Datasheet

LM2678
www.ti.com SNVS029I –MARCH 2000–REVISED APRIL 2013
Step 3: To use the nomograph for the adjustable device, Figure 24, requires a calculation of the inductor
Volt•microsecond constant (E•T expressed in V•μS) from the following formula:
where
• V
SAT
is the voltage drop across the internal power switch which is R
ds(ON)
times I
load
(3)
In this example this would be typically 0.12Ω x 3.5A or 0.42V and V
D
is the voltage drop 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 energy is stored in the inductor.
For this example E•T is found to be:
(4)
(5)
Using Figure 24, the intersection of 27V•μS horizontally and the 3.5A vertical line (I
load
max) indicates that L48 , a
47μH 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.
Step 4: Use Table 12 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 solutions. Table 3 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)
NOTE
When using the adjustable device in low voltage applications (less than 3V output), if the
nomograph, Figure 24, selects an inductance of 22μH or less, Table 12 and Table 13 do
not provide an output capacitor solution. With these conditions 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 12 and Table 13.
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 3 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
adequate.
Step 6: From Table 10 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.
VSON PACKAGE DEVICES
The LM2678 is offered in the 14 lead VSON surface mount package to allow for a significantly decreased
footprint with equivalent power dissipation compared to the DDPAK. For details on mounting and soldering
specifications, refer to Application Note AN-1187 at
www.ti.com/lsds/ti/analog/powermanagement/power_portal.page.
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