LM2596 Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (Adjustable Output Voltage Version)
Given:
VOUT = Regulated Output Voltage
IN(max) = Maximum Input Voltage
LOAD(max) = Maximum Load Current
EXAMPLE (Adjustable Output Voltage Version)
Given:
VOUT = 20V
IN(max) = 28V
ILOAD(max) = 3A
V
V
I
F = Switching Frequency (Fixed at a nominal 150 kHz).
F = Switching Frequency (Fixed at a nominal 150 kHz).
1. Programming Output Voltage (Selecting R1 and R2, as
1. Programming Output Voltage (Selecting R1 and R2, as
shown in Figure 1 )
shown in Figure 1 )
Use the following formula to select the appropriate resistor
values.
Select R1 to be 1 kΩ, 1%. Solve for R2.
R2 = 1k (16.26 − 1) = 15.26k, closest 1% value is 15.4 kΩ.
R2 = 15.4 kΩ.
Select a value for R1 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.)
2. Inductor Selection (L1)
2. Inductor Selection (L1)
A. Calculate the inductor Volt • microsecond constant
(E • T),
A. Calculate the inductor Volt • microsecond constant E • T (V
• µs), from the following formula:
where VSAT = internal switch saturation voltage = 1.16V
and VD = diode forward voltage drop = 0.5V
B. E • T = 34.2 (V • µs)
C. ILOAD(max) = 3A
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.
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.
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. From the table in Figure 8, locate line L39, and select an
inductor part number from the list of manufacturers part num-
bers.
E. Select an appropriate inductor from the four manufacturer’s
part numbers listed in Figure 8.
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