stressful
condition for this diode is a shorted output condition.
provide the best performance. If the circuit must withstand a
continuous shorted output, a higher current Schottky diode is
recommended.
B. The reverse voltage rating of the diode should be at least 1.25
times the maximum input voltage.
C. Because of their fast switching speed and low forward voltage
drop, Schottky diodes provide the best performance and efficiency.
This Schottky diode must be located close to the LM2674 using
short leads and short printed circuit traces.
4. Input Capacitor (CIN
)
4. Input Capacitor (CIN)
12
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