LM2641MTC-ADJ National Semiconductor, LM2641MTC-ADJ Datasheet - Page 16

• Manufacturer Part Number: LM2641MTC-ADJ
• Description: Power Supply IC
• Manufacturer: National Semiconductor

Specifications of LM2641MTC-ADJ

• Power Dissipation Pd: 883mW
• No. Of Pins: 28
• Peak Reflow Compatible (260 C): No
• Leaded Process Compatible: No
• Mounting Type: Surface Mount
• Package / Case: 28-TSSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Design Procedure
It is also recommended that a 0.1µF ceramic capacitor be
placed from V
cated as close as possible to the V
OUTPUT CAPACITORS
The output capacitor(s) are critical in loop stability (covered
in a previous section) and also output voltage ripple.
The types best suited for use as output capacitors are alumi-
num electrolytics and solid Tantalum.
Aluminum Electrolytics
The primary advantage of aluminum electrolytics is that they
typically give the maximum capacitance-to-size ratio, and
they are reasonably priced. However, it must be noted that
aluminum electrolytics used in high-performance switching
regulator designs must be high frequency, low ESR types
such as Sanyo OSCON or Panasonic HFQ which are spe-
cifically designed for switching applications. Capacitors such
as these with good high frequency ( 100kHz) specifications
are not cheap.
Aluminum electrolytic capacitors should generally not be
used in switching regulator applications where the ambient
temperature goes below 0˚C. A typical low-voltage aluminum
electrolytic has an ESR vs. Temperature curve that is fairly
flat from 25˚C to 125˚C. However, a temperature change
from 25˚C to 0˚C will approximately double the ESR, and it
will double again going from 0˚C down to −20˚C.
Tantalum
Solid Tantalum capacitors are best in applications which
must operate over a wide temperature range. A good quality
Tantalum will typically exhibit less than 2:1 change in ESR
over the temperature range of +125˚C to −40˚C. Recom-
mended types are Sprague 593D, Sprague 594D, and AVX
TPS series.
Selecting An Output Capacitor
The required value of output capacitance is directly related
to the specification for the maximum amount of output volt-
age ripple allowed in the application. Since ESR effects the
ripple voltage, it is important to have a guideline for ESR.
The maximum allowed ESR can be calculated as follows.
V
Using V = Ldi/dt
V
I
ESR(max) = V
A reasonable value for C
capacitors with net ESR less than
Hence,
The value of C
specification can be found using the approximation:
Where:
I
RIPPLE
RIPPLE
RIPPLE
OUT
= L
is the inductor ripple current.
= V
= I
ESR(max) = V
*
I
RIPPLE
OUT
RIPPLE
IN
RIPPLE
*
OUT
(1−D)/)L
to ground for high frequency bypassing, lo-
/{(1−D)T
*
ESR(max)
necessary to meet the voltage ripple
/I
RIPPLE
RIPPLE
*
OUT
F
S
} = L
S
)
can be obtained by choosing
*
L
(Continued)
*
*
F
I
1
RIPPLE
IN
⁄
S
2
/ {V
of ESR(max).
pin.
OUT
*
F
(1−D)}
S
/(1−D)
16
V
ESR is the equivalent series resistance of the output capaci-
tor.
F is the switching frequency, F
T
The ESR term predominates in determining output ripple
voltage. Good quality Tantalum capacitors have guaranteed
maximum specifications for ESR, but the typical values for
ESR are usually considerably lower than the maximum limit.
S
RIPPLE
= 1/F
is the output ripple voltage.
S
.
S
.