LM2595T-ADJ National Semiconductor, LM2595T-ADJ Datasheet - Page 21

• Manufacturer Part Number: LM2595T-ADJ
• Description: DC/DC Converter IC
• Manufacturer: National Semiconductor

Specifications of LM2595T-ADJ

• Input Voltage: 40V
• Output Current: 1A
• Output Voltage: 37V
• No. Of Pins: 5
• Termination Type: Through Hole
• Mounting Type: Through Hole
• Voltage Regulator Type: Buck Switching
• Output Current Max: 1A
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Application Information
eters such as, peak inductor or peak switch current, mini-
mum load current before the circuit becomes discontinuous,
output ripple voltage and output capacitor ESR can all be
calculated from the peak-to-peak I
nomographs shown in Figure 4 through Figure 7 are used to
select an inductor value, the peak-to-peak inductor ripple
current can immediately be determined. The curve shown in
Figure 18 shows the range of ( I
for different load currents. The curve also shows how the
peak-to-peak inductor ripple current ( I
go from the lower border to the upper border (for a given load
current) within an inductance region. The upper border rep-
resents a higher input voltage, while the lower border repre-
sents a lower input voltage (see Inductor Selection Guides).
These curves are only correct for continuous mode opera-
tion, and only if the inductor selection guides are used to se-
lect the inductor value
Consider the following example:
The selection guide in Figure 5 shows that the vertical line
for a 0.8A load current, and the horizontal line for the 12V in-
put voltage intersect approximately midway between the up-
per and lower borders of the 68 µH inductance region. A 68
µH inductor will allow a peak-to-peak inductor current ( I
to flow that will be a percentage of the maximum load cur-
rent. Referring to Figure 18 , follow the 0.8A line approxi-
mately midway into the inductance region, and read the
peak-to-peak inductor ripple current ( I
axis (approximately 300 mA p-p).
As the input voltage increases to 14V, it approaches the up-
per border of the inductance region, and the inductor ripple
current increases. Referring to the curve in Figure 18 , it can
be seen that for a load current of 0.8A, the peak-to-peak in-
ductor ripple current ( I
range from 340 mA at the upper border (14V in) to 225 mA at
the lower border (10V in).
Once the I
used to calculate additional information about the switching
regulator circuit.
1. Peak Inductor or peak switch current
2. Minimum load current before the circuit becomes dis-
3. Output Ripple Voltage = ( I
4. ESR of C
V
V
OUT
IN
continuous
= 0.30Ax0.16 = 48 mV p-p
= 12V, nominal, varying between 10V and 14V.
= 5V, maximum load current of 800 mA
IND
OUT
value is known, the following formulas can be
IND
) is 300 mA with 12V in, and can
IND
IND
)x(ESR of C
IND
) that can be expected
IND
IND
. When the inductor
(Continued)
) changes as you
) on the left hand
OUT
)
IND
)
21
OPEN CORE INDUCTORS
Another possible source of increased output ripple voltage or
unstable operation is from an open core inductor. Ferrite
bobbin or stick inductors have magnetic lines of flux flowing
through the air from one end of the bobbin to the other end.
These magnetic lines of flux will induce a voltage into any
wire or PC board copper trace that comes within the induc-
tor’s magnetic field. The strength of the magnetic field, the
orientation and location of the PC copper trace to the mag-
netic field, and the distance between the copper trace and
the inductor, determine the amount of voltage generated in
the copper trace. Another way of looking at this inductive
coupling is to consider the PC board copper trace as one
turn of a transformer (secondary) with the inductor winding
as the primary. Many millivolts can be generated in a copper
trace located near an open core inductor which can cause
stability problems or high output ripple voltage problems.
If unstable operation is seen, and an open core inductor is
used, it’s possible that the location of the inductor with re-
spect to other PC traces may be the problem. To determine
if this is the problem, temporarily raise the inductor away
from the board by several inches and then check circuit op-
eration. If the circuit now operates correctly, then the mag-
netic flux from the open core inductor is causing the problem.
Substituting a closed core inductor such as a torroid or
E-core will correct the problem, or re-arranging the PC layout
may be necessary. Magnetic flux cutting the IC device
ground trace, feedback trace, or the positive or negative
traces of the output capacitor should be minimized.
Sometimes, locating a trace directly beneath a bobbin induc-
tor will provide good results, provided it is exactly in the cen-
ter of the inductor (because the induced voltages cancel
themselves out), but if it is off center one direction or the
other, then problems could arise. If flux problems are
present, even the direction of the inductor winding can make
a difference in some circuits.
This discussion on open core inductors is not to frighten the
user, but to alert the user on what kind of problems to watch
out for when using them. Open core bobbin or “stick” induc-
tors are an inexpensive, simple way of making a compact ef-
ficient inductor, and they are used by the millions in many dif-
ferent applications.
THERMAL CONSIDERATIONS
The LM2595 is available in two packages, a 5-pin TO-220
(T) and a 5-pin surface mount TO-263 (S).
The TO-220 package can be used without a heat sink for
ambient temperatures up to approximately 50˚C (depending
on the output voltage and load current). The curves in Figure
19 show the LM2595T junction temperature rises above am-
bient temperature for different input and output voltages. The
data tor these curves was taken with the LM2595T (TO-220
package) operating as a switching regutator in an ambient
temperature of 25˚C (still air). These temperature rise num-
bers are all approximate and there are many factors that can
affect these temperatures. Higher ambient temperatures re-
quire some heat sinking, either to the PC board or a small
external heat sink.
The TO-263 surface mount package tab is designed to be
soldered to the copper on a printed circuit board. The copper
and the board are the heat sink for this package and the
other heat producing components, such as the catch diode
and inductor. The PC board copper area that the package is
soldered to should be at least 0.4 in
have 2 or more square inches of 2 oz. (0.0028 in) copper.
Additional copper area improves the thermal characteristics,
2
, and ideally should
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