AN937 Datasheet, PDF (21/29 Pages) STMicroelectronics – The L4971 is a 1.5 A monolithic dc-dc converter

AN937
3.5
Typical application
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
mΩ at 20 °C. The ideal ripple voltage over the input voltage range is shown in Figure 26.
Figure 26. Ideal ripple voltage as a function of input and output voltage (f_SW=200
kHz)
90
82
74
66
58
50
42
34
26
18
10
10
15
20
25
30
35
40
45
Vin
Vout = 1.5V
Vout = 2.5V
Vout = 3.3V
Vout = 5V
Vout = 12V
51 mV
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
ΔV_O
=
-----------------------⁽---^Δ----^I--^O----⁾--²----^â‹---^L----^O-------------------------
2 ⋅ C_O ⋅ (V_inmin ⋅ D_max – V_O)
where ΔI_o is the load current change, from 0.5 A to 1.5 A, Dmax is the max. duty cycle, 95%,
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.
Doc ID 5655 Rev 15
21/29