COG LCD with Negative Mode LCD

COG LCD with Negative Mode LCD
March 16, 2017 Eric Hawkins
In Journals

I received a call from an engineer who was designing a battery powered C02 sensor with a digital display. He had settled on a segmented display with a combination of seven segments and icons. The display was to have light colored letters with a dark black background, also called negative mode. The backlight would either be blue or white LED.

Note: Negative mode LCD means that the characters are light colored
and the background is dark. Negative mode LCDs
must have the backlight ON to read the display.

The overall dimension of the glass was 50mm in the X and 70mm in the Y. It was still early in the design stage, so the number of segments ranged from 82 to 88.

From this information, we were able to create the first draft drawing of this LCD module as seen below.

Since Focus Displays will customize this display to match the requirements of the engineer, the next step was to define the details of the LCD module.

Negative Mode LCD | VATN vs EBT Technology

Two types of negative mode LCD technology that provides a black background are:

  • EBT (Excellent Black Technology)
  • VATN (Vertically Aligned Twisted Nematic)

EBT Displays have fallen out of favor due to their limited operating temperature and higher cost.

The VATN, (also called a Black Twisted Nematic – BTN display), contains a fluid that is aligned vertically to the viewing plane. This provides the widest viewing angle and a faster response time than a FSTN (Film Super Twisted super-twisted nematic) display.

We recommend VATN over EBT since EBT lacked in several key areas including:

  • Operating temperature
  • NRE (Non-Recurring Engineering) cost
  • Viewing angle
  • Lead times

Negative Mode LCD: COG or COB

The two types of display construction are COB (Chip on Board) and COG (Chip on Glass). COB contains a PCB (Printed Circuit Board) attached to the glass and adds additional thickness in the Z direction. The average COB segmented display has a thickness of 12mm to 16mm.

COG technology is thinner than COB since it is the comprised only of LCD glass and a controller chip. A FPC (Flexible Printed Circuit) cable is attached to connect the LCD logic, if needed a separate cable can be attached to power the LED backlight.

The total thickness of the COG version of the VATN with backlight ranges from 6mm to 8mm.

In the engineer’s case (shown to the right), the COB version was too thick to fit into his design, so he went with a COG.

Negative Mode LCD: LCD interfaces

There are several interfaces, or bus types, available for a VATN LCD Display. The three most popular being: Direct Drive, 4:1 Multiplex, and SPI.

Direct Drive segment LCDs

A direct drive, also called a static drive, display contains one pin for each segment; this display would require up to 80+ pins on this design. The engineer felt this was a poor option due to the assembly cost required to place the display on the PCB.

4:1 multiplex

A Multiplex, or Mux display, is the most popular option and contains one pin for every four segments (plus the four commons). This reduces the pin count to around 23 pins.

The engineer liked the reduced pin count, but had a limited number of outputs on his micro and since his current processor did support a SPI interface he choose SPI.

  • NRE cost is the same for Direct Drive & 4:1 Mux.
  • Unit cost for Direct Drive is slightly more due to the higher number of pins.

SPI interface

A SPI (Serial Peripheral Interface) bus is the second most popular option for a segment display. A driver chip is placed on the LCD glass that allows the LCD to be driven with fewer connections.

The NRE cost is slightly higher than a 4:1 multiplex and the unit cost is higher since each unit contains its own driver chip.

5V or 3.3V operating voltage

The two most common driving voltages for segment displays are 5V (4.7V) and 3.3V. The favorite choice being 3.3V for most battery applications.

We recommended 5V operation for this VATN display. The primary reason is that his display will be operating in lower ambient temperatures. In lower temperatures a higher voltage is required to drive the display.

There is no cost difference between a 3.3V and 5V and the choice of voltage does not affect the one-time tooling cost.

He went with the 5V display.

Temperature range

VATN / BTN is able to operate down to -30C, with a storage temperature of -40C.

If cost is a concern, the VATN / BTN can incorporate a -20C fluid that will lower the cost by and estimated 3%. NRE, or Tooling, cost is the same for either temperature range.

If you have any questions or need help with your next design,  contact our US based support.

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Project Data:

 

  • Project number 2214BHJT_05
  • FPC cable 50mm X 70mm, 86 segments.
  • VATN Display with LED backlight and SPI interface
  • Vs. 4:1 mux,