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Part Number U6809B

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Rev. 4764A­AUTO­10/03
Features
·
Digital Self-supervising Watchdog with Hysteresis
·
Three 250-mA Output Drivers
·
One Relay Driver, Two Lamp Drivers
·
Lamp Drivers with Auxiliary Ground
·
Lamp Drivers Short-circuit Protected
·
Lamp Drivers with Status Feedback
·
Enable Output
·
Over-/Undervoltage Detection and Reset
·
All Power Outputs Protected Against Standard Transients
·
All Power Outputs Protected Against 40-V Load Dump
·
Automatically Activated Lamp Drivers if V
S
is Disconnected
·
Automatically Activated Lamp Drivers Via AUX GND if Standard
Ground is Disconnected
Description
The U6809B is designed to support the fail-safe function of safety-critical systems
such as ABS. It includes a relay driver, two independent short-circuit-protected lamp
drivers which are supplied by redundant ground lines, two monitoring circuits for the
lamp driver output voltage and output current, a watchdog controlled by an external
RC network and a reset circuit initiated by an over- and undervoltage condition of the
5-V supply providing a positive and a negative reset signal.
Fail-safe IC with
Relay Driver
and Lamp
Driver
U6809B
2
U6809B
4764A­AUTO­10/03
Figure 1. Block Diagram
Pin Configuration
Figure 2. Pinning SO20
Digital input lamp 1
Digital input relay
Digital input lamp 2
Digital input wd
Feedback lamp 1
Feedback lamp 2
Debouncing of
over- and under-
voltage detection
Short-circuit
detection and
temperature
monitor
Oscillator
monitoring
RC-oscillator
Watchdog
p reset
n reset
Logic
Open-collector
250 mA
lamp driver 1
Temperature
Open-collector
250 mA
lamp driver 2
Open-collector
250 mA
relay driver
Open-collector
25 mA
enable driver
LA1I
LA2I
RELI
WDI
FBLA1
FBLA2
PRES
NRES
Micro-
controller
OSC
GND
AUX GND
LA1O
LA2O
RELO
ENO
VS
Loads
1
2
3
4
5
6
7
8
10
9
19
18
17
16
14
15
13
12
11
20
RELO
GND
GND
FBLA1
NRES
PRES
LA1I
LA2I
GND
GND
GND
LA2O
AUX
GND
ENO
VS
LA1O
FBLA2
OSC
REL1
WDI
U6809B
3
U6809B
4764A­AUTO­10/03
Pin Description
Pin
Name
Type
Function
Logic
1
RELI
Digital input
Activation of relay driver
Driver on: L
Driver off: H
2
LA1I
Digital input
Activation of lamp driver 1
Driver off: L
Driver on: H
3
LA2I
Digital input
Activation of lamp driver 2
Driver off: L
Driver on: H
4
RELO
Open-collector driver output
Fail-safe relay driver
Driver off:---
Driver on: L
5, 6
GND
Supply
Standard ground
7
FBLA1
Digital output
Feedback lamp1
See Table 1 on page 5 and
Table 2 on page 5
8
NRES
Digital output
Negative reset signal
Reset: L
No reset: H
9
PRES
Digital output
Positive reset signal
Reset: H
No reset: L
10
FBLA2
Digital output
Feedback lamp 2
See Table 1 on page 5 and
Table 2 on page 5
11
OSC
Analog input
External RC for watchdog timer
12
ENO
Open-collector output
Watchdog disable output
Watchdog ok: ---
Watchdog n ok: L
13
AUX, GND
Supply
Auxiliary ground of lamp drivers
14
LA2O
Open-collector driver output
Warning lamp driver
Driver off: ---
Driver on: L
15, 16, 17
GND
Supply
Standard ground
18
LA1O
Open-collector driver output
Warning lamp driver
Driver off: ---
Driver on: L
19
VS
Supply
5 V supply
20
WDI
Digital input
Watchdog trigger signal
Pulse sequence
4
U6809B
4764A­AUTO­10/03
Detailed Block Diagram with External Components
Figure 3. Detailed Block Diagram
+
-
V
Ref 4
+
-
V
Ref 5
+
-
+
-
+
-
Oscillator
Watchdog
+
-
V
Ref 1
Failure
detection
lamp 1
+
-
+
-
+
-
+
-
V
Ref2
V
Ref3
19
14
GND
15
13
VS
RELI
OSC
+ PRES
Temperature
shut down
A
C
Ground backup
AUX GND
GND
Integrated
oscillator
Internal
timing
Short-circuit
protection
D
B
D
C
V
S
V
Batt
V
Batt
Push pull
R-
Push pull
R+
Reset
delay
Push pull
L1
Failure
detection
lamp 2
Push pull
L2
A
D
V
S
V
S
B
C
E
V
Ref2
E
E
Reset
debouncing
1
2
3
20
11
LA1I
LA2I
WDI
18
7
4
8
9
10
12
- NRES
RELO
V
Batt
FBLA1
LA1O
LA2O
FBLA2
ENO
5 6
16 17
AUX GND
V
Ref3
D
e
b
o
u
n
c
i
n
g
D
e
b
o
u
n
c
i
n
g
5
U6809B
4764A­AUTO­10/03
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
0
1
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
Batt
and faulty input level
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
1
1
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)
Lamp output shorted to V
Batt
1 (= no detection)
1 (= detection)
Lamp output open
1 (= no detection)
1 (= detection)
Feedback shorted to GND
0 (= detection)
0 (= no detection)
Feedback shorted to V
S
1 (= no detection)
1 (= detection)
Lamp input shorted to GND
1 (= no detection)
1 (= detection)
Lamp input shorted to V
S
0 (= detection)
0 (= no detection)