Datasheet

LM2679
SNVS026N –MARCH 2000–REVISED APRIL 2013 www.ti.com
Step 2: Select an LM2679T-3.3. The output voltage will have a tolerance of
±2% at room temperature and ±3% over the full operating temperature range.
Step 3: Use the nomograph for the 3.3V device, Figure 18. The intersection of the 16V horizontal line (V
in
max)
and the 4A vertical line (I
load
max) indicates that L46, a 15μH inductor, is required.
From Table 1, L46 in a through-hole component is available from Renco with part number RL-1283-15-43.
Step 4: Use Table 6 and Table 7 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 capacitors to be paralleled and an
identifying capacitor code given. Table 3 and Table 4 provide 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 8 and Table 9 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
load
max). Again using Table 3 and Table 4 for specific component characteristics the following 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 10, a 5A or more Schottky diode must be selected. For through-hole components only 40V
rated diodes are indicated and 4 part types are suitable:
1N5825
MBR745
80SQ045
6TQ045
Step 7: A 0.01μF capacitor will be used for Cboost. For the 50mS softstart delay the following parameters are to
be used:
I
SST
: 3.7μA
t
SS
: 50mS
V
SST
: 0.63V
V
OUT
: 3.3V
V
SCHOTTKY
: 0.5V
V
IN
: 16V
Using Vin max ensures that the softstart delay time will be at least the desired 50mS.
Using the formula for Css a value of 0.148μF is determined to be required. Use of a standard value 0.22μF
capacitor will produce more than sufficient softstart delay.
Step 8: Determine a value for R
ADJ
to provide a peak switch current limit of at least 4A plus 50% or 6A.
(2)
Use a value of 6.2KΩ.
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