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| 型號： | LM2673 |
| 文件頁數(shù)： | 12/25頁 |
| 文件大小： | 495K |
| 代理商： | LM2673 |
LM2671 Series Buck Regulator Design Procedure (Fixed Output)
(Continued)
PROCEDURE (Fixed Output Voltage Version)
4. 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
1
2
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. 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. Soft-Start Capacitor (C
SS
- optional)
This capacitor controls the rate at which the device starts up.
The formula for the soft-start capacitor C
SS
is:
EXAMPLE (Fixed Output Voltage Version)
4. 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 12V, an aluminum electrolytic capacitor with a
voltage rating greater than 15V (1.25 x V
IN
) would be needed.
The next higher capacitor voltage rating is 16V.
The RMS current rating requirement for the input capacitor in a
buck regulator is approximately
1
2
the DC load current. In this
example, with a 500 mA load, a capacitor with a RMS current
rating of at least 250 mA is needed. The curves shown in Figure
14 can be used to select an appropriate input capacitor. From
the curves, locate the 16V line and note which capacitor values
have RMS current ratings greater than 250 mA.
For a through hole design, a 100 μF/16V 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, 25V capacitor is adequate.
5. Boost Capacitor (C
B
)
For this application, and all applications, use a 0.01 μF, 50V
ceramic capacitor.
6. Soft-Start Capacitor (C
SS
- optional)
For this application, selecting a start-up time of 10 ms and using
the formula for C
SS
results in a value of:
where:
I
SS
t
SS
V
SSTH
V
OUT
V
SCHOTTKY
V
IN
If this feature is not desired, leave this pin open.
= Soft-Start Current
= Soft-Start Time
= Soft-Start Threshold Voltage
= Output Voltage
= Schottky Diode Voltage Drop
= Input Voltage
:Selected.
:4.5 μA typical.
:Selected.
:0.63V typical.
:Selected.
:0.4V typical.
L
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