Datasheet

LM2598
www.ti.com SNVS125C –MARCH 1998–REVISED APRIL 2013
Table 3. LM2598 Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
Given: Given:
V
OUT
= Regulated Output Voltage V
OUT
= 20V
V
IN
(max) = Maximum Input Voltage V
IN
(max) = 28V
I
LOAD
(max) = Maximum Load Current I
LOAD
(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 R
1
and R
2
, as shown in 1. Programming Output Voltage (Selecting R
1
and R
2
, as shown in
Figure 24) Figure 24)
Use the following formula to select the appropriate resistor values. Select R
1
to be 1 kΩ, 1%. Solve for R
2
.
(1)
(3)
Select a value for R
1
between 240Ω and 1.5 kΩ. The lower resistor
R
2
= 1k (16.26 − 1) = 15.26k, closest 1% value is 15.4 kΩ.
values minimize noise pickup in the sensitive feedback pin. (For the
R
2
= 15.4 kΩ.
lowest temperature coefficient and the best stability with time, use
1% metal film resistors.)
(2)
2. Inductor Selection (L1) 2. Inductor Selection (L1)
A. Calculate the inductor Volt • microsecond constant E • T (V • μs), A. Calculate the inductor Volt • microsecond constant (E • T),
from the following formula:
(5)
where
• V
SAT
= internal switch saturation voltage = 1V
• V
D
= diode forward voltage drop = 0.5V (4)
B. Use the E • T value from the previous formula and match it with B. E • T = 34.8 (V • μs)
the E • T number on the vertical axis of the Inductor Value Selection
Guide shown in INDUCTOR VALUE SELECTION GUIDES.
C. on the horizontal axis, select the maximum load current. C. I
LOAD
(max) = 1A
D. Identify the inductance region intersected by the E • T value and D. From the inductor value selection guide shown in INDUCTOR
the Maximum Load Current value. Each region is identified by an VALUE SELECTION GUIDES, the inductance region intersected by
inductance value and an inductor code (LXX). the 35 (V • μs) horizontal line and the 1A vertical line is 100 μH, and
the inductor code is L29.
E. Select an appropriate inductor from the four manufacturer's part E. From the table in Table 5, locate line L29, and select an inductor
numbers listed in Table 5. part number from the list of manufacturers part numbers.
3. Output Capacitor Selection (C
OUT
) 3. Output Capacitor Selection (C
OUT
)
A. In the majority of applications, low ESR electrolytic or solid A. See section on C
OUT
in Application Information section.
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 information, see section on
output capacitors in Application Information section.
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