LM2598 Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (Adjustable Output Voltage Version)
EXAMPLE (Adjustable Output Voltage Version)
Given:
Given:
VOUT = Regulated Output Voltage
VOUT = 20V
VIN(max) = Maximum Input Voltage
VIN(max) = 28V
I
LOAD(max) = Maximum Load Current
ILOAD(max) = 1A
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
(V • µs), from the following formula:
(E • T),
where VSAT = internal switch saturation voltage = 1V
and VD = 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 Figure 7.
B. E • T = 34.8 (V • µs)
C. ILOAD(max) = 1A
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
identified by an inductance value and an inductor code
(LXX).
D. From the inductor value selection guide shown in Figure 7,
the inductance region intersected by the 35 (V • µs) horizon-
tal line and the 1A vertical line is 100 µH, and the inductor
code is L29.
E. From the table in Figure 8, locate line L29, and select an
inductor part number from the list of manufacturers part
numbers.
E. Select an appropriate inductor from the four manufactur-
er’s part numbers listed in Figure 8.
3. Output Capacitor Selection (COUT
)
3. Output Capacitor SeIection (COUT
)
A. In the majority of applications, low ESR electrolytic or solid
tantalum capacitors between 82 µF and 220 µF provide the
best results. This capacitor should be located close to the IC
using short capacitor leads and short copper traces. Do not
use capacitors larger than 220 µF. For additional informa-
tion, see section on output capacitors in application
information section.
A. See section on COUT in Application Information section.
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