6

ML4790

Figure 7 shows efficiency under the conditions used to

create Figure 6. 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 10

µH, the

efficiency drops to between 70% and 75%. With 47

µH,

the efficiency reaches approximately 90% and there is

little room for improvement. At values greater than 47

µH,

the operation of the synchronous rectifier becomes

unreliable at low input voltages because the inductor

current is so small that it is difficult for the control circuitry

to detect as shown for the 5.5V output.

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

()

()

()

=

×

(2)

It is important to note that for reliable operation, make

current rating. In the two cell application previously

described, a maximum input voltage of 3V would give a

peak current of 880mA. 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 ML4790 to

determine which inductor is the best choice. Check

Figure 6. Output Current versus Input Voltage.

200

180

160

140

120

100

80

60

40

20

0

1.0

2.0

3.0

4.0

5.0

L = 10

µH

L = 22

µH

L = 47

µH

35

30

25

20

15

10

5

0

1.0

1.2

1.4

1.6

1.8

2.0

L = 10

µH

L = 22

µH

L = 47

µH

140

120

100

80

60

40

20

0

1.0

1.5

2.0

2.5

3.0

L = 10

µH

L = 22

µH

L = 47

µH

180

160

140

120

100

80

60

40

20

0

1.0

1.5

2.0

2.5

3.0

3.5

4.0

L = 10

µH

L = 22

µH

L = 47

µH