laptop lcd panel voltage supplier

Monochrome character, graphic and static displays require different input voltages. All the different LCD voltage symbols can be confusing, but believe it or not, there is a system to the madness.

The voltages VCC, VDD, VSS and VEE are used in describing voltages at various common power supply terminals. The differences between these voltages stem from their origins in the transistor circuits they were originally used for.

This LCD voltage terminology originated from the terminals of each type of transistor and their common connections in logic circuits. In other words, VCC is often applied to BJT (Bipolar Junction Transistor) collectors, VEE to BJT emitters, VDD to FET (Field-Effect Transistor) drains and VSS to FET sources. Most CMOS (Complementary metal–oxide–semiconductor) IC data sheets now use VCC and GND to designate the positive and negative supply pins.

In the Pleistocene era (1960’s or earlier), logic was implemented with bipolar transistors. NPN (Negative-Positive-Negative) were used because they were faster. It made sense to call positive supply voltage VCC where the “C” stands for collector. The negative supply was called VEE where “E” stands for emitter.

When FET transistor logic came around a similar naming convention was used, but now positive supply was VDD where “D” stands for drain. The negative supply was called VSS where “S” stands for source. Now that CMOS is the most common logic this makes no sense. The “C” in CMOS is for “complementary” but the naming convention still persists. In practice today VCC/VDD means positive power supply voltage and VEE/VSS is for negative supply or ground.

The convention of VAB means the voltage potential between VA and VB. The convention of using 3 letters was used to show power supply and ground reference voltages as well. In some cases a processor may have both an analog and digital power supply. In this case VCCA/VCCD and VSSA/VSSD are used. Another reason for the 3 letters is in an NPN circuit with a load resister between the collector and VCC. VC would be the collector voltage. In this case VCC is the positive power supply voltage and would be higher than VC.

Pin three (3) is Vo and is the difference in voltage between VDD and VSS. This LCD voltage is adjusted to provide the sharpest contrast. The adjustment can be accomplished through a fixed resistor or a variable potentiometer. Many products have firmware that monitor the temperature and automatically adjust the contrast voltage.

In a Liquid Crystal Display (LCD), V0 is used to vary the screen brightness or contrast. Contrast, simply put is the ratio of the light areas to the dark areas in a LCD. This is usually done in a production setting with values which are optimized for most users. Temperature can have an undesirable effect on the display brightness and for this reason a varying resister or potentiometer is used to accommodate the desires of the user.

Below is a data sheet of a 16x2 Character LCD module that shows various recommended driving voltages. The LCD voltage can range from MIN (minimum) to TYP (Typical) to Max (maximum).

If the supplied LCD voltage drops too low, the display is ‘under-driven’ and will produce segments that are ‘grey’. The lower the LCD voltage falls below the acceptable threshold, the lower the contrast will be.

If the LCD is over-driven, you may see ghosting. This is where segments that should not be ‘on’ are gray. They are not as dark as the segments that should be on, but they can be seen and may cause confusion for the end user.

There are times when a customer needs to replace a display that has been discontinued or EOL (End-Of -Life) by their previous LCD supplier. The previous LCD’s pin-outs may be different than Focus’ standard, off-the-shelf display. This is not a large problem to overcome.

The third option is to pull power from pins one and two. This is the same location from which the LCD is pulling its power. Focus does not recommend this option and can modify the PCB for the customer to connect the backlight from a different location.

Many LCD Modules will require more than one internal voltage/current. This may make it necessary for the customer to supply the needed inputs. They may need to supply 3V, 5V, 9V, -12V etc.

The solution for this is to integrate a charge pump (or booster circuit) into the LCD circuitry. This solution works in most applications, but if the product will be operating in an intrinsic environment, care must be taken with layout of the circuit board.

Intrinsically-safe LCDs are Liquid Crystal Displays that are designed to operate in conditions where an arc or spark can cause an explosion. In these cases, charge pumps cannot be employed. In fact, the total capacitive value of the display needs to be kept to a minimum.

Focus Display Solutions does not build a display that is labeled ‘Intrinsically safe’ but we do design the LCD to meet the requirements of the engineer. In meeting the design engineer’s requirements, the display may need to contain two or three independent inputs. Focus can redesign the PCB and lay out the traces to allow for these additional inputs.

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When starting a laptop, the LCD screen isn’t displaying any video but the laptop’s lights are working and you can hear the fan and hard drive running. Which of these options would be appropriate troubleshooting steps?

D) An operating system update may have caused the LCD display to stop working. Restore the operating system from backup or reload the operating system.

When there’s nothing on a laptop display, the problem is most-likely related to the operation of the LCD panel or LCD hardware itself. Many laptops include function keys that will send the video signal to the external video port, but those function keys won’t usually blank the LCD panel unless a physical device is connected to the laptop’s external monitor port. To ensure the laptop can “see” the monitor, it’s usually a good best practice to shut down the laptop, connect the external monitor, and power up the laptop. At the very minimum, the laptop’s BIOS information should appear on one of the displays.

If the laptop’s memory wasn’t properly seated, the laptop LCD display will continue to work, and you’ll probably see a BIOS error noting a memory problem.

If the power supply wasn’t providing enough power, the hard drive and fans would not be operating. If a laptop power supply isn’t providing enough power to run the laptop and the battery of the laptop is fully discharged, the laptop usually won’t power on at all. Newer laptops check the power supply when starting the system and will tell you if the power supply isn’t adequate for the system. That’s useful, isn’t it?

When your portable device starts having problems, where do you start the troubleshooting process. Unlike traditional desktop personal computers, laptops and portable devices have additional steps for diagnosing troubles. In this video module, we’ll troubleshoot the boot process, video settings, stylus and digitizer issues, keypad problems, and antenna wires.

With the laptop turned on and booted up, try shining a torch at an angle close to the screen and check for any images. They will be very faint if they are there so trying this in a darkened room may help to see them.

If there is no voltage then there"s a problem with the power supply for the backlights. Depending on the motherboard circuit sometimes there is a fuse and sometimes not. If there is one it will be close to the LVDS cable connector on the motherboard. Look for a component with an "F" designation and test it for continuity.

If there is voltage there then there may be a problem with the backlight enable lead (BL_EN). The LVDS pin for this lead can vary so the schematics will be needed to find out which one it is.

It here is no BL voltage or even if there is here’s a link that may help with the schematics. Type the motherboard model info in the search box at the top right to hopefully get results. If none then search online for them with the motherboard information e.g. (insert motherboard make and model info) schematics

Here’s the maintenance and service guide for the laptop, taken from this support webpage. This details how to access the LCD panel unit, so that you can check.

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V_LED or VBL is VDD where we need to connect main supply voltage i.e 12V which will be boosted to the required voltage by the boost circuit and finally will be driven by the LED driver.

LED_PWM or PWM pin is where you provide the brightness control to the display. This is how your laptop display brightness is varied on your input Pulse Width Modulated signal. The measure is in terms of the duty cycle for the square wave PWM and is calculated as Ton/(Ton+Toff) i.e if Toff is zero PWM is 1 i.e 100%.

LCD screen tester (for testing eDP/embedded Display Port interface). With its built-in Color bar generator, it can test resolution from 1280X800 to 3840X2160.