8

Rev. B, July 2000

Equation (3-8) can be obtained as in Equation (3-6) by referring to Figure 3-5:

Rearranging the above equation (3-9) in terms of D,

The efficiency (

η) can be obtained from equation (3-11) using the input to output current ratio,

As shown by equations (3-9) and (3-11), the forward voltage drop of the switch MOSFET and

the freewheeling diode reduces the output voltage and the entire system efficiency. Besides

these factors, the inductor coil resistance and the capacitor ESR also drop the efficiency.

Hence, a low coil resistance inductor and low ESR capacitors must be used. In the case of the

capacitor, it is advantageous to use several capacitors in parallel.

A high quality output DC voltage with low ripple, if possible, is considered the best. The follow-

ing is a description of the Buck converter output voltage ripple.

Figure 3-6. Inductor Current

Ripples appear in the output voltage as the L2 current ripple component,

∆i

output voltage ripple component. First, the following equation calculates the

∆i

V

in

v

()

–

V

out

–

()DT

s

V

out

v

()

+

() 1D

–

()T

–

()

=

V

out

DV

in

Dv

() v

() 1D

–

()

+

[]… 39

–

()

–

=

D

V

out

v

() 1D

–

()

+

V

in

v

()

–

-----------------------------------------------------

………………… 310

–

()

=

η

V

V

-----------------------

V

out

V

in

------------

V

in

v

()

–

V

out

v

() 1D

–

()

+

-----------------------------------------------------

1

……… 311

–

()

<

==

∆i

∆i

L

t

∆Q

t1

t2

t3

∆i

L

DT

s

V

in

V

out

–

L

---------------------------

D1

D

–

()V

L

-----------------------------------------

……… 312

–

()

=

=