LM2674LDX-3.3 National Semiconductor, LM2674LDX-3.3 Datasheet - Page 18

• Manufacturer Part Number: LM2674LDX-3.3
• Manufacturer: National Semiconductor

Specifications of LM2674LDX-3.3

• Dc To Dc Converter Type: Step Down
• Number Of Outputs: 1
• Pin Count: 16
• Input Voltage: 40V
• Output Voltage: 3.3V
• Switching Freq: 275KHz
• Output Current: 500mA
• Efficiency: 86%
• Package Type: LLP EP
• Output Type: Fixed
• Switching Regulator: Yes
• Mounting: Surface Mount
• Input Voltage (min): 8V
• Operating Temp Range: -40C to 125C
• Operating Temperature Classification: Automotive
• Lead Free Status / Rohs Status: Not Compliant

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To simplify the buck regulator design procedure, National
Semiconductor is making available computer design software to
be used with the SIMPLE SWITCHER line of switching
regulators. LM267X Made Simple (version 6.0) is available for
use on Windows 3.1, NT, or 95 operating systems.
Given:
1. Programming Output Voltage (Selecting R
shown in Figure 3)
Use the following formula to select the appropriate resistor
values.
Select a value for R
resistor 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
diode forward voltage drop = 0.5V
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 Selection (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.
where V
LM2674 Series Buck Regulator Design Procedure (Adjustable Output)
V
V
I
F = Switching Frequency (Fixed at a nominal 260 kHz).
LOAD
OUT
IN
PROCEDURE (Adjustable Output Voltage Version)
(max) = Maximum Input Voltage
(max) = Maximum Load Current
= Regulated Output Voltage
SAT
REF
=internal switch saturation voltage=0.25V and V
= 1.21V
1
between 240Ω and 1.5 kΩ. The lower
OUT
)
1
and R
2
, as
D
=
18
Given:
1. Programming Output Voltage (Selecting R
shown in Figure 3)
Select R
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 500mA 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.
2
2
V
V
I
F = Switching Frequency (Fixed at a nominal 260 kHz).
LOAD
= 1k (16.53 − 1) = 15.53 kΩ, closest 1% value is 15.4 kΩ.
= 15.4 kΩ.
LOAD
OUT
IN
(max) = 28V
EXAMPLE (Adjustable Output Voltage Version)
(max) = 500 mA
= 20V
(max) = 500 mA
1
to be 1 kΩ, 1%. Solve for R
OUT
2
)
.
1
and R
2
, as