LM2670 datasheet(14/26 Pages) NSC

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Application Hints (Continued)

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

Step 6: From Table5a3A Schottky diode must be selected.

For through-hole components 20V rated diodes are sufficient

and 2 part types are suitable:

1N5820

SR302

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 2A 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 = 2A

Step 2: Select an LM2670S-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.

The switching frequency of 260KHz is the nominal value to

use to estimate the ON time of the switch during which

energy is stored in the inductor.

For this example E

•T is found to be:

Using Figure 6, the intersection of 27V

•µS horizontally and

the 2A vertical line (I

load max) indicates that L38 , a 68µH

inductor, should be used.

From Table 1, L38 in a surface mount component is available

from Pulse Engineering with part number PE-54038S.

Step 4: Use Table 6 to determine an output capacitor. With a

14.8V output the 12.5 to 15V row is used and with a 68µH

inductor there are three surface mount output capacitor so-

lutions. Table 2 provides the actual capacitor characteristics

based on the C Code number. Any of the following choices

can be used:

1 x 33µF/20V AVX TPS (code C6)

1 x 47µF/20V Sprague 594 (code C8)

1 x 47µF/20V Kemet T495 (code C8)

Important Note: When using the adjustable device in low

voltage applications (less than 3V output), if the nomograph,

Figure 6, selects an inductance of 22µH or less, Table 6 does

not provide an output capacitor solution. With these condi-

tions the number of output capacitors required for stable

operation becomes impractical. It is recommended to use

either a 33µH or 47µH inductor and the output capacitors

from Table 6.

Step 5: An input capacitor for this example will require at

least a 35V WV rating with an rms current rating of 1A (1/2

Iout max). From Table 2 it can be seen that C12, a 33µF/35V

capacitor from Sprague, has the required voltage/current

rating of the surface mount components.

Step 6: From Table5a3A Schottky diode must be selected.

For surface mount diodes with a margin of safety on the

voltage rating one of five diodes can be used:

SK34

30BQ040

30WQ04F

MBRS340

MBRD340

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

LLP PACKAGE DEVICES

The LM2670 is offered in the 14 lead LLP surface mount

package to allow for a significantly decreased footprint with

equivalent power dissipation compared to the TO-263. For

details on mounting and soldering specifications, refer to

Application Note AN-1187.

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