Datasheet

PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
5. Input Capacitor (C
IN
)
A low ESR aluminum or tantalum bypass capacitor is needed
between the input pin and ground to prevent large voltage
transients from appearing at the input. This capacitor should be
located close to the IC using short leads. In addition, the RMS
current rating of the input capacitor should be selected to be at least
½ the DC load current. The capacitor manufacturer data sheet must
be checked to assure that this current rating is not exceeded. The
curves shown in Figure 14 show typical RMS current ratings for
several different aluminum electrolytic capacitor values. A parallel
connection of two or more capacitors may be required to increase
the total minimum RMS current rating to suit the application
requirements.
For an aluminum electrolytic capacitor, the voltage rating should be
at least 1.25 times the maximum input voltage. Caution must be
exercised if solid tantalum capacitors are used. The tantalum
capacitor voltage rating should be twice the maximum input
voltage. The tables in Figure 15 show the recommended
application voltage for AVX TPS and Sprague 594D tantalum
capacitors. It is also recommended that they be surge current
tested by the manufacturer. The TPS series available from AVX,
and the 593D and 594D series from Sprague are all surge current
tested. Another approach to minimize the surge current stresses
on the input capacitor is to add a small inductor in series with the
input supply line.
Use caution when using ceramic capacitors for input bypassing,
because it may cause severe ringing at the V
IN
pin.
5. Input Capacitor (C
IN
)
The important parameters for the input capacitor are the input
voltage rating and the RMS current rating. With a maximum input
voltage of 28V, an aluminum electrolytic capacitor with a voltage
rating of at least 35V (1.25 × V
IN
) would be needed.
The RMS current rating requirement for the input capacitor in a
buck regulator is approximately ½ the DC load current. In this
example, with a 1A load, a capacitor with a RMS current rating of
at least 500 mA is needed. The curves shown in Figure 14 can be
used to select an appropriate input capacitor. From the curves,
locate the 35V line and note which capacitor values have RMS
current ratings greater than 500 mA.
For a through hole design, a 330 μF/35V electrolytic capacitor
(Panasonic HFQ series, Nichicon PL, Sanyo MV-GX series or
equivalent) would be adequate. Other types or other
manufacturers' capacitors can be used provided the RMS ripple
current ratings are adequate. Additionally, for a complete surface
mount design, electrolytic capacitors such as the Sanyo CV-C or
CV-BS and the Nichicon WF or UR and the NIC Components NACZ
series could be considered.
For surface mount designs, solid tantalum capacitors can be used,
but caution must be exercised with regard to the capacitor surge
current rating and voltage rating. In this example, checking Figure
15, and the Sprague 594D series datasheet, a Sprague 594D 15
μF, 50V capacitor is adequate.
6. Boost Capacitor (C
B
)
This capacitor develops the necessary voltage to turn the switch
gate on fully. All applications should use a 0.01 μF, 50V ceramic
capacitor.
6. Boost Capacitor (C
B
)
For this application, and all applications, use a 0.01 μF, 50V
ceramic capacitor.
If the soft-start and frequency synchronization features are desired,
look at steps 6 and 7 in the fixed output design procedure.
Case
Style (Note 7)
Output
Voltage (V)
Inductance (μH)
22 33 47 68 100 150 220
SM and TH 1.21–2.50
— — — —
C1 C2 C3
SM and TH 2.50–3.75
— — —
C1 C2 C3 C3
SM and TH 3.75–5.0
— —
C4 C5 C6 C6 C6
SM and TH 5.0–6.25
—
C4 C7 C6 C6 C6 C6
SM and TH 6.25–7.5 C8 C4 C7 C6 C6 C6 C6
SM and TH 7.5–10.0 C9 C10 C11 C12 C13 C13 C13
SM and TH 10.0–12.5 C14 C11 C12 C12 C13 C13 C13
SM and TH 12.5–15.0 C15 C16 C17 C17 C17 C17 C17
SM and TH 15.0–20.0 C18 C19 C20 C20 C20 C20 C20
SM and TH 20.0–30.0 C21 C22 C22 C22 C22 C22 C22
TH 30.0–37.0 C23 C24 C24 C25 C25 C25 C25
Note 7: SM - Surface Mount, TH - Through Hole
FIGURE 16. Capacitor Code Selection Guide
19 www.national.com
LM2672