7

ML4790

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 5.

The DC resistance of the inductor should be kept to a

minimum to reduce losses. A good rule of thumb is to

allow 5 to 10m

Ω of resistance for each µH of inductance.

Also, be aware that the DC resistance of an inductor

usually isn‘t specified tightly, so an inductor with a

maximum DC resistance spec of 150m

Ω may actually

have 100m

Ω of resistance.

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

BOOST CAPACITOR

The boost capacitor (C2) supplies current to the load

during the ON-time of Q1 and will limit the ripple the

LDO stage has to contend with. The ripple on C2 is

influenced by three capacitor parameters: capacitance,

ESL, and ESR. The contribution due to capacitance can be

determined by looking at the change in the capacitor

voltage required to store the energy delivered by the

inductor in a single charge-discharge cycle, as given by

the following formula:

C

TV

LV

V

V

in Farads

ON

IN

BOOST

OUT

IN

2

2

22

≥

×

××

×

∆

(–

)

()

(3)

For example, a 2.4V input, a 5V output, a 22

µH inductor,

and an allowance of 100mV of ripple on the boost

capacitor results in a minimum C2 value of 15

µF.

100

95

90

85

80

75

70

65

60

55

50

1.0

2.0

3.0

4.0

5.0

L = 10

µH

L = 22

µH

L = 47

µH

100

95

90

85

80

75

70

65

60

55

50

1.0

1.2

1.4

1.6

1.8

2.0

L = 10

µH

L = 22

µH

L = 47

µH

100

95

90

85

80

75

70

65

60

55

50

1.0

1.5

2.0

2.5

3.0

3.5

4.0

L = 22

µH

L = 10

µH

L = 47

µH

100

95

90

85

80

75

70

65

60

55

50

1.0

1.5

2.0

2.5

3.0

L = 10

µH

L = 47

µH

L = 22

µH