5

ML4866

DESIGN CONSIDERATIONS

INDUCTOR SELECTION

Figure 1 shows the inductor current in a step-down

converter operating in CCM. Note that the inductor

current does not reach zero during each switching cycle.

This is unlike discontinuous conduction mode (DCM)

where the inductor current is allowed to reach zero. CCM

operation generally results in lower peak to peak output

ripple voltage and higher circuit efficiencies because of

lower peak and RMS currents in the switching FETs and

buck inductor. The minimum value of inductance required

for CCM operation with a 6.5V input and a load range of

100mA to 500mA is:

L

VV

V

VI

f

OUT

IN MAX

OUT

IN MAX

OUT MIN

SW

>

™-

™™

™

()

()

()

(

)

2

(1)

L

VV

V

VmAkHz

H

>

™-

™™

™

>

33

6 5

33

2 6 5

100

120

68

.( .

.

)

.

m

To guarantee reliable operation, the peak inductor current

must be between 80% and 85% of its maximum rated

value. This value is the sum of the inductor peak to peak

current and the maximum output current:

I

VV

V

Vf

L

LP P

OUT MAX

IN MIN

OUT MAX

IN MIN

SW MIN

()

()

(

)

()

()

(

)

()

–

=

™™

-

™™

2

(2)

I

VV

V

VkHz

H

mA

LP P

()

.( .

.

)

.

–

=

™™

-

™™

m

=

2 3 465

4 0

3 465

40

90

100

103

II

VV

V

Vf

L

LPEAK

OUT MAX

OUT MAX

IN MIN

OUT MAX

IN MIN

SW

()

(

)

()

(

)

()

()

()

=+

™-

™™

(3)

II

VV

V

VkHz

H

mA

L PEAK

OUT MAX

()

(

)

.( .

.

)

.

=+

™-

™™

m

=

3465

40

3465

40

120

100

550

For the highest efficiency, inductor core and copper losses

must be minimized. Good high frequency core material

such as Kool-Mu, ferrite or Molyperm are popular choices

for this converter. Disregarding physical size

requirements, the lowest loss inductor will generally be

the one with the highest peak current rating.

Figure 2 displays the efficiency of the ML4866 under

various input voltage and output current conditions. These

results were obtained using a Coiltronics CTX100-4

inductor having the following specifications:

Nominal Inductance - 100µH

Peak Current Rating - 950mA

DC Resistance - 175m

W

A partial listing of inductor manufacturers with standard

parts which meet the criteria for use with the ML4866 is

given below.

Coiltronics

(561) 241-7876

Dale

(605) 665-9301

Coilcraft

(847) 639-6400

XFMRS, Inc (317) 834-1066

Sumida

(847) 956-0666

CAPACITOR SELECTION

A typical digital system requires a peak to peak output

ripple voltage of no greater than 1% to 3% of the nominal

output voltage. In a step-down converter, the largest

contributor to ripple voltage is almost always the product

of the inductor peak-to-peak current times the output

capacitor’s equivalent series resistance. To select the

correct capacitor, first calculate the minimum

capacitance value required:

C

VV

V

VV

L

f

OUT

OUT

IN MAX

OUT

P P MAX

IN MAX

SW

>

™-

™™

™ ™

-

()

()

()

()

8

2

(4)

C

VV

mV

V

H

kHz

F

™-

™™

™

m ™

>

33

6 5

33

8 33

6 5

100

120

427

2

.( .

.

)

.

.

m

Next, calculate the maximum permissible ESR of the

output capacitor:

ESR

<<

(.

)

(. )

.

0033

01

033

W

(5)

When limited space is available, tantalum capacitors are

the best choice. Electrolytic capacitors can be used and

will be less expensive, but the ESR for low capacitance

values as needed here will be much higher than for the

same value tantalum. Table 2 lists the ESR values for a

number of general purpose tantalum capacitors which are

widely available from a number of sources. A 47µF

capacitor was chosen for the design example.

Figure 2. Efficiency vs. Input Voltage

INPUT VOLTAGE (V)

3.5

4.5

6.0

4.0

5.0

6.5

5.5

100

98

96

94

92

90

88

86

IOUT = 500mA

IOUT = 100mA

IOUT = 10mA