LED brightness Issue Using Matrix Method

Thread Starter

vinoth.ganesan

Joined Aug 9, 2010
9
It is a User Interface product , which contains more than 50 LED's and it is matrix driven which contains 16 column and 3 rows.Currently we are using this LED part number LW QH8G-Q2S2-3K5L.

We have difficulty in lighting up the LED’s with the desired intensity.
We have three rows, when we handle single row through software we get the desired brightness, but if we handle 2 or 3 rows the brightness of leds comes down. Because we are selecting rows one by one, and hence each row gets 33% of the time slot.
Duty cycle is 0.33 and scan time of single row is 2ms , from the permissible pulse handling capacity graph we conclude to go for 22mA to increase the brightness,as per datasheet maximum forward current of the LED is 15mA but our value is 22mA
is that will affect the reliablity ,life time and junction temperture of LED?
whether we are going in the right way?
Whether the LED will handle that much current ?
Please guide us to proceed further in this analysis.

Regards
Vinoth G
 

crutschow

Joined Mar 14, 2008
23,500
There are graphs of "Permissible Pulse Handling Capability" on page 10 of the LED data sheet. That tells you the maximum current you can run through the diode versus duty-cycle and pulse-width. To be safe use the 85C graph.
 

wayneh

Joined Sep 9, 2010
16,114
Sounds to me like a power supply issue where the supply can't keep up with more than one string pulsing. You haven't said anything about the supply. The problem with increasing the LED current could come if the power supply is improved, causing the LED current to exceed a limit.
 

thatoneguy

Joined Feb 19, 2009
6,359
What are you using for a driver on the matrix?

Constant current sink/driver, or resistors?

Maxim has come out with some outstanding LED Driver ICs, MAX6979AUG (20 port, 4 wire serial), and MAX6956ANI+ 28 port, I2C. Constant current driver and I/O expander. Stable drive over varying supply voltage (as long as LED Voltage meets minimum for 10-25mA adjustable current)
 

Thread Starter

vinoth.ganesan

Joined Aug 9, 2010
9
There are graphs of "Permissible Pulse Handling Capability" on page 10 of the LED data sheet. That tells you the maximum current you can run through the diode versus duty-cycle and pulse-width. To be safe use the 85C graph.

Thanks for the reply, If i do like that (using 85C graph), I am not able to acheive the enough brightness using 33% duty cycle.
 

Thread Starter

vinoth.ganesan

Joined Aug 9, 2010
9
Sounds to me like a power supply issue where the supply can't keep up with more than one string pulsing. You haven't said anything about the supply. The problem with increasing the LED current could come if the power supply is improved, causing the LED current to exceed a limit.
For driving the LED matrix, we are using the current source driver (A2981). and Driver output is connected to anode of led and cathode is connected to series resistor of 100ohms and finally it connected to ground. But in that, we are not able to get the voltage properly (only 3.8V are coming)
 

thatoneguy

Joined Feb 19, 2009
6,359
Are using resistors in the LED string in addition to a constant current source driver? Does the driver's datasheet state the need for additional resistors?
 
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