AN938 Datasheet, PDF (27/33 Pages) STMicroelectronics – The L4973 family is a 3.5 A monolithic DC-DC converter

AN938
4
Applications
Applications
4.1
Mains transformer power supply with output adjustable from
0 V to 24 V
Figure 37 shows a power supply including mains transformer working at 50 Hz 220Vac or 60
Hz 110Vac, bridge rectifier and filtering capacitor. The output voltage is adjustable from 0 to
24 V.
Figure 37. Typical adjustable 0-24 V power supply
220VAC
28VAC
R2
C1 C2 C7
7,8
12
1
9
17 L4973V3.3
16
4,5,6 ^2,3
11 10 13,14,15
C3 C4 C5
R1
C6
R5 4.3K
C8
L1
3x
D1 _C0
V_O
R3
47K
C12
R4
10K
The formula to evaluate the output voltage is:
AM03519v1
Equation 21
V_o
=
3.336
+
3.336
⋅
⎛
⎝
RR_-----34_--
+
RR_-----35_{--⎠⎞}
–
5.1
⋅
R_-----3_--
R5
Figure 38 shows the formula plot in which it is possible to program the output voltage from 0
V to 24 V using a 10 kΩ potentiometer as R4. R5 is fixed at 4.3 kΩ and R3 is 47 kΩ.
Output capacitors have to be chosen with low ESR to reduce the output ripple voltage and
load transients.
Doc ID 5410 Rev 10
27/33