Datasheet

LM2679

www.ti.com

SNVS026N –MARCH 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 3.5A 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 = 3.5A

Step 2: Select an LM2679S-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:

(3)

Where V

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:

(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:

(5)

where V

SAT

is the voltage drop across the internal power switch which is R

ds(ON)

times I

load

. In this example this

would be typically 0.12Ω x 3.5A or 0.42V 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 3.5A vertical line (I

load

max) indicates that L48 , a

47μH inductor, or L49, a 33μH inductor could be used. Either inductor will be suitable, but for this example

selecting the larger inductance will result in lower ripple current.

From Table 1, L48 in a surface mount component is available from Pulse Engineering with part number P0848.

Product Folder Links: LM2679