Newhaven Display Forum

Newhaven Products => TFTs => Topic started by: vamsi on February 10, 2015, 11:38:23 PM

Title: NHD-5.0-800480TF-ATXL#-CTP backlight Driver
Post by: vamsi on February 10, 2015, 11:38:23 PM
we are using NHD-5.0-800480TF-ATXL#-CTP LCD, which is interfacing with arm9 processor, LCD interface with my processor is clear.
i am not able to choose the part number for Backlight driver that can support 19.2V @ 60mA.
can you please suggest the part number of  backlight driver for this LCD.
Title: Re: NHD-5.0-800480TF-ATXL#-CTP backlight Driver
Post by: Paul_B on February 11, 2015, 07:49:17 AM

We do not have a specific part number for driving the backlight, if you search on Mouser for "LED Lighting Drivers" you will find what you are looking for.

Please see the link below:

Hope this helps!
Title: Re: NHD-5.0-800480TF-ATXL#-CTP backlight Driver
Post by: GeoffE on May 13, 2015, 08:45:20 AM

We are currently incorporating the NHD-5.0-800480TF-ATXL into an OEM product, and are designing the power supplies for the product (including the 19V required for the LCD backlight).

The LCD has switching power supplies generating both the 3V3 (measured as 3.295 Volts) and 19V2 (measured as 18.75 Volts) rails connected to a 5.0" LCD.   All other signals are grounded (for now).

The LCD backlight comes on OK, however the current we are measuring on the 19V rail is around 250 mA  - rather than the maximum of 75mA (specified in the datasheet).    The LCD is also getting warm to touch.

(A) Is there a mistake in the NHD-5.0-800480TF-ATXL datasheet ?
(B) Should the LCD be getting warm to the touch ?

I hope someone can help us soon.
Title: Re: NHD-5.0-800480TF-ATXL#-CTP backlight Driver
Post by: Paul_B on May 13, 2015, 05:05:39 PM

There are no discrepancies within the NHD-5.0-800480TF-ATXL datasheet.

The backlight supply current values are: Min - 45, Typ. - 60 and Max. 75 mA

There must be an issue with your wiring or backlight circuit. Please note you have to limit your current. The reason your display is getting hot is because the supply current is 250mA.  :o

Title: Re: NHD-5.0-800480TF-ATXL#-CTP backlight Driver
Post by: GeoffE on May 15, 2015, 05:48:51 PM
Hello Paul_B,

Thank you for your response.

From the datasheet, we had assumed that the LCD would draw 60mA from a 19.2 volt supply rail, since the specifications are not defined as Vf=19.2V @60mA (as per standard LED definitions).

Should the backlight specification be changed to state the following ?
  - Backlight supply Current (ILED)       -  45mA (min), 60mA (typ), 75mA (max)
  - Backlight Supply Voltage @ ILED=60mA  -  16.2V (min), 19.2V (typ), 20.4V (max)

Based on your comments, we have re-configured the LCD backlight power supply output to include a series resistor between the power supply output and the voltage to the LCD.   Please note that the power supply has been successfully load tested up to an output current of 500mA (with purely resistive loads).

Is there any application note on how to determine the PSU output voltage and the series resistance between the PSU and LCD ?

Could you please comment on these test results ?

1. The PSU output was set to 21.88 Volts with a series 20 ohm resistor.  The resulting series current of 175mA is too high.
2. The PSU voltage was reduced to 20.78 Volts and the series resistor was increased to 56 ohms.  The resulting current (66mA) is within spec, but the LCD voltage (17.4V) is at the lower end of spec.
3. Increase the PSU output to raise the LCD voltage.  PSU voltage set to 22.74 Volts, but LCD voltage only increased slightly, and series current (95mA) is now too high.

     PSU Output  |  Series R    |   VLCD    |    Vres     |    ILCD     |  Comment
  1.    21.88 V    |  20.2 ohm  |  18.30 V  |   3.58 V   |   176 mA  |  current is too high
  2.    20.78 V    |  56.5 ohm  |  17.03 V  |   3.75 V   |   66 mA    |  current is within range, voltage at lower end of spec
  3.    22.74 V    |  56.5 ohm  |  17.41 V  |   5.34 V   |   95 mA    |  increased PSU voltage does not proportionally increase VLCD, current now above spec