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AN2846 Scheda tecnica(PDF) 7 Page - STMicroelectronics

Il numero della parte AN2846
Spiegazioni elettronici  The serial current limited termination device
PDF  35 Pages
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Produttore elettronici  STMICROELECTRONICS [STMicroelectronics]
Homepage  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

AN2846 Scheda tecnica(HTML) 7 Page - STMicroelectronics

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AN2846
Application guidelines
Doc ID 15130 Rev 1
7/35
Figure 5.
Input characteristic according to IEC 61131-2 type 3
Current limited inputs allow reduced power dissipation into the device as well as reduced
power needed by the external supplies. A typical application circuit schematic is shown in
Figure 31 in the application section.
Figure 6 displays the SCLT3-8 input stage configuration and its typical input threshold
voltages. Low frequency triangular waveform as the input voltage has been used to better
highlight the voltage thresholds. Input current (IIN) and voltage across the LEDs are also
displayed in Figure 6. The VLED wave shape shows clearly the on-off states of the SCLT3-8.
Figure 6.
Input stage
With RIN = 2.2 kΩ, the typical VI_ON and VI_OFF threshold voltages are respectively 9.5 V
and 8.5 V, consistent with the 11 V min. and 5 V max specified in the IEC 61131-2 standard.
The hysteresis (1 V) improves the input noise immunity.
The module input thresholds are the results of drop voltage across the input resistor RIN,
into which flows the input current IIN, and the SCLT3-8 input thresholds VTH_ON and
VTH_OFF.
The input current limiter is activated typically when VIN = 3.7 V, before the VTH_ON threshold
is reached.
In all cases the following formula can be applied:
When the input current limiter is activated, the formula becomes:
The typical module input threshold voltage can be calculated as follow:
11V
5V
IIN (mA)
2.0
1.5
ON
OFF
ON
0.1
11V
5V
ON Region
OFF Region
Transition Region
SCLT3-8 (including R
IN)
input characteristic
ILIMIT
OFF
OFF
LED
R
IN
V
I
INPUT STATUS
V
TH
I
IN
DIGITAL
FILTER
V
IN
: 2V/div
VI: 2V/div
I IN: 1mA/div
V
LED
VIN_ON = 9.5 V
VIN_OFF =8.0 V
I
LIM = 2.35 mA
Input stage configuration
Typical input voltage thresholds
IN
IN
IN
I
V
I
R
V+
×
=
IN
ILIM
IN
I
V
I
R
V+
×
=
Math Composer 1.1.5
ON
VTH
ILIM
IN
ON
IN
V
I
R
V
_
_
+
×
=



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