Datasheet

LM2673

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SNVS030N –APRIL 2000–REVISED APRIL 2013

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 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 17). For the adjustable device the output voltage is set by the following relationship:

where

• V

is the feedback voltage of typically 1.21V (3)

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

(4)

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.

Step 3: To use the nomograph for the adjustable device, Figure 21, requires a calculation of 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 I

load

(5)

In this example this would be typically 0.15Ω x 2A or 0.3V and V

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:

(6)

(7)

Using Figure 21, 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.

Product Folder Links: LM2673