5

U6809B

4764A–AUTO–10/03

Explanation

Lamp voltage is logic 1 if output voltage > threshold voltage detection

Lamp voltage is logic 0 if output voltage < threshold voltage detection

Lamp current is logic 1 if output current > threshold current detection

Lamp current is logic 0 if output current < threshold current detection

Fail-safe Functions

A fail-safe IC has to maintain its monitoring function even if there is a fault condition at

one of the pins (e.g., short circuit). This ensures that a microcontroller system would not

be brought into a critical status. A critical status is reached if the system is not able to

actuate a warning lamp and switch off the relay. The following table shows fault condi-

tions for different pins during which the IC still works as a fail-safe device.

Table 1. Truth Table for Lamp Drivers and Lamp Feedback

Inputs

Outputs

Comment

Lamp (I)

Lamp

Voltage

Lamp

Current

Lamp

Driver

Current

Lamp

Current

Feedback

Lamp

01

0

off

off

1

Output ok or open (internal pull-up) or shorted to

V

Batt

0

1

1

on

off

1

Output shorted to V

0

0

1

on

on

0

Internal driver activated due to internal failure

0

0

0

off

on

0

Output shorted to GND

1

0

1

on

on

0

Output ok

1

1

1

on

off

1

Output shorted to V

Batt

11

0

off

off

1

Internal driver deactivated due to internal failure or

thermal shutdown

1

0

0

off

on/off

1

Output shorted to GND or open

Table 2. Table of Fault Detection

Condition

Feedback Lamp

Lamp Input is 0 (Lamp off)

Lamp Input is 1 (Lamp on)

Normal operation

1

0

Lamp output shorted to GND

0 (= detection)

1 (= detection)

1 (= no detection)

1 (= detection)

Lamp output open

1 (= no detection)

1 (= detection)

Feedback shorted to GND

0 (= detection)

0 (= no detection)

1 (= no detection)

1 (= detection)

Lamp input shorted to GND

1 (= no detection)

1 (= detection)

0 (= detection)

0 (= no detection)