AN937

Typical application

Doc ID 5655 Rev 15

21/29

3.5

Output capacitors

The output capacitors selection, Co, is mainly driven by the output ripple voltage that has to

be guaranteed, in this case 1% max. of Vo.

The output ripple is defined by the ESR of Co and by the ripple current flowing through it.

The output capacitance is of 150

μF/35 V KY (Nippon Chemi-con), having an ESR of 150

m

Ω at 20 °C. The ideal ripple voltage over the input voltage range is shown in Figure 26.

Figure 26.

kHz)

Co has also to support load transients. An idea of the magnitude of the output voltage drop

during load transients is given below:

Equation 2

where

ΔI

Vo nominal is 5.1 V, and finally L is 68

μH.

Equation 2, normalized at Vo, is represented in the following diagram, Fig. 27, as a function

of the minimum input voltage.

These curves are represented for different output capacitor 2x150

μF, 2x220 μF, 2x330 μF

all KY, 35 V.

10

15

20

25

30

35

40

45

10

18

26

34

42

50

58

66

74

82

90

Vout = 1.5V

Vout = 2.5V

Vout = 3.3V

Vout = 5V

Vout = 12V

51 mV

Vin

ΔV

O

ΔI

O

()

2

L

O

⋅

2C

O

V

inmin

D

max

V

O

–

⋅

()

⋅⋅

--------------------------------------------------------------------------

=