LM2678T-3.3 National Semiconductor, LM2678T-3.3 Datasheet - Page 14

• Manufacturer Part Number: LM2678T-3.3
• Description: IC, STEP-DOWN REGULATOR, TO-220-7
• Manufacturer: National Semiconductor

Specifications of LM2678T-3.3

• Primary Input Voltage: 12V
• No. Of Outputs: 1
• Output Voltage: 3.3V
• Output Current: 5A
• No. Of Pins: 7
• Operating Temperature Range: -40°C To +125°C
• Supply Voltage Range: 8V To 40V
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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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 in-
ductor 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.
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 solu-
tions. 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,
load
max) indicates that L48 , a 47μH
14
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 ei-
ther 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 1.75A (1/2 Iout
max). From Table 2 it can be seen that C12, a 33μF/35V ca-
pacitor from Sprague, has the highest voltage/current rating
of the surface mount components and that two of these ca-
pacitor in parallel will be adquate.
Step 6: From Table 5 a 5A or more Schottky diode must be
selected. For surface mount diodes with a margin of safety on
the voltage rating one of two diodes can be used:
MBRD1545CT
6TQ045S
Step 7: A 0.01μF capacitor will be used for Cboost.
LLP PACKAGE DEVICES
The LM2678 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 Ap-
plication Note AN-1187.