LM2674 Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (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:
EXAMPLE (Adjustable Output Voltage Version)
Given:
VOUT = 20V
VOUT = Regulated Output Voltage
V
IN(max) = Maximum Input Voltage
VIN(max) = 28V
I
LOAD(max) = Maximum Load Current
ILOAD(max) = 500 mA
F = Switching Frequency (Fixed at a nominal 260 kHz).
1. Programming Output Voltage (Selecting R1 and R2, as
shown in Figure 3)
F = Switching Frequency (Fixed at a nominal 260 kHz).
1. Programming Output Voltage (Selecting R1 and R2, as
shown in Figure 3)
Use the following formula to select the appropriate resistor
values.
Select R1 to be 1 kΩ, 1%. Solve for R2.
where VREF = 1.21V
Select a value for R1 between 240Ω and 1.5 kΩ. The lower
R2 = 1k (16.53 − 1) = 15.53 kΩ, closest 1% value is 15.4 kΩ.
resistor values minimize noise pickup in the sensitive feedback R2 = 15.4 kΩ.
pin. (For the lowest temperature coefficient and the best stability
with time, use 1% metal film resistors.)
2. Inductor Selection (L1)
2. Inductor Selection (L1)
A. Calculate the inductor Volt • microsecond constant E • T (V • A. Calculate the inductor Volt • microsecond constant (E • T),
µs), from the following formula:
where VSAT=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. ILOAD(max) = 500 mA
C. On the horizontal axis, select the maximum load current.
D. Identify the inductance region intersected by the E • T value D. From the inductor value selection guide shown in Figure 7,
and the Maximum Load Current value. Each region is identified the inductance region intersected by the 21.6 (V • µs) horizontal
by an inductance value and an inductor code (LXX).
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
E. From the table in Figure 8, locate line L20, and select an
part numbers listed in Figure 8. For information on the different inductor part number from the list of manufacturers part
types of inductors, see the inductor selection in the fixed output numbers.
voltage design procedure.
3. Output Capacitor Selection (COUT
A. Select an output capacitor from the capacitor code selection A. Use the appropriate row of the capacitor code selection
guide in Figure 16. Using the inductance value found in the guide, in Figure 16. For this example, use the 15–20V row. The
inductor selection guide, step 1, locate the appropriate capacitor capacitor code corresponding to an inductance of 100 µH is
code corresponding to the desired output voltage. C20.
)
3. Output Capacitor SeIection (COUT)
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