LM2674 Series Buck Regulator Design Procedure (Adjustable Output)
(Continued)
PROCEDURE (Adjustable Output Voltage Version)
B. Select an appropriate capacitor value and voltage rating,
using the capacitor code, from the output capacitor selection
EXAMPLE (Adjustable Output Voltage Version)
B. From the output capacitor selection table in Figure 17,
choose a capacitor value (and voltage rating) that intersects the
table in Figure 17. There are two solid tantalum (surface mount) capacitor code(s) selected in section A, C20.
capacitor manufacturers and four electrolytic (through hole)
The capacitance and voltage rating values corresponding to the
capacitor manufacturers to choose from. It is recommended that capacitor code C20 are the:
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 Figure 11.
Surface Mount:
33 µF/25V Sprague 594D Series.
33 µF/25V AVX TPS Series.
Through Hole:
33 µF/25V Sanyo OS-CON SC Series.
120 µF/35V Sanyo MV-GX Series.
120 µF/35V Nichicon PL Series.
120 µF/35V Panasonic HFQ Series.
Other manufacturers or other types of capacitors may also be
used, provided the capacitor specifications (especially the 100
kHz ESR) closely match the characteristics of the capacitors
listed in the output capacitor table. Refer to the capacitor
manufacturers’ data sheet for this information.
4. Catch Diode Selection (D1)
4. Catch Diode Selection (D1)
A. In normal operation, the average current of the catch diode is A. Refer to the table shown in Figure 12. Schottky diodes
the load current times the catch diode duty cycle, 1-D (D is the provide the best performance, and in this example a 500mA,
switch duty cycle, which is approximately VOUT/VIN). The largest 40V Schottky diode would be a good choice. If the circuit must
value of the catch diode average current occurs at the maximum withstand a continuous shorted output, a higher current (at least
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 greater than the maximum current limit of the
LM2674. The most stressful condition for this diode is a shorted
output condition.
1.2A) 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. The Schottky diode must be located close to the
LM2674 using short leads and short printed circuit traces.
19
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