LM2574 Series Buck Regulator Design Procedure (Continued)
PROCEDURE (Adjustable Output Voltage Versions)
3. Output Capacitor Selection (COUT
EXAMPLE (Adjustable Output Voltage Versions)
3. Output Capacitor Selection (COUT
)
)
A. The value of the output capacitor together with the inductor
defines the dominate pole-pair of the switching regulator loop.
For stable operation, the capacitor must satisfy the following
requirement:
However, for acceptable output ripple voltage select
COUT ≥ 100 µF
=
COUT 100 µF electrolytic capacitor
The above formula yields capacitor values between 5 µF and
1000 µF that will satisfy the loop requirements for stable op-
eration. But to achieve an acceptable output ripple voltage,
(approximately 1% of the output voltage) and transient re-
sponse, the output capacitor may need to be several times
larger than the above formula yields.
B. The capacitor’s voltage rating should be at last 1.5 times
greater than the output voltage. For a 24V regulator, a rating
of at least 35V is recommended.
Higher voltage electrolytic capacitors generally have lower
ESR numbers, and for this reasion it may be necessary to se-
lect a capacitor rate for a higher voltage than would normally
be needed.
4. Catch Diode Selection (D1)
4. Catch Diode Selection (D1)
A. The catch-diode current rating must be at least 1.5 times
greater than the maximum load current. Also, if the power
supply design must withstand a continuous output short, the
diode should have a current rating equal to the maximum cur-
rent limit of the LM2574. The most stressful condition for this
diode is an overload or shorted output condition. Suitable di-
odes are shown in the selection guide of Figure 9.
A. For this example, a 1A current rating is adequate.
B. Use a 50V MBR150 or 11DQ05 Schottky diode, or any of
the suggested fast-recovery diodes in Figure 9.
B. The reverse voltage rating of the diode should be at least
1.25 times the maximum input voltage.
5. Input Capacitor (CIN
)
5. Input Capacitor (CIN)
An aluminum or tantalum electrolytic bypass capacitor located
close to the regulator is needed for stable operation.
A 22 µF aluminum electrolytic capacitor located near the input
and ground pins provides sufficient bypassing. See (Figure 9).
To further simplify the buck regulator design procedure, Na-
tional Semiconductor is making available computer design
software to be used with the Simple Switcher line of switching
regulators. Switchers Made Simple (version 3.3) is available
on a (31⁄
tional Semiconductor sales office in your area.
2
") diskette for IBM compatible computers from a Na-
15
www.national.com
NSC [ National Semiconductor ]
