DescriptionIC, STEP-DOWN VOLTAGE REGULATOR, TO-220
ManufacturerNational Semiconductor

Primary Input Voltage40VNo. Of Outputs1
Output Voltage37VOutput Current3A
No. Of Pins5Operating Temperature Range-40Â°C To +125Â°C
Supply Voltage Range4.5V To 40VFilter TerminalsThrough Hole
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LM2596 Series Buck Regulator Design Procedure (Adjustable Output)
Given:
V
= Regulated Output Voltage
OUT
V
(max) = Maximum Input Voltage
IN
I
F = Switching Frequency (Fixed at a nominal 150 kHz).
1. Programming Output Voltage (Selecting R
shown in Figure 1 )
Use the following formula to select the appropriate resistor
values.
Select a value for R
between 240
and 1.5 k . The lower
1
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.)
2. Inductor Selection (L1)
A. Calculate the inductor Volt • microsecond constant E • T (V
• µs), from the following formula:
where V
= internal switch saturation voltage = 1.16V
SAT
and V
= diode forward voltage drop = 0.5V
D
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 .
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 Figure 8 .
Given:
V
= 20V
OUT
V
(max) = 28V
IN
I
(max) = 3A
F = Switching Frequency (Fixed at a nominal 150 kHz).
and R
, as
1. Programming Output Voltage (Selecting R
1
2
shown in Figure 1 )
Select R
to be 1 k , 1%. Solve for R
1
R
= 1k (16.26 − 1) = 15.26k, closest 1% value is 15.4 k .
2
R
= 15.4 k .
2
2. Inductor Selection (L1)
A. Calculate the inductor Volt • microsecond constant
(E • T),
B. E • T = 34.2 (V • µs)
C. I
(max) = 3A
D. From the inductor value selection guide shown in Figure 7 ,
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 Figure 8 , locate line L39, and select an
inductor part number from the list of manufacturers part num-
bers.
13
and R
, as
1
2
.
2
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