AN937 Datasheet, PDF (19/29 Pages) STMicroelectronics – The L4971 is a 1.5 A monolithic dc-dc converter

AN937
Figure 23. Input capacitance rms current vs. duty cycle
I_RMS
D97IN801
0.75
0.5
I_O=1.5A
I_O=1.2A
I_O=1A
0.25
I_O=0.5A
I_O=0.2A
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 D
Typical application
3.3
Output voltage selection
An external voltage divider is required to set the regulated output voltage: I
V_o
=
V_R
E
F
⋅
⎛
⎝
1
+
RR_-----23_{--⎠⎞}
where V_REF is 3.3 V
3.4
Inductor selection
The criteria used in fixing the inductor value has been dictated by the wanted output ripple
voltage, 51mV max., performance obtained of course in combination with output capacitors
too.
The inductor ripple current, fixed at 30% of Iomax, i.e., 0.45 A, requires an inductor value of:
Equation 1
L_O
=
V_O
⋅
(_---1_{-----–----}D_{-----m----i-n----})
ΔI_L ⋅ f_sw
It is possible to plot as a function of Vo and Vccmax at 150 kHz and 200 kHz (see Figure 24
and Figure 25)
Doc ID 5655 Rev 15
19/29