Datasheet

LM2674
www.ti.com SNVS007F –SEPTEMBER 1998–REVISED APRIL 2013
PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)
A low ESR aluminum or tantalum bypass capacitor is needed The important parameters for the input capacitor are the input
between the input pin and ground to prevent large voltage transients voltage rating and the RMS current rating. With a maximum input
from appearing at the input. This capacitor should be located close voltage of 12V, an aluminum electrolytic capacitor with a voltage
to the IC using short leads. In addition, the RMS current rating of the rating greater than 15V (1.25 × V
IN
) would be needed. The next
input capacitor should be selected to be at least ½ the DC load higher capacitor voltage rating is 16V.
current. The capacitor manufacturer data sheet must be checked to The RMS current rating requirement for the input capacitor in a buck
assure that this current rating is not exceeded. The curves shown in regulator is approximately ½ the DC load current. In this example,
Figure 28 show typical RMS current ratings for several different with a 500mA load, a capacitor with an RMS current rating of at least
aluminum electrolytic capacitor values. A parallel connection of two 250 mA is needed. The curves shown in Figure 28 can be used to
or more capacitors may be required to increase the total minimum select an appropriate input capacitor. From the curves, locate the
RMS current rating to suit the application requirements. 16V line and note which capacitor values have RMS current ratings
For an aluminum electrolytic capacitor, the voltage rating should be greater than 250 mA.
at least 1.25 times the maximum input voltage. Caution must be For a through hole design, a 100 μF/16V electrolytic capacitor
exercised if solid tantalum capacitors are used. The tantalum (Panasonic HFQ series, Nichicon PL, Sanyo MV-GX series or
capacitor voltage rating should be twice the maximum input voltage. equivalent) would be adequate. Other types or other manufacturers'
Tables 7 and 8 show the recommended application voltage for AVX capacitors can be used provided the RMS ripple current ratings are
TPS and Sprague 594D tantalum capacitors. It is also recommended adequate. Additionally, for a complete surface mount design,
that they be surge current tested by the manufacturer. The TPS electrolytic capacitors such as the Sanyo CV-C or CV-BS and the
series available from AVX, and the 593D and 594D series from Nichicon WF or UR and the NIC Components NACZ series could be
Sprague are all surge current tested. Another approach to minimize considered.
the surge current stresses on the input capacitor is to add a small For surface mount designs, solid tantalum capacitors can be used,
inductor in series with the input supply line. but caution must be exercised with regard to the capacitor surge
Use caution when using only ceramic capacitors for input bypassing, current rating and voltage rating. In this example, checking Tables 7
because it may cause severe ringing at the V
IN
pin. and 8, and the Sprague 594D series datasheet, a Sprague 594D 15
μF, 25V capacitor is adequate.
5. Boost Capacitor (C
B
) 5. Boost Capacitor (C
B
)
This capacitor develops the necessary voltage to turn the switch For this application, and all applications, use a 0.01 μF, 50V ceramic
gate on fully. All applications should use a 0.01 μF, 50V ceramic capacitor.
capacitor.
Inductor Value Selection Guides
(For Continuous Mode Operation)
Figure 24. LM2674-5.0 Figure 25. LM2674-3.3
Figure 26. LM2674-12 Figure 27. LM2674-ADJ
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