Datasheet

LM2674
SNVS007F –SEPTEMBER 1998–REVISED APRIL 2013 www.ti.com
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, Texas instruments
is making available computer design software to be used with the
SIMPLE SWITCHERline 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 R
2
, as shown in 1. Programming Output Voltage (Selecting R
1
and R
2
, as shown in
Figure 23) Figure 23)
Use the following formula to select the appropriate resistor values. Select R
1
to be 1 kΩ, 1%. Solve for R
2
.
where
where
• R
2
= 1k (16.53 − 1) = 15.53 kΩ, closest 1%
• V
REF
= 1.21V (1)
value is 15.4 kΩ.
R
2
= 15.4 kΩ. (2)
Select a value for R
1
between 240Ω 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.)
(3)
2. Inductor Selection (L1) 2. Inductor Selection (L1)
A. Calculate the inductor Volt • microsecond constant E • T (V • μs), A. Calculate the inductor Volt • microsecond constant (E • T),
from the following formula:
(5)
where
• V
SAT
=internal switch saturation voltage=0.25V
and V
D
= diode forward voltage drop =
0.5V (4)
B. Use the E • T value from the previous formula and match it with B. E • T = 21.6 (V • μs)
the E • T number on the vertical axis of the Inductor Value Selection
Guide shown in Figure 27.
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 D. From the inductor value selection guide shown in Figure 27, the
the Maximum Load Current value. Each region is identified by an inductance region intersected by the 21.6 (V • μs) horizontal line and
inductance value and an inductor code (LXX). the 500mA vertical line is 100 μH, and the inductor code is L20.
E. Select an appropriate inductor from the four manufacturer's part E. From Table 1, locate line L20, and select an inductor part number
numbers listed in Table 1. For information on the different types of from the list of manufacturers part numbers.
inductors, see the inductor selection in the fixed output voltage
design procedure.
3. Output Capacitor Selection (C
OUT
) 3. Output Capacitor SeIection (C
OUT
)
A. Select an output capacitor from the capacitor code selection guide A. Use the appropriate row of the capacitor code selection guide, in
in Table 9. Using the inductance value found in the inductor Table 9. For this example, use the 15–20V row. The capacitor code
selection guide, step 1, locate the appropriate capacitor code corresponding to an inductance of 100 μH is C20.
corresponding to the desired output voltage.
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