LM2671MX-3.3/NOPB National Semiconductor, LM2671MX-3.3/NOPB Datasheet - Page 17

• Manufacturer Part Number: LM2671MX-3.3/NOPB
• Description: IC REG SW 3.3V 500MA STPDN 8SOIC
• Manufacturer: National Semiconductor
• Series: SIMPLE SWITCHER®r
• Type: Step-Down (Buck)r

Specifications of LM2671MX-3.3/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 3.3V
• Current - Output: 500mA
• Frequency - Switching: 260kHz
• Voltage - Input: 6.5 ~ 40 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: *LM2671MX-3.3; *LM2671MX-3.3/NOPB; LM2671MX-3.3

Select a value for R
values minimize noise pickup in the sensitive feedback pin. (For the
lowest temperature coefficient and the best stability with time, use
1% metal film resistors.)
2. Inductor Selection (L1)
A. Calculate the inductor Volt • microsecond constant E • T
(V • μs), from the following formula:
where V
V
B. Use the E • T value from the previous formula and match it with
the E • T number on the vertical axis of the Inductor Value Selection
Guide shown in Figure 7.
C. On the horizontal axis, select the maximum load current.
D. Identify the inductance region intersected by the E • T value and
the Maximum Load Current value. Each region is identified by an
inductance value and an inductor code (LXX).
E. Select an appropriate inductor from the four manufacturer's part
numbers listed in Figure 8. For information on the different types of
inductors, see the inductor selection in the fixed output voltage
design procedure.
3. Output Capacitor SeIection (C
A. Select an output capacitor from the capacitor code selection
guide in Figure 16. Using the inductance value found in the inductor
selection guide, step 1, locate the appropriate capacitor code
corresponding to the desired output voltage.
B. Select an appropriate capacitor value and voltage rating, using
the capacitor code, from the output capacitor selection table in
Figure 17. There are two solid tantalum (surface mount) capacitor
manufacturers and four electrolytic (through hole) capacitor
manufacturers to choose from. It is recommended that both the
manufacturers and the manufacturer's series that are listed in the
table be used. A table listing the manufacturers' phone numbers is
located in Figure 11.
D
= diode forward voltage drop = 0.5V
PROCEDURE (Adjustable Output Voltage Version)
SAT
=internal switch saturation voltage=0.25V and
1
between 240Ω and 1.5 kΩ. The lower resistor
where V
REF
OUT
= 1.21V
)
17
R
R
2. Inductor Selection (L1)
A. Calculate the inductor Volt • microsecond constant (E • T),
B. E • T = 21.6 (V • μs)
C. I
D. From the inductor value selection guide shown in Figure 7, the
inductance region intersected by the 21.6 (V • μs) horizontal line
and the 500 mA vertical line is 100 μH, and the inductor code is
L20.
E. From the table in Figure 8, locate line L20, and select an inductor
part number from the list of manufacturers' part numbers.
3. Output Capacitor SeIection (C
A. Use the appropriate row of the capacitor code selection guide,
in Figure 16. For this example, use the 15–20V row. The capacitor
code corresponding to an inductance of 100 μH is C20.
B. From the output capacitor selection table in Figure 17, choose
a capacitor value (and voltage rating) that intersects the capacitor
code(s) selected in section A, C20.
The capacitance and voltage rating values corresponding to the
capacitor code C20 are the:
Surface Mount:
33 μF/25V Sprague 594D Series.
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.
2
2
= 1 kΩ (16.53 − 1) = 15.53 kΩ, closest 1% value is 15.4 kΩ.
= 15.4 kΩ.
LOAD
EXAMPLE (Adjustable Output Voltage Version)
(max) = 500 mA
OUT
)
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