6

ML4875

1.0

3.0

2.0

400

300

200

100

0

0

ML4875-3.3

4.0

5.0

L = 27µH

L = 15µH

L = 56µH

1.0

3.0

2.0

400

300

200

100

0

0

ML4875-5.0

4.0

5.0

L = 27µH

L = 15µH

L = 56µH

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 ML4875 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.

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

tolerances. Assuming that 2V is the end of life voltage of a

two cell input, Figure 4 shows that with a 2V input, the

ML4875-5 delivers 99mA with a 27µH inductor.

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. The data used to generate

Figures 4 and 5 is provided in Table 1.

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)

Figure 4. Output Current vs Input Voltage.

1.0

3.0

2.0

400

300

200

100

0

0

ML4875-3.0

4.0

L = 27µH

L = 15µH

L = 56µH