Diagnostic for LED systems DownloadLink 3792

Application note MLX10801
Power LED driver for automotive applications
Power saving low side coil driver
Electronic fuse
1.
Scope
The following document describes the use of the MLX10801 on systems needing a diagnostic function
2.
General
A Diagnostic function has not been implemented in the low cost Automotive LED Driver MLX10801. This is not
required in the majority of single source (e.g. interior lighting) applications.
Different lighting systems and applications will require different system diagnostic solutions. One possible solution
for a typical 'bulb outage' detection is described.
3.
Application diagrams
3.1.
VBAT
LED module with a diagnostic function using an ADC in the diagnosing
system
Cap for EMC
directly on the
connector
100nF...1uF
10...100uF
100nF
Calibration point
CONTROL
47k
Calibration
point
VS
CONTR
GND
DRVOUT
TEST
RSENSE
CALIB
DSENSE
Calibration points
DIAG
>47k
GND
On the IC pin RSENSE a low pass filter has been added. The module pin DIAG will represent now a DC voltage
proportional to the average current of the LED.
This DC voltage can be measured by the diagnosing system by means of an ADC.
390111080103 Rev. 005
Author: TFR
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Application note MLX10801
Power LED driver for automotive applications
Power saving low side coil driver
Electronic fuse
The minimum value of the 47k in the low pass filter is needed due to short circuit protection of this module pin
against VBAT and GND.
The disadvantage of this method is, that for every LED module a separate ADC channel is needed as shown in the
following figure. Anyway this method allows to diagnose, which module is failing.
CONTROL
DIAG
LED-Module
VBAT
Ignition
LED-Module 1
LED-Module
VBAT
LED-Module 2
VBAT
INPUT
VBAT
ADCIN1
CONTROL
ADCIN2
DIAG
ADCINn
GND
VBAT
CONTROL
DIAG
GND
LED-Module
...
GND
PWMOUT
...
Light
information
input
CPU-Module
GND
LED-Module n
GND
A simplified method using only 2 ADC channels for n LED-Modules would be to measure just the voltage drop in
the supply line as shown in the following figure. The needed resolution depends on the number of connected
modules.
CONTROL
LED-Module
VBAT
Ignition
LED-Module 1
LED-Module
VBAT
LED-Module 2
Light
information
input
VBAT
INPUT
PWMOUT
ADCIN1
VBAT
CONTROL
ADCIN2
GND
VBAT
CONTROL
GND
LED-Module
...
GND
CPU-Module
GND
LED-Module n
GND
The current measurement in the supply could be changed to the current measurement in the ground lines of the
LED modules. Just one ADC channel is needed as the next figure shows,
390111080103 Rev. 005
Author: TFR
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14/Mar/03
Application note MLX10801
Power LED driver for automotive applications
Power saving low side coil driver
Electronic fuse
CONTROL
LED-Module
VBAT
Ignition
LED-Module 1
LED-Module
VBAT
LED-Module 2
Light
information
input
VBAT
INPUT
PWMOUT
ADCIN1
VBAT
CONTROL
GND
VBAT
CONTROL
GND
LED-Module
...
GND
CPU-Module
GND
LED-Module n
GND
The ground shift of the LED modules due to the sense resistor is not a problem, in the LED-Modules is an
internal protection resistor on the LED-Module pin CONTR.
3.2.
LED module with a diagnostic function using just a logic information in the
diagnosing system
Some small electronic must be added to the LED-Module in order to build up this function.
The idea is to realise a hard wired OR of all n LED-Modules connected to the diagnosing system.
In case of a failure a logic “L” is given to the diagnosing system.
Attention must be drawn to the fact:
• to keep the LED-Module pin DIAG short circuit protected against GND and VBAT. It depends on the
architecture of the whole system, if this request is really needed.
• to add no additional current in sleep mode
• to have one central pull up, which can be connected to any voltage.
The electronic just observes the voltage drop over the coil. Due to the switching principle, only ½ period is
observed. The non observed period is handled by a capacitor keeping the actual state.
The electronic insures too, that the sleep mode is not interpreted as failure due to the fact, that the voltage drop
over the coil is zero in this case.
390111080103 Rev. 005
Author: TFR
Page 3/5
14/Mar/03
Application note MLX10801
Power LED driver for automotive applications
Power saving low side coil driver
Electronic fuse
Cap for EMC
directly on the
connector
100nF...1uF
VBAT
10...100uF
Calibration point
100nF
DIAG
47k
CONTROL
<100 Ohm
Protection
Resistor
VS
Calibration
point
CONTR
GND
DRVOUT
TEST
RSENSE
CALIB
DSENSE
Calibration points
GND
CONTROL
DIAG
LED-Module
VBAT
Ignition
LED-Module 1
LED-Module
VBAT
LED-Module 2
Light
information
input
VBAT
INPUT
PWMOUT
VBAT
DIAGINPUT
CONTROL
DIAG
GND
VBAT
CONTROL
GND
DIAG
LED-Module
...
GND
CPU-Module
GND
LED-Module n
GND
390111080103 Rev. 005
Author: TFR
Page 4/5
14/Mar/03
Application note MLX10801
Power LED driver for automotive applications
Power saving low side coil driver
Electronic fuse
3.3.
Conclusion
Diagnosis, in case it is requested, can be added in different manners by means of some external electronics.
For the latest version of this document, go to our website at:
www.melexis.com
Or for additional information contact Melexis Direct:
Europe and Japan:
Phone: +32 13 61 16 31
E-mail: [email protected]
All other locations:
Phone: +1 603 223 2362
E-mail: [email protected]
QS9000, VDA6.1 and ISO14001 Certified
390111080103 Rev. 005
Author: TFR
Page 5/5
14/Mar/03
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