LM2674
LM2674 Series Buck Regulator Design Procedure (Fixed Output)
PROCEDURE (Fixed Output Voltage Version)
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
table listing the manufacturers’ phone numbers is located in
3. Catch Diode Selection (D1)
A.
In normal operation, the average current of the catch diode is
the 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.
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 (C
IN
)
4. Input Capacitor (C
IN
)
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.
(Continued)
EXAMPLE (Fixed Output Voltage Version)
3. Catch Diode Selection (D1)
A.
Refer to the table shown in
In this example, a 1A,
20V Schottky diode will provide the best performance. If the
circuit must withstand a continuous shorted output, a higher
current Schottky diode is recommended.
11
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