AN2846 datasheet(11/35 Pages) STMICROELECTRONICS

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AN2846

Application guidelines

Doc ID 15130 Rev 1

11/35

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 ILIM

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 COMP to allow the input current to flow back to

the ground.

The LEDs must be chosen with a VF voltage less then 2.7 V (at minimum operating

temperature -25 °C).

1.3

Monitoring functions and regulator

1.3.1

VCC monitoring

The power bus voltage connected to VCC is sensed by the VCS pin through a resistor bridge.

The VCS 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:

Figure 12.

UVA comparator

For example, the resistor bridge consisting of RS = 1.5 MΩ and RPD = 120 kΩ produces UVA

activation when VCC 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.

)

(

min

V+

RPD

VBG=1.25V

VDD

1ms delay

UVA

COM

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