Datasheet

LM2594, LM2594HV
SNVS118C –DECEMBER 1999–REVISED APRIL 2013 www.ti.com
Table 3. LM2594/LM2594HV 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)
where V
SAT
= internal switch saturation voltage = 0.9V (5)
and V
D
= diode forward voltage drop = 0.5V
B. E • T = 35.2 (V • μs)
B. Use the E • T value from the previous formula and match it with
C. I
LOAD
(max) = 0.5A
the E • T number on the vertical axis of the Inductor Value Selection
D. From the inductor value selection guide shown in Figure 24, the
Guide shown in Figure 24.
inductance region intersected by the 35 (V • μs) horizontal line and
C. on the horizontal axis, select the maximum load current.
the 0.5A vertical line is 150 μH, and the inductor code is L19.
D. Identify the inductance region intersected by the E • T value and
E. From Table 5, locate line L19, and select an inductor part number
the Maximum Load Current value. Each region is identified by an
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 OUTPUT CAPACITORsection on C
OUT
in Application
tantalum capacitors between 82 μF and 220 μF provide the best Information section.
results. This capacitor should be located close to the IC using short
B. From the quick design table shown in Table 4, locate the output
capacitor leads and short copper traces. Do not use capacitors
voltage column. From that column, locate the output voltage closest
larger than 220 μF. For additional information, see OUTPUT
to the output voltage in your application. In this example, select the
CAPACITOR section in Application Information section.
24V line. Under the OUTPUT CAPACITORoutput capacitor, select a
B. To simplify the capacitor selection procedure, refer to the quick capacitor from the list of through hole electrolytic or surface mount
design table shown in Table 4. This table contains different output tantalum types from four different capacitor manufacturers. It is
voltages, and lists various output capacitors that will provide the best recommended that both the manufacturers and the manufacturers
design solutions. series that are listed in the table be used.
C. The capacitor voltage rating should be at least 1.5 times greater In this example, through hole aluminum electrolytic capacitors from
than the output voltage, and often much higher voltage ratings are several different manufacturers are available.
needed to satisfy the low ESR requirements needed for low output
82 μF 50V Panasonic HFQ Series
ripple voltage.
120 μF 50V Nichicon PL Series
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 20 ) 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: LM2594 LM2594HV