CLT3-4B Datasheet, PDF (7/13 Pages) STMicroelectronics – Current limited over-voltage protected quad digital termination

CLT3-4B
2
Application information
Application information
2.1
Application description
The CLT device is used between the input module and the opto-coupler of an I/O module. To
secure the coupler off state and the non-conduction of its diode, the maximum CLT output
current is 40 µA with R opto = 15 kΩ. When the CLT input voltage V_IN is higher than 5 V, a
minimum CLT output current of 1.5 mA will secure the opto-coupler on state.
The CLT limiter is insensitive to the output voltage up to 2.0V. Therefore, its output is able to
drive any state of the art opto-coupler. Some additional device can be added in series with
the opto-coupler if the output voltage remains within its functional limits.
A stand-alone sensor is normally connected on the CLT input. For a good off state control,
its quiescent current has to be less than 1.5 mA (EN60947-5-2 standard compatible)
keeping then the opto-coupler off thanks to the CLT device (I_OUT < 40 µA). And its maximum
ON state voltage V_S-ON will be lower than 8 V (EN60947-5-2 standard compatible)
corresponding to the difference between the minimum supply voltage (V_{CC MIN} = 19.2 V)
and the maximum current limitation activation voltage:
V_{CC MIN} = V_S-ON + V_I
V_I = V_IN + R_I x I_{IN MAX}
(V_I = 11 V when V_IN = 5 V, R_I = 1.2 kΩ, I_IN < 3.7 mA).
2.2
Input reverse polarity robustness
Each input of the CLT device may be biased to a reverse polarity. This case corresponds to
a connection mistake or a reverse biasing that is generated by the demagnetization of a
monitored inductive solenoid.
The involved input can withstand the reverse current up to 20 mA (in the configuration
shown in Figure 10) through its input protection diode while its opto-coupler remains OFF.
Such a case is not critical for the CLT3-4 itself, as it represents less than 15 mW per
disturbed input. But the input resistors should be designed with care regardless of their
thermal characteristics (refer to Application note AN2527).
When the reverse polarity operation is required in types 3 input mode, it is recommended to
increase the input resistance R_I to 1.8 kΩ in order to reduce its dissipation and keep the
input current below its maximum allowed level (20 mA). The surge and transient immunity
robustness of the CLT3 is then also increased.
A 1.8 kΩ value allows the reverse input current to be reduced to 16.2 mA and the resistor
dissipation to be reduced to 0.47 W, for 30 V V_CC and a - 30 V V_I voltages.
The CLT3 normal operation in type 3 is fully compatible with this recommended resistance
since its internal state threshold is monitored at V_IN = 4.2 V and V_C = 10V - corresponding to
V_I = 11 V and V_CC = 19 V.
In type 1 input operation where R_I = 2.2 kΩ the dissipation of this resistor is less than
0.39 W.
The other inputs remain operational, as long as the power supply is working properly
(V_CC >19 V). Refer to Application note AN 2527 for representative application design case.
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