LM2676SX-5.0 National Semiconductor, LM2676SX-5.0 Datasheet - Page 14

• Manufacturer Part Number: LM2676SX-5.0
• Description: IC,SMPS CONTROLLER,VOLTAGE-MODE,BIPOLAR/MOS,SIP,7PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM2676SX-5.0

• Rohs Compliant: NO

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Application Hints
Step 6: From Table 5 a 3A Schottky diode must be selected.
For through-hole components 20V rated diodes are sufficient
and 2 part types are suitable:
1N5820
SR302
Step 7: A 0.01µF capacitor will be used for Cboost.
ADJUSTABLE OUTPUT DESIGN EXAMPLE
In this example it is desired to convert the voltage from a two
battery automotive power supply (voltage range of 20V to
28V, typical in large truck applications) to the 14.8VDC alter-
nator supply typically used to power electronic equipment
from single battery 12V vehicle systems. The load current
required is 2A maximum. It is also desired to implement the
power supply with all surface mount components.
Step 1: Operating conditions are:
Vout = 14.8V
Vin max = 28V
Iload max = 2A
Step 2: Select an LM2676S-ADJ. To set the output voltage
to 14.9V two resistors need to be chosen (R1 and R2 in
Figure 2). For the adjustable device the output voltage is set
by the following relationship:
Where V
A recommended value to use for R1 is 1K. In this example
then R2 is determined to be:
R2 = 11.23KΩ
The closest standard 1% tolerance value to use is 11.3KΩ
This will set the nominal output voltage to 14.88V which is
within 0.5% of the target value.
Step 3: To use the nomograph for the adjustable device,
Figure
Volt • microsecond constant (E • T expressed in V • µS) from
the following formula:
where V
switch which is R
be typically 0.15Ω x 2A or 0.3V and V
across the forward bisased Schottky diode, typically 0.5V.
The switching frequency of 260KHz is the nominal value to
SAT
FB
6,
is the feedback voltage of typically 1.21V.
is the voltage drop across the internal power
requires
ds(ON)
times I
a
calculation
load
(Continued)
. In this example this would
D
is the voltage drop
of
the
inductor
14
use to estimate the ON time of the switch during which
energy is stored in the inductor.
For this example E • T is found to be:
Using Figure 6, the intersection of 27V • µS horizontally and
the 2A vertical line (I
inductor, should be used.
From Table 1, L38 in a surface mount component is available
from Pulse Engineering with part number PE-54038S.
Step 4: Use Table 6 to determine an output capacitor. With a
14.8V output the 12.5 to 15V row is used and with a 68µH
inductor there are three surface mount output capacitor so-
lutions. Table 2 provides the actual capacitor characteristics
based on the C Code number. Any of the following choices
can be used:
1 x 33µF/20V AVX TPS (code C6)
1 x 47µF/20V Sprague 594 (code C8)
1 x 47µF/20V Kemet T495 (code C8)
Important Note: When using the adjustable device in low
voltage applications (less than 3V output), if the nomograph,
Figure 6, selects an inductance of 22µH or less, Table 6 does
not provide an output capacitor solution. With these condi-
tions the number of output capacitors required for stable
operation becomes impractical. It is recommended to use
either a 33µH or 47µH inductor and the output capacitors
from Table 6.
Step 5: An input capacitor for this example will require at
least a 35V WV rating with an rms current rating of 1A (1/2
Iout max). From Table 2 it can be seen that C12, a 33µF/35V
capacitor from Sprague, has the required voltage/current
rating of the surface mount components.
Step 6: From Table 5 a 3A Schottky diode must be selected.
For surface mount diodes with a margin of safety on the
voltage rating one of five diodes can be used:
SK34
30BQ040
30WQ04F
MBRS340
MBRD340
Step 7: A 0.01µF capacitor will be used for Cboost.
LLP PACKAGE DEVICES
The LM2676 is offered in the 14 lead LLP surface mount
package to allow for a significantly decreased footprint with
equivalent power dissipation compared to the TO-263. For
details on mounting and soldering specifications, refer to
Application Note AN-1187.
load
max) indicates that L38 , a 68µH