AN2846 Datasheet, PDF (11/35 Pages) STMicroelectronics – The serial current limited termination device

AN2846
Application guidelines
1.2.5
Input state monitoring
The state of each of the 8 monitoring LEDs is an image of the 8 filtered input states. All the
monitoring LED cathodes have to be connected to ground. In the on state a current of I_LIM
reduced by 0.15 mA is available for each LED. In case of a LED not being used, the LED
output pin must be connected to the ground COM_P to allow the input current to flow back to
the ground.
The LEDs must be chosen with a V_F voltage less then 2.7 V (at minimum operating
temperature -25 °C).
1.3
1.3.1
Monitoring functions and regulator
V_CC monitoring
The power bus voltage connected to V_CC is sensed by the V_CS pin through a resistor bridge.
The V_CS threshold voltage is typically 1.25 V with a hysteresis of 100 mV. Designers can
easily set their own alarm detection voltage by an appropriate resistor bridge (see
Figure 12) using the formula:
V_{CC _ min}
= V _CS
(1 +
R_{S )}
R _PD
Figure 12. UVA comparator
V_CC
R_S
V_CS
R _PD
V_DD
V_BG=1.25V
UVA
1ms delay
COM
For example, the resistor bridge consisting of R_S = 1.5 MΩ and R_PD = 120 kΩ produces UVA
activation when V_CC drops below 17 V. The UVA activation has no effect on SCLT3-8
behavior but the information is transmitted trough the SPI bus by setting bit 7 to low state in
the control bits register (see Figure 13 and Figure 14).
To eliminate any short voltage disturbances that could trigger the UVA, a 1 ms delay circuit
has been inserted in the output line of the UVA comparator.
Doc ID 15130 Rev 1
11/35