Datasheet

LM2595
SNVS122B –MAY 1999–REVISED APRIL 2013 www.ti.com
Table 3. LM2595 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 = 1V
and V
D
= diode forward voltage drop = 0.5V
B. E • T = 34.8 (V • μs)
B. Use the E • T value from the previous formula and match it with
C. I
LOAD
(max) = 1A
the E • T number on the vertical axis of the Inductor Value Selection
D. From the inductor value selection guide shown in Figure 25, the
Guide shown in Figure 25.
inductance region intersected by the 35 (V • μs) horizontal line and
C. on the horizontal axis, select the maximum load current.
the 1A vertical line is 100 μH, and the inductor code is L29.
D. Identify the inductance region intersected by the E • T value and
E. From the table in Table 5, locate line L29, and select an inductor
the Maximum Load Current value. Each region is identified by an
part number from the list of manufacturers part numbers.
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 47 μF and 330 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 330 μF. For additional information, see section on
24V line. Under the output capacitor section, select a capacitor from
output capacitors in Application Information section.
the list of through hole electrolytic or surface mount tantalum types
B. To simplify the capacitor selection procedure, refer to the quick from four different capacitor manufacturers. It is recommended that
design table shown in Table 4. This table contains different output both the manufacturers and the manufacturers series that are listed
voltages, and lists various output capacitors that will provide the best 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
82 μF, 35V Panasonic HFQ Series
needed to satisfy the low ESR requirements needed for low output
82 μF, 35V 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 35V rating was chosen, although a 50V rating could also be used
if a lower output ripple voltage is needed.
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 21) 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 1 nF capacitor is
between 50 pF and 10 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.)
14 Submit Documentation Feedback Copyright © 1999–2013, Texas Instruments Incorporated
Product Folder Links: LM2595