A system logic power supply bus of 3.3 V is to be generated from a wall adapter which provides an unregulated DC voltage of 13 V to 16 V. The maximum load current is 2.5 A. Through-hole components are preferred.

Step 1: Operating conditions are:

• V_OUT = 3.3 V
• V_IN maximum = 16 V
• I_LOAD maximum = 2.5 A

Step 2: Select an LM2676T-3.3. The output voltage has a tolerance of ±2% at room temperature and ±3% over the full operating temperature range.

Step 3: Use the nomograph for the 3.3-V device (Figure 8-2). The intersection of the 16-V horizontal line (V_IN max) and the 2.5-A vertical line (I_LOAD max) indicates that L33, a 22-µH inductor, is required.

From Table 8-3, L33 in a through-hole component is available from Renco with part number RL-1283-22-43 or part number PE-53933 from Pulse Engineering.

Step 4: Use Table 8-5 or Table 8-6 to determine an output capacitor. With a 3.3-V output and a 22-µH 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 8-1 or Table 8-2 provides the actual capacitor characteristics. Any of the following choices work in the circuit:

• 1 × 220-µF, 10-V Sanyo OS-CON (code C5)
• 1 × 1000-µF, 35-V Sanyo MV-GX (code C10)
• 1 × 2200-µF, 10-V Nichicon PL (code C5)
• 1 × 1000-µF, 35-V Panasonic HFQ (code C7)

Step 5: Use Table 8-7 or Table 8-8 to select an input capacitor. With a 3.3-V output and 22 µH, there are three through-hole solutions. These capacitors provide a sufficient voltage rating and an RMS current rating greater than 1.25 A (1/2 I_LOAD max). Again, using Table 8-1 or Table 8-2 for specific component characteristics, the following choices are suitable:

• 1 × 1000-µF, 63-V Sanyo MV-GX (code C14)
• 1 × 820-µF, 63-V Nichicon PL (code C24)
• 1 × 560-µF, 50-V Panasonic HFQ (code C13)

Step 6: From Table 8-4, a 3-A Schottky diode must be selected. For through-hole components, 20-V rated diodes are sufficient and two part types are suitable:

• 1N5820
• SR302

Step 7: A 0.01-µF capacitor is used for C_BOOST.