Datasheet

LM2599
www.ti.com SNVS123C –APRIL 1998–REVISED APRIL 2013
Table 3. LM2599 Series Buck Regulator Design Procedure (Adjustable Output) (continued)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
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:
(4)
(5)
where V
SAT
= internal switch saturation voltage = 1.16V and V
D
=
B. E • T = 34.2 (V • μs)
diode forward voltage drop = 0.5V
C. I
LOAD
(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
D. From the inductor value selection guide shown in Figure 28, the
Guide shown in Figure 28.
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.
E. From the table in Table 5, locate line L39, and select an inductor
D. Identify the inductance region intersected by the E • T value and
part number from the list of manufacturers part numbers.
the Maximum Load Current value. Each region is identified by an
inductance value and an inductor code (LXX).
E. Select an appropriate inductor from the four manufacturer's part
numbers listed in Table 5.
3.Output Capacitor Selection (C
OUT
) 3. Output Capacitor SeIection (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 820 μF provide the best
B. From the quick design table shown in Table 4, locate the output
results. This capacitor should be located close to the IC using short
voltage column. From that column, locate the output voltage closest
capacitor leads and short copper traces. Do not use capacitors
to the output voltage in your application. In this example, select the
larger than 820 μF. For additional information, see section on
24V line. Under the OUTPUT CAPACITOR section, select a
output capacitors in Application Information section.
capacitor from the list of through hole electrolytic or surface mount
B. To simplify the capacitor selection procedure, refer to the quick tantalum types from four different capacitor manufacturers. It is
design table shown in Table 4. This table contains different output recommended that both the manufacturers and the manufacturers
voltages, and lists various output capacitors that will provide the best series that are listed in the table be used.
design solutions.
In this example, through hole aluminum electrolytic capacitors from
C. The capacitor voltage rating should be at least 1.5 times greater several different manufacturers are available.
than the output voltage, and often much higher voltage ratings are
220/35 Panasonic HFQ Series
needed to satisfy the low ESR requirements needed for low output
150/35 Nichicon PL Series
ripple voltage.
C. For a 20V output, a capacitor rating of at least 30V or more is
needed. In this example, either a 35V or 50V capacitor would work.
A 50V rating was chosen because it has a lower ESR which
provides a lower output ripple voltage.
Other manufacturers or other types of capacitors may also be used,
provided the capacitor specifications (especially the 100 kHz ESR)
closely match the types listed in the table. Refer to the capacitor
manufacturers data sheet for this information.
4. Feedforward Capacitor (C
FF
) (See Figure 24) 4. Feedforward Capacitor (C
FF
)
For output voltages greater than approximately 10V, an additional The table shown in Table 4 contains feed forward capacitor values
capacitor is required. The compensation capacitor is typically for various output voltages. In this example, a 560 pF capacitor is
between 100 pF and 33 nF, and is wired in parallel with the output needed.
voltage setting resistor, R
2
. It provides additional stability for high
output voltages, low input-output voltages, and/or very low ESR
output capacitors, such as solid tantalum capacitors.
(6)
This capacitor type can be ceramic, plastic, silver mica, etc.
(Because of the unstable characteristics of ceramic capacitors made
with Z5U material, they are not recommended.)
Copyright © 1998–2013, Texas Instruments Incorporated Submit Documentation Feedback 15
Product Folder Links: LM2599