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LM2674 Datasheet(PDF) 18 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Part No. LM2674
Description  SIMPLE SWITCHER Power Converter High Efficiency 500 mA Step-Down Voltage Regulator
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Manufacturer  NSC [National Semiconductor (TI)]
Direct Link  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM2674 Datasheet(HTML) 18 Page - National Semiconductor (TI)

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LM2674 Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (Adjustable Output Voltage Version)
EXAMPLE (Adjustable Output Voltage Version)
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:
Given:
V
OUT = Regulated Output Voltage
V
OUT = 20V
V
IN(max) = Maximum Input Voltage
V
IN(max) = 28V
I
LOAD(max) = Maximum Load Current
I
LOAD(max) = 500 mA
F = Switching Frequency (Fixed at a nominal 260 kHz).
F = Switching Frequency (Fixed at a nominal 260 kHz).
1. Programming Output Voltage (Selecting R
1 and R2,as
shown in Figure 3)
1. Programming Output Voltage (Selecting R
1 and R2,as
shown in Figure 3)
Use the following formula to select the appropriate resistor
values.
Select R
1 to be 1 k
Ω, 1%. Solve for R
2.
where V
REF = 1.21V
Select a value for R
1 between 240
Ω and 1.5 kΩ. The lower
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.)
R
2 = 1k (16.53 − 1) = 15.53 k
Ω, closest 1% value is 15.4 kΩ.
R
2 = 15.4 k
Ω.
2. Inductor Selection (L1)
2. Inductor Selection (L1)
A. Calculate the inductor Volt
microsecond constant E T(V
µs), from the following formula:
A. Calculate the inductor Volt
microsecond constant (E T),
where V
SAT=internal switch saturation voltage=0.25V and VD =
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.
B. E
T = 21.6 (V µs)
C. On the horizontal axis, select the maximum load current.
C. I
LOAD(max) = 500 mA
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).
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. 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.
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 Selection (C
OUT)
3. Output Capacitor SeIection (C
OUT)
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.
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.
www.national.com
18


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