LM2574 Series Buck Regulator Design Procedure
(Continued)
DS011394-16
FIGURE 8. LM2574HV-ADJ Inductor Selection Guide
PROCEDURE (Adjustable Output Voltage Versions)
Given:
V
OUT
= Regulated Output Voltage
V
IN
(Max) = Maximum Input Voltage
I
LOAD
(Max) = Maximum Load Current
F = Switching Frequency
(Fixed at 52 kHz)
1. Programming Output Voltage
(Selecting R1 and R2, as
shown in Figure 2)
Use the following formula to select the appropriate resistor
values.
EXAMPLE (Adjustable Output Voltage Versions)
Given:
V
OUT
= 24V
V
IN
(Max) = 40V
I
LOAD
(Max) = 0.4A
F = 52 kHz
1. Programming Output Voltage
(Selecting R1 and R2)
R
1
can be between 1k and 5k.
(For best temperature coeffi-
cient and stability with time, use 1% metal film resistors)
R
2
= 1k (19.51−1) = 18.51k, closest 1% value is 18.7k
2. Inductor Selection (L1)
A.
Calculate the inductor Volt
•
microsecond constant,
E
•
T (V
•
µs), from the following formula:
2. Inductor Selection (L1)
A.
Calculate E
•
T (V
•
µs)
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 8.
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, and note the in-
ductor value for that region.
E.
Select an appropriate inductor from the table shown in
Fig-
ure 3.
Part numbers are listed for three inductor manufactur-
ers. The inductor chosen must be rated for operation at the
LM2574 switching frequency (52 kHz) and for a current rating
of 1.5 x I
LOAD
. For additional inductor information, see the in-
ductor section in the application hints section of this data
sheet.
B.
E
•
T = 185 V
•
µs
C.
I
LOAD
(Max) = 0.4A
D.
Inductance Region = 1000
E.
Inductor Value = 1000 µH
Choose from
Pulse Engineer-
ing
Part
#PE-52631,
or
Renco
Part
#RL-1283-1000.
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14
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