LM2678S-3.3 National Semiconductor, LM2678S-3.3 Datasheet - Page 14

• Manufacturer Part Number: LM2678S-3.3
• Description: DC/DC Converter IC
• Manufacturer: National Semiconductor

Specifications of LM2678S-3.3

• Input Voltage: 40V
• Output Voltage: 3.3V
• No. Of Pins: 7
• Termination Type: SMD
• Mounting Type: Through Hole
• Voltage Regulator Type: Buck Switching
• Output Current Max: 5A
• Peak Reflow Compatible (260 C): No
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Application Hints
The switching frequency of 260KHz is the nominal value to
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 3.5A vertical line (I
inductor, or L49, a 33µH inductor could be used. Either
inductor will be suitable, but for this example selecting the
larger inductance will result in lower ripple current.
From Table 1, L48 in a surface mount component is available
from Pulse Engineering with part number P0848.
load
max) indicates that L48 , a 47µH
(Continued)
14
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 47µH
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.