LM2596
LM2596 Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (Adjustable Output Voltage Version)
Given:
V
OUT
= Regulated Output Voltage
V
IN
(max) = Maximum Input Voltage
I
LOAD
(max) = Maximum Load Current
F = Switching Frequency
(Fixed at a nominal 150 kHz).
1. Programming Output Voltage
(Selecting R
1
and R
2
, as
shown in
)
Use the following formula to select the appropriate resistor
values.
EXAMPLE (Adjustable Output Voltage Version)
Given:
V
OUT
= 20V
V
IN
(max) = 28V
I
LOAD
(max) = 3A
F = Switching Frequency
(Fixed at a nominal 150 kHz).
1. Programming Output Voltage
(Selecting R
1
and R
2
, as
shown in
)
Select R
1
to be 1 kΩ, 1%. Solve for R
2
.
Select a value for R
1
between 240Ω and 1.5 kΩ. The lower
resistor values minimize noise pickup in the sensitive feed-
back pin. (For the lowest temperature coefficient and the best
stability with time, use 1% metal film resistors.)
R
2
= 1k (16.26 − 1) = 15.26k, closest 1% value is 15.4 kΩ.
R
2
= 15.4 kΩ.
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 the inductor Volt
•
microsecond constant
(E
•
T),
where V
SAT
= internal switch saturation voltage = 1.16V
and V
D
= 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
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. Each region is identi-
fied by an inductance value and an inductor code (LXX).
E.
Select an appropriate inductor from the four manufacturer’s
part numbers listed in
B.
E
•
T = 34.2 (V
•
µs)
C.
I
LOAD
(max) = 3A
D.
From the inductor value selection guide shown in
the inductance region intersected by the 34 (V
•
µs) horizontal
line and the 3A vertical line is 47 µH, and the inductor code is
L39.
E.
From the table in
locate line L39, and select an
inductor part number from the list of manufacturers part num-
bers.
13
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