Datasheet

LM2674
www.ti.com SNVS007F –SEPTEMBER 1998–REVISED APRIL 2013
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
B. Select an appropriate capacitor value and voltage rating, using B. From the output capacitor selection in Table 10, choose a
the capacitor code, from the output capacitor selection in Table 10. capacitor value (and voltage rating) that intersects the capacitor
There are two solid tantalum (surface mount) capacitor code(s) selected in section A, C20.
manufacturers and four electrolytic (through hole) capacitor The capacitance and voltage rating values corresponding to the
manufacturers to choose from. It is recommended that both the capacitor code C20 are the:
manufacturers and the manufacturer's series that are listed in the Surface Mount:
table be used. A table listing the manufacturers' phone numbers is 33 μF/25V Sprague 594D Series.
located in Table 4. 33 μF/25V AVX TPS Series.
Through Hole:
33 μF/25V Sanyo OS-CON SC Series.
120 μF/35V Sanyo MV-GX Series.
120 μF/35V Nichicon PL Series.
120 μF/35V Panasonic HFQ Series.
Other manufacturers or other types of capacitors may also be used,
provided the capacitor specifications (especially the 100 kHz ESR)
closely match the characteristics of the capacitors listed in the output
capacitor table. Refer to the capacitor manufacturers' data sheet for
this information.
4. Catch Diode Selection (D1) 4. Catch Diode Selection (D1)
A. In normal operation, the average current of the catch diode is the A. Refer to Table 5. Schottky diodes provide the best performance,
load current times the catch diode duty cycle, 1-D (D is the switch and in this example a 500mA, 40V Schottky diode would be a good
duty cycle, which is approximately V
OUT
/V
IN
). The largest value of choice. If the circuit must withstand a continuous shorted output, a
the catch diode average current occurs at the maximum input higher current (at least 1.2A) Schottky diode is recommended.
voltage (minimum D). For normal operation, the catch diode current
rating must be at least 1.3 times greater than its maximum average
current. However, if the power supply design must withstand a
continuous output short, the diode should have a current rating
greater than the maximum current limit of the LM2674. The most
stressful condition for this diode is a shorted output condition.
B. The reverse voltage rating of the diode should be at least 1.25
times the maximum input voltage.
C. Because of their fast switching speed and low forward voltage
drop, Schottky diodes provide the best performance and efficiency.
The Schottky diode must be located close to the LM2674 using short
leads and short printed circuit traces.
5. Input Capacitor (C
IN
) 5. Input Capacitor (C
IN
)
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 28V, an aluminum electrolytic capacitor with a voltage
to the IC using short leads. In addition, the RMS current rating of the rating of at least 35V (1.25 × V
IN
) would be needed.
input capacitor should be selected to be at least ½ the DC load The RMS current rating requirement for the input capacitor in a buck
current. The capacitor manufacturer data sheet must be checked to regulator is approximately ½ the DC load current. In this example,
assure that this current rating is not exceeded. The curves shown in with a 500mA load, a capacitor with an RMS current rating of at least
Figure 28 show typical RMS current ratings for several different 250 mA is needed. The curves shown in Figure 28 can be used to
aluminum electrolytic capacitor values. A parallel connection of two select an appropriate input capacitor. From the curves, locate the
or more capacitors may be required to increase the total minimum 35V line and note which capacitor values have RMS current ratings
RMS current rating to suit the application requirements. greater than 250 mA.
For an aluminum electrolytic capacitor, the voltage rating should be For a through hole design, a 68 μF/35V electrolytic capacitor
at least 1.25 times the maximum input voltage. Caution must be (Panasonic HFQ series, Nichicon PL, Sanyo MV-GX series or
exercised if solid tantalum capacitors are used. The tantalum equivalent) would be adequate. Other types or other manufacturers'
capacitor voltage rating should be twice the maximum input voltage. capacitors can be used provided the RMS ripple current ratings are
Table 7 and Table 8 show the recommended application voltage for adequate. Additionally, for a complete surface mount design,
AVX TPS and Sprague 594D tantalum capacitors. It is also electrolytic capacitors such as the Sanyo CV-C or CV-BS, and the
recommended that they be surge current tested by the manufacturer. Nichicon WF or UR and the NIC Components NACZ series could be
The TPS series available from AVX, and the 593D and 594D series considered.
from Sprague are all surge current tested. Another approach to For surface mount designs, solid tantalum capacitors can be used,
minimize the surge current stresses on the input capacitor is to add but caution must be exercised with regard to the capacitor surge
a small inductor in series with the input supply line. current rating and voltage rating. In this example, checking note 1 of
Use caution when using only ceramic capacitors for input bypassing, Table 8, and the Sprague 594D series datasheet, a Sprague 594D
because it may cause severe ringing at the V
IN
pin. 15 μF, 50V capacitor is adequate.
6. Boost Capacitor (C
B
) 6. 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.
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