Datasheet

LM2678

SNVS029I –MARCH 2000–REVISED APRIL 2013

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Step 4: Use Table 6 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 provides 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 6 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 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.

ADJUSTABLE OUTPUT DESIGN EXAMPLE

In this example it is desired to convert the voltage from a two battery automotive power supply (voltage range of

20V to 28V, typical in large truck applications) to the 14.8VDC alternator supply typically used to power electronic

equipment from single battery 12V vehicle systems. The load current required is 3.5A maximum. It is also

desired to implement the power supply with all surface mount components.

Step 1: Operating conditions are:

Vout = 14.8V

Vin max = 28V

Iload max = 3.5A

Step 2: Select an LM2678S-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

• V

is the feedback voltage of typically 1.21V (1)

A recommended value to use for R1 is 1K. In this example then R2 is determined to be:

(2)

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.

Product Folder Links: LM2678