Datasheet

LM2675
www.ti.com SNVS129E –MAY 2004–REVISED JUNE 2005
LM2675 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 SWITCHER line of switching regulators.LM267X Made
Simpleversion 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) = 1A
• F = Switching Frequency (Fixed at a nominal 260 • F = Switching Frequency (Fixed at a nominal 260
kHz). 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 V
REF
= 1.21V
Select a value for R
1
between 240Ω and 1.5 kΩ. The lower resistor R
2
= 1k (16.53 − 1) = 15.53 kΩ, closest 1% value is 15.4 kΩ.
values minimize noise pickup in the sensitive feedback pin. (For the R
2
= 15.4 kΩ.
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 • μs), A. Calculate the inductor Volt • microsecond constant (E • T),
from the following formula:
where
• V
SAT
= internal switch saturation voltage =
0.25V
• V
D
= diode forward voltage drop = 0.5V
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) = 1A
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 1A vertical line is 68 μH, and the inductor code is L30.
E. Select an appropriate inductor from the four manufacturer's part E. From the table in Table 1, locate line L30, and select an inductor
numbers listed in Table 1. For information on the different types of part number 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 68 μH is C20.
corresponding to the desired output voltage.
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