AN938 Datasheet, PDF (30/33 Pages) STMicroelectronics – The L4973 family is a 3.5 A monolithic DC-DC converter

Applications
AN938
4.4
Current generator
Sometimes the application specifications requires generating constant current, fixed or
adjustable, for chemical processes, lamp powering, or battery chargers for lead acids, ni-cd
and ni-me-hyd batteries.
We give one suggestion for obtaining an accurate constant current generator.
The schematic in Figure 41 shows a simple solution for constant or variable current
generator, with good accuracy of the current, using a simple op/amp supplied by the 5.1 V
available reference voltage.The threshold current is fixed by the voltage divider connected to
the n.i. input op/amp, at about 100 mV. This is also the maximum voltage dropping on the
current sense resistor R_s. Adjusting the 4.7 kΩ, the threshold can be easily changed.
The formula to set the output current is:
Equation 24
I_o = ₍---_R----₁-⁵---^.-_â‹-¹--_R---^â‹-₂--^R-₎---²-_â‹---_R----_s-
Figure 41. Constant current generator up to 3.5 A
13V<Vin<45V
470μF
7,8
2,3
140μH
6V to 12V/2A
5.1
+
E/A
-
PWM
OUTPUT
STAGE
12
4,5,6
1
11 13,14,15
16
17
1μF
1μF
STPS
745D
220μF
5.1K
100nF
R2 100
R1 4.7K
+
-
LM358
55mΩ
Rs
V+
100
AM03523v1
4.5
From positive input to negative output
Figure 42 shows how to obtain a -12 V, when only a positive supply is available.
The maximum output current is given by the formula Iout = 3.5 × (1-D) according to the
relation for the buck-boost topology.
This negative output has to show good precision, stability and regulation, and it must be
protected from output short-circuit.
With the suggested application schematic, one of the aims is to guarantee the performance
as specified above and to contribute to the simplification of the power transformer, mains or
high frequency.
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Doc ID 5410 Rev 10