AN938 Datasheet, PDF (19/33 Pages) STMicroelectronics – The L4973 family is a 3.5 A monolithic DC-DC converter

AN938
Typical application
3.3
Output voltage selection
The two available devices can directly regulate the output voltage of 3.3 V for the L4973V3.3
and 5.1 V for the L4973V5.1. Each of the two devices can regulate an output voltage higher
than the nominal value, up to 40 V, by adding an external voltage divider.
In case of requested output voltage lower than 3.3 V, it is possible to use the L4973V3.3 with
the external connections as shown in Figure 24 with the function below plotted in Figure 25.
Equation 10
V_o
=
V_fb
–
(V_ref
–
V_fb)
⋅
R_-----3_--
R5
Figure 24. Example of output regulated
voltage lower than 3.3 V
Figure 25. Output voltage vs. R5 using R3 as
parameter
2_
6REF
,
6&"
2_
2_/54
!-V
6_/
6








+
+
+
+
+
27
    27
!-V
Only two resistors are used and no curve intercepts 3.3 V, but all curves have an asymptotic
trend to this value.
3.4
Inductor selection
The criteria used in fixing the inductor value have been dictated by the desired output ripple
voltage, 50 mV max., performance obtained of course in combination with output capacitors
too.
The inductor ripple current, fixed at 30% of Iomax, i.e., 1.05 A, requires an inductor value of:
Equation 11
L_O
=
V_O
⋅
(_---1_{-----–----}D_{-----m----i-n---})_-
ΔI_L ⋅ f_sw
It is possible to plot Equation 11 as a function of Vo and V_ccmax at 150 kHz and 200 kHz (see
Figure 26, 27).
Doc ID 5410 Rev 10
19/33