6

ML4775

1.0

3.0

2.0

400

300

200

100

0

0

4.0

5.0

L = 27µH

L = 15µH

L = 56µH

1.0

3.0

2.0

400

300

200

100

0

0

4.0

5.0

L = 27µH

L = 15µH

L = 56µH

Figure 4. Output Current vs Input Voltage.

1.0

3.0

4.0

2.0

100

90

80

70

60

50

0

5.0

L = 56

µH

L = 27

µH

L = 15

µH

1.0

3.0

2.0

100

90

80

70

60

50

0

L = 56

µH

L = 27

µH

L = 15

µH

In the two cell application previously described, a

maximum input voltage of 3V would give a peak current

of 1.2A. 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 ML4775 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 Applications

Note 29, “Choosing an Inductor for Your ML4861

Application.”

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 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 15µH, the

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

the efficiency approaches 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)