6

ML4761

100

1.0

3.0

2.0

200

300

400

500

0

L = 10µH

L = 15µH

L = 27µH

L = 56µH

100

1.0

3.0

4.0

2.0

200

300

400

500

0

L = 10µH

L = 15µH

L = 27µH

L = 56µH

70%

13

2

75%

80%

85%

90%

65%

0

L = 10µH

L = 15µH

L = 27µH

L = 56µH

95%

75%

1.0

3.0

4.0

2.0

80%

85%

90%

95%

70%

0

L = 10µH

L = 15µH

L = 27µH

L = 57µH

Figure 5 shows efficiency under the conditions used to

create Figure 4. It can be seen that efficiency is mostly

independent of input voltage and is closely related to

inductor value. This illustrates the need to keep the

inductor value as high as possible to attain peak system

efficiency. As the inductor value goes down to 10µH, the

efficiency drops to between 70% and 75%. With 56µH,

the efficiency exceeds 90% and there is little room for

improvement. At values greater than 100µH, the operation

of the synchronous rectifier becomes unreliable because

the inductor current is so small that it is difficult for the

control circuitry to detect.

After the appropriate inductor value is chosen, it is

necessary to find the minimum inductor current rating

required. Peak inductor current is determined from the

following formula:

I

TV

L

L PEAK

ON MAX

IN MAX

MIN

()

()

()

=

×

(3)

When comparing various inductors, it is important to keep

in mind that suppliers use different criteria to determine

their ratings. Many use a conservative current level, where

inductance has dropped to 90% of its normal level. In any

case, it is a good idea to try inductors of various current

ratings with the ML4761 to determine which inductor is

the best choice. Check efficiency and maximum output

current, and if a current probe is available, look at the

inductor current to see if it looks like the waveform shown

in Figure 3. For additional information, see Application

Note 29.

Suitable inductors can be purchased from the following

suppliers:

Coilcraft

(708) 639-6400

Coiltronics

(407) 241-7876

Dale

(605) 665-9301

Sumida

(708) 956-0666

Figure 4. Output Current vs. Input Voltage.