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Recommended Backlight current

I have a constant current source driving the backlight of the DT070BTFT-PTS1 display at 135 mA.  Per the datasheet, the typical current of 140 mA is specified and the absolute MAX is 175 mA.  The display at 135 mA is way too dim and am wondering what backlight current level  Engineering recommends?

I am curious if there exists any data that indicates the expected % increase in brightness per mA of current above the 140 mA tyical value.  I need to drive more current through the LEDs but I don't want to damage any of the parts.  Do most customers drive the display at 175 mA and PWM the control pin to dim the display from that baseline?  How critical is the 175 mA MAX level?  Is this a conservative MAX value?

Your input would be appreciated.

Thanks

Jim

Replies

engineering_seacomp's picture
engineering_seacomp
July 18, 2018

Hi,

It is possible to increase the supply current in order to increase brightness, however since the total hours of operation available to an LED decrease with increased current (and temperature) you will get less hours out of the LEDs. For example, at the typical LED supply current for this module, 140mA, you could get 25000 hours out of these LEDs before you begin to see a decrease in brightness of the backlight. Increasing the current to 175 or 210 mA will result in a brighter backlight, but reduced hours of operation (about 25000 & 18000 hours,  respectively).

If backlight brightness is a concern, a better option might be to use the "High Bright" version of this LCD module (DT070BTFT-HB), in which case you could drive the LEDs with the "typical" supply current of 200mA, resulting in a luminance of 500 nits (as opposed to 250 nits at the typical current of the original module) with no change in the available hours of operation. 

Thanks,

rfdes's picture
rfdes
July 19, 2018

Thanks for the response.  I understand that increasing the current to > 175mA will shorten the life span of the LEDs but I was hoping that I could get some idea of just how much brighter that I can expect the display to be when driven to 175mA as opposed to 140mA.  Is there any data available to help me judge what to expect?

i.e. does 175 mA equate to a 25% (for example) increase in brightness?  My hope is that it will be substantially much brighter.

Thanks

Jim

engineering_seacomp's picture
engineering_seacomp
July 19, 2018

Hi,

I appolgize.  I do not have the numbers for that.  I can only suggest the obvious, trying it.

Have you considered the High Bright?

Thanks,

rfdeswb5kye's picture
rfdeswb5kye
August 1, 2018

Hi -

I am still having serious issues with the brightness.  I have tried both the HB version of the display (300-400 nits) and the normal (200 nits) and I see NO difference in brightness with the led current set for about 190 mA on both. I've measure approx 9V across the LED+ and LED- pins.  The drive current is a constant current source, so I am comfortable that I am driving them correctly.   I can visually 'see' the leds burning throught the sides of the units but the rendered screen is way too dim.   Is there something basic that I am missing?  I've only tried a sample of one DT070BTFT-PTS1 and one DT070BTFT-HB-PTS1, so I am wondering if the non-HB is working OK but the HB version is faulty.   I am not sure what the issue is but if you have any suggestions, it would be most appreciated.

I remember using some NEC displays several years ago which had 'light films' over the display to enhance brightness.  Do the Displaytech displays use similar light films?  Anyway, something is wrong and I am struggling to understand.

Looking forward to your help

Jim

engineering_seacomp's picture
engineering_seacomp
August 3, 2018

Hi,

To see a difference in the brightness of the backlight between the standard and high-bright displays you will have to apply more overall current to the high bright. The difference between the two back lights is that one is made up of 21 LEDs (7 rows of 3) and the other is made up of 30 LEDs (10 rows of 3). If you are driving the LEDs with 190mA in both instances you will not see a difference in the brightness between the two because you are simply distributing the same overall current over more LEDs  (i.e. at 190mA you are supplying about 27mA to each of the standard LEDs, but only about 19mA to each of the high-bright LEDs). What you want is to apply the same current to each of the LEDs, not the same overall current. In other words, increasing the high-bright supply to about 270mA will give you 27mA across each of the 30 LEDs (same as the 190mA applied to the standard version) and therefore a brighter back light.

There is light film or other to enhance.

Thanks,