LM2678-5.0 National Semiconductor, LM2678-5.0 Datasheet - Page 12

• Manufacturer Part Number: LM2678-5.0
• Description: SIMPLE SWITCHER High Efficiency 5A Step-Down Voltage Regulator
• Manufacturer: National Semiconductor

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Application Hints
Step 4: Use Table 3 to determine an output capacitor. With a
3.3V output and a 15µH inductor there are four through-hole
output capacitor solutions with the number of same type ca-
pacitors to be paralleled and an identifying capacitor code
given. Table 2 provides the actual capacitor characteristics.
Any of the following choices will work in the circuit:
2 x 220µF/10V Sanyo OS-CON (code C5)
2 x 820µF/16V Sanyo MV-GX (code C5)
1 x 3900µF/10V Nichicon PL (code C7)
2 x 560µF/35V Panasonic HFQ (code C5)
Step 5: Use Table 4 to select an input capacitor. With 3.3V
output and 15µH there are three through-hole solutions.
These capacitors provide a sufficient voltage rating and an
rms current rating greater than 2A (1/2 I
ing Table 2 for specific component characteristics the follow-
ing choices are suitable:
2 x 680µF/63V Sanyo MV-GX (code C13)
1 x 1200µF/63V Nichicon PL (code C25)
1 x 1500µF/63V Panasonic HFQ (code C16)
Step 6: From Table 5 a 5A or more Schottky diode must be
selected. For through-hole components only 40V rated di-
odes are indicated and 4 part types are suitable:
1N5825
MBR745
80SQ045
6TQ045
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 re-
quired is 3.5A 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 = 3.5A
Step 2: Select an LM2678S-ADJ. To set the output voltage
to 14.9V two resistors need to be chosen (R1 and R2 in Fig-
ure 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 = 11.23K
The closest standard 1% tolerance value to use is 11.3K
This will set the nominal output voltage to 14.88V which is
within 0.5% of the target value.
FB
is the feedback voltage of typically 1.21V.
(Continued)
load
max). Again us-
12
Step 3: To use the nomograph for the adjustable device, Fig-
ure
Volt • microsecond constant (E • T expressed in V • µS) from
the following formula:
where V
switch which is R
be typically 0.12 x 3.5A or 0.42V and V
across the forward bisased Schottky diode, typically 0.5V.
The switching frequency of 260KHz is the nominal value to
use to estimate the ON time of the switch during which en-
ergy 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 in-
ductor 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,
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 op-
eration 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 capacitor from Sprague, has the highest
voltage/current rating of the surface mount components and
that two of these capacitor 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.
6 ,
SAT
requires
is the voltage drop across the internal power
ds(ON)
a
load
times I
max) indicates that L48 , a 47µH
calculation
load
. In this example this would
D
of
is the voltage drop
the
inductor