LM2673 datasheet(13/26 Pages) NSC

13 / 26 page

Application Hints (Continued)

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

design and then determine the value of a softstart capacitor

if desired.

Step 8: Define a value for R

ADJ to set the peak switch

current limit to be at least 20% greater than Iout max to allow

for at least 30% inductor ripple current (±15% of Iout). For

designs that must operate over the full temperature range

the switch current limit should be set to at least 50% greater

than Iout max (1.5 x I

out max).

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 2.5A. A

softstart delay time of 50mS is desired. Through-hole com-

ponents are preferred.

Step 1: Operating conditions are:

Vout = 3.3V

Vin max = 16V

Iload max = 2.5A

Step 2: Select an LM2673T-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

2.5A vertical line (I

load max) indicates that L33, a 22µH

inductor, is required.

From Table 1, 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 3 to determine an output capacitor. With a

3.3V output and a 33µH 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:

1 x 220µF/10V Sanyo OS-CON (code C5)

1 x 1000µF/35V Sanyo MV-GX (code C10)

1 x 2200µF/10V Nichicon PL (code C5)

1 x 1000µF/35V Panasonic HFQ (code C7)

Step 5: Use Table 4 to select an input capacitor. With 3.3V

output and 22µH there are three through-hole solutions.

These capacitors provide a sufficient voltage rating and an

rms current rating greater than 1.25A (1/2 I

load max). Again

using Table 2 for specific component characteristics the

following choices are suitable:

1 x 1000µF/63V Sanyo MV-GX (code C14)

1 x 820µF/63V Nichicon PL (code C24)

1 x 560µF/50V Panasonic HFQ (code C13)

Step 6: From Table5a3A or more Schottky diode must be

selected. The 20V rated diodes are sufficient for the appli-

cation and for through-hole components two part types are

suitable:

1N5820

SR302

Step 7: A 0.01µF capacitor will be used for Cboost. For the

50mS softstart delay the following parameters are to be

used:

SST: 3.7µA

SS: 50mS

SST: 0.63V

OUT: 3.3V

SCHOTTKY: 0.5V

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 2.5A plus 50% or 3.75A.

Use a value of 10K

Ω.

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 2A maximum. It is also desired to implement the

power supply with all surface mount components. Softstart is

not required.

Step 1: Operating conditions are:

Vout = 14.8V

Vin max = 28V

Iload max = 2A

Step 2: Select an LM2673S-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 0.15

Ω x 2A or 0.3V and V

D is the voltage drop

across the forward bisased Schottky diode, typically 0.5V.

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