LM2676
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SNVS031J –APRIL 2000–REVISED APRIL 2013
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 20). For the adjustable device the output voltage is set by the following relationship:
where
•
VFB is the feedback voltage of typically 1.21V
(1)
(2)
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 24, requires a calculation of the inductor
Volt•microsecond constant (E•T expressed in V•μS) from the following formula:
where
•
VSAT is the voltage drop across the internal power switch which is Rds(ON) times Iload
(3)
In this example this would be typically 0.15Ω x 2A or 0.3V and VD is the voltage drop across the forward biased
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 2A vertical line (Iload max) indicates that L38 , a
68μH 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 12 or Table 13 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 solutions. Table 3 or Table 4
provide 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 or Table 13.
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 3 or Table 4 it can be seen that C12, a 33μF/35V capacitor from Sprague, has the
required voltage/current rating of the surface mount components.
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