Datasheet

LM2677
www.ti.com SNVS077I –MAY 2004–REVISED JUNE 2012
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
In this example this would be typically 0.15Ω x 2A or 0.3V 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:
Using Figure 24, the intersection of 27V•μS horizontally and the 2A vertical line (I
load
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 10 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 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 24, selects an inductance of 22μH or less, Table 10 does 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 10.
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 it can be seen that C12, a 33μF/35V capacitor from Sprague, has the required
voltage/current rating of the surface mount components.
Step 6: From Table 8 a 3A Schottky diode must be selected. For surface mount diodes with a margin of safety
on the voltage rating one of five diodes can be used:
SK34
30BQ040
30WQ04F
MBRS340
MBRD340
Step 7: A 0.01μF capacitor will be used for Cboost.
VSON PACKAGE DEVICES
The LM2677 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/TO-263. For details on mounting and
soldering specifications, refer to Application Note AN-1187 (SNOA401).
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