LM2678 datasheet(13/27 Pages) NSC

13 / 27 page

Application Hints (Continued)

Using the nomographs and tables in this data sheet (or use

the available design software at http://www.national.com) a

complete step-down regulator can be designed in a few

simple steps.

Step 1: Define the power supply operating conditions:

Required output voltage

Maximum DC input voltage

Maximum output load current

Step 2: Set the output voltage by selecting a fixed output

LM2678 (3.3V, 5V or 12V applications) or determine the

required feedback resistors for use with the adjustable

LM2678−ADJ

Step 3: Determine the inductor required by using one of the

four nomographs, Figure 3 through Figure 6. Table 1 pro-

vides a specific manufacturer and part number for the induc-

tor.

Step 4: Using Table 3 (fixed output voltage) or Table 6

(adjustable output voltage), determine the output capaci-

tance required for stable operation. Table 2 provides the

specific capacitor type from the manufacturer of choice.

Step 5: Determine an input capacitor from Table 4 for fixed

output voltage applications. Use Table 2 to find the specific

capacitor type. For adjustable output circuits select a capaci-

tor from Table 2 with a sufficient working voltage (WV) rating

greater than Vin max, and an rms current rating greater than

one-half the maximum load current (2 or more capacitors in

parallel may be required).

Step 6: Select a diode from Table 5. The current rating of the

diode must be greater than I load max and the Reverse

Voltage rating must be greater than Vin max.

Step 7: Include a 0.01µF/50V capacitor for Cboost in the

design.

FIXED OUTPUT VOLTAGE DESIGN EXAMPLE

A system logic power supply bus of 3.3V is to be generated

from a wall adapter which provides an unregulated DC volt-

age of 13V to 16V. The maximum load current is 4A.

Through-hole components are preferred.

Step 1: Operating conditions are:

Vout = 3.3V

Vin max = 16V

Iload max = 4A

Step 2: Select an LM2678T-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 3.

The intersection of the 16V horizontal line (V

in max) and the

4A vertical line (I

load max) indicates that L46, a 15µH induc-

tor, 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 3 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 2 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 4 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 2 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 Table5a5A or more Schottky diode must be

selected. For through-hole components only 40V rated di-

odes 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 alter-

nator 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 2). For the adjustable device the output voltage is set

by the following relationship:

Where V

FB is the feedback voltage of typically 1.21V.

A recommended value to use for R1 is 1K. In this example

then R2 is determined to be:

R2 = 11.23K

Ω

The closest standard 1% tolerance value to use is 11.3K

Ω

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

requires

calculation

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 Iload. In this example this would

be typically 0.12

Ω x 3.5A or 0.42V and V

D is the voltage drop

across the forward bisased Schottky diode, typically 0.5V.

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