LM2574 Series Buck Regulator Design Procedure (Continued)
VR
1 Amp Diodes
Schottky
1N5817
SR102
Fast Recovery
20V
MBR120P
1N5818
SR103
30V
40V
11DQ03
MBR130P
10JQ030
1N5819
SR104
The
following
diodes
are all
rated to
100V
11DQ04
11JQ04
MBR140P
MBR150
SR105
50V
60V
90V
11DF1
10JF1
11DQ05
11JQ05
MBR160
SR106
MUR110
HER102
11DQ06
11JQ06
11DQ09
FIGURE 9. Diode Selection Guide
INDUCTOR SELECTION
Application Hints
All switching regulators have two basic modes of operation:
continuous and discontinuous. The difference between the
two types relates to the inductor current, whether it is flowing
continuously, or if it drops to zero for a period of time in the
normal switching cycle. Each mode has distinctively different
operating characteristics, which can affect the regulator per-
formance and requirements.
INPUT CAPACITOR (CIN
)
To maintain stability, the regulator input pin must be by-
passed with at least a 22 µF electrolytic capacitor. The ca-
pacitor’s leads must be kept short, and located near the
regulator.
If the operating temperature range includes temperatures
below −25˚C, the input capacitor value may need to be
larger. With most electrolytic capacitors, the capacitance
value decreases and the ESR increases with lower tempera-
tures and age. Paralleling a ceramic or solid tantalum ca-
pacitor will increase the regulator stability at cold tempera-
tures. For maximum capacitor operating lifetime, the
capacitor’s RMS ripple current rating should be greater than
The LM2574 (or any of the Simple Switcher family) can be
used for both continuous and discontinuous modes of opera-
tion.
In many cases the preferred mode of operation is in the con-
tinuous mode. It offers better load regulation, lower peak
switch, inductor and diode currents, and can have lower out-
put ripple voltage. But it does require relatively large inductor
values to keep the inductor current flowing continuously, es-
pecially at low output load currents.
To simplify the inductor selection process, an inductor selec-
tion guide (nomograph) was designed (see Figure 4 through
Figure 8). This guide assumes continuous mode operation,
and selects an inductor that will allow a peak-to-peak induc-
tor ripple current (∆IIND) to be a certain percentage of the
maximum design load current. In the LM2574 SIMPLE
SWITCHER, the peak-to-peak inductor ripple current per-
centage (of load current) is allowed to change as different
design load currents are selected. By allowing the percent-
age of inductor ripple current to increase for lower current
applications, the inductor size and value can be kept rela-
tively low.
www.national.com
16
NSC [ National Semiconductor ]
