AN938 Datasheet, PDF (20/33 Pages) STMicroelectronics – The L4973 family is a 3.5 A monolithic DC-DC converter

Typical application
AN938
Figure 26.
Ideal inductor value requested for Figure 27.
30% ripple current, as a function of
max. input voltage and output
(f_SW=150 kHz)
Ideal inductor value requested for
30% ripple current, as a function of
max. input voltage and output
(f_SW=200 kHz)





















6OUT
6IN 6
6IN 6
6IN 6
6IN 6
6IN 6
6IN 6
6IN 6
!-V





















6OUT
6IN   6
6IN   6
6IN   6
6IN   6
6IN   6
6IN   6
6IN   6
!-V
The 68 µH value of the selected inductor is useful to keep the current ripple below 0.3*I_LOAD
over the input / output voltage range.
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 chosen total capacitance is 2 x 150 µF/35 V KY (Nippon Chemi-con), each of which has
an ESR of 150 mΩ at 20°C. The ideal ripple voltage over the input voltage range is shown in
Figure 28.
Figure 28. Ideal ripple voltage as a function of input and output voltage (f_SW=150
kHz)
60
55
50
45
40
35
30
25
20
15
10
10
15
20
25
30
35
40
45
Vin
Vout = 1.5 V
Vout = 2.5 V
Vout = 3.3 V
Vout = 5 V
Vout = 12 V
51 mV
AM03510v1
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Doc ID 5410 Rev 10