Active Matrix VFD

1.1 Basic Structure

Active Matrix VFDs are a special type of Chip-in-Glass (CIG) VFD. They contain small silicon chips about 5x5mm in size which include the phosphor matrix, driver and memory functions.

In order to construct a wide display area, these small square silicon chips are precisely “tiled” on the glass plate with each having a 16×16 dot matrix phosphor pattern precisely formed on the top surface.

Only 2 chips can be put vertically (Y direction) due to the space required for wire bonding as shown in Fig. 1.

As with conventional VFDs, the Active matrix VFDs have cathode (filaments) to emit electrons, mesh grids to diffuse electrons, and anodes coated with phosphor to which attract electrons and emit light.

As with other CIG VFDs, a reduced number of lead pins contribute to easy assembly compared to the conventional graphic VFDs.

The Active Matrix technology has only one grid structure allowing a high resolution graphic display without the need for the complex multiple grid structure found in other graphic VFDs.

Active Matrix VFDs are a special type of Chip-in-Glass (CIG) VFD. They contain small silicon chips about 5x5mm in size which include the phosphor matrix, driver and memory functions.

In order to construct a wide display area, these small square silicon chips are precisely “tiled” on the glass plate with each having a 16×16 dot matrix phosphor pattern precisely formed on the top surface.

Only 2 chips can be put vertically (Y direction) due to the space required for wire bonding as shown in Fig. 1.

1.2 The Silicon Chip

The silicon chip used in Active Matrix VFDs contains a 256 bit (16 x 16 matrix) driver, latch and shift-register circuit as shown in Fig. 4. which is similar to ordinary discrete VFD driver ICs. When the chips are formed into a large graphic array, the serial data output from each chip is fed to serial input of the next chip and can therefore be controlled as “one” large shift register and driver circuit. Once data is loaded into the shift registers it can remain unchanged until the image on the display is required to be changed. This makes it possible to control the display from the simplest of CPUs.

2.1 Supply Voltages

The supply voltage requirement of active matrix VFDs is a filament supply, (as in ordinary VFDs), a logic supply voltage “VDD1” and a display supply voltage “VDD2”. The table lists the typical termination names and function. Please check the individual specification for current consumption and actual voltage value.

2.2 Power Supply Circuit

The grid and anode supply voltage is typically 12V to 15V DC and applied to G directly and to VDD2 via a limiting resistor of 22R to 100R to prevent current surges causing false operation. Please refer to the specification for the recommended value. Do not fit a capacitor between the VDD2 input and GND as this will obsolete the resistor.

The filament voltage applied to F1,F2 of the VFD is critical in terms of it’s effect on life-time if operated outside the specified operating voltages. The filament transformer is normally supplied with a centre tap which is biased at 0.6VDC above GND. Too high a bias may cause uneven illumination.

The logic supply terminal VDD1 should have a noise filtering capacitor of 0.01uF to 0.22uF mounted between it and GND to prevent false operation.

Although adjustment of VDD2 will change the display brightness, it is preferred to use pulse width control applied to the enable input to prevent uneven illumination at low brightness. This method is described later.

2.3 Power Supply Sequence

2 types of power supply sequence apply at power ON and power OFF due to the different internal construction of the silicon chips employed in Active Matrix VFDs. The chips may become damaged if the sequence is not followed.

TYPE 1 (Dot pitch=0.347mm Type)

Power On	“VDD1” and “VDD2” should be ON at the same time, or “VDD1” should be ON after “VDD2” is ON. The VDD2″-VDD1″ delay time should be small as possible (Less than 200msec.).
Power Off	“VDD1” and “VDD2” should be OFF at the same time, or “VDD2” should be OFF after “VDD1” is OFF. The VDD1″-VDD2″ delay time should be small as possible (Less than 200msec.).

TYPE 2 (Dot pitch=0.308mm Type)

Power On	“VDD1” and “VDD2” should be ON at the same time, or “VDD2” should be ON after “VDD1” is ON.
Power Off	“VDD1” and “VDD2” should be OFF at the same time, or “VDD1” should be OFF after “VDD2” is OFF.

There is no specific restriction for the on/off timing between “VDD1″/”VDD2” and the filament voltage Ef.
Please note that it takes one to two seconds for the filament to rise to its optimum temperature after the filament voltage is applied, during which time the brightness will rise.

3.1 Interface Signals

The interface of Active Matrix VFDs is “C-MOS” level clock synchronized serial data.
The functions are described in the adjacent.

3.2 Interface Characteristics

This table shows the basic threshold voltages and response times of the interface.
Please check the individual specification for details.

3.3 Interface Timing

Fig 8 shows the timing waveforms for the interface signals. Please check the individual specification for details.

3.4 CPU Interface

A single line chip array Active Matrix VFD typically has one serial I/O and can be controlled by 4 ports on the CPU.

A dual line chip array type has two serial I/Os, “a” for the upper array and “b” for the lower array.

This dual line array type can be controlled by one I/O by connecting the “SOa” output to “SIb” input as shown in Fig. 10

4.1 Data Transfer Protocol

Figure 11 and 12 show an example of the data transfer protocol for an Active Matrix VFD with 16 silicon chips in a single line which produces a graphic display of 256×16 dots. The inputs and outputs of each chip are cascaded to form one long shift register chain of 4096 bits.

Each bit in shift register is assigned to each dot on the silicon chip on a one-on-one basis. The relevant assignment order is explained in “Dot Assignment and Shift Register” or “Data Sending Order” on the individual specification.
Each dot is turned on/off by setting each related bit in shift register as “High” = ON or “Low” = OFF.
The silicon chip has a data latch control function, so the content of latched data is kept until it is latched again. This allows one image to be displayed while the next image is being loaded.

4.2 Display Enable

The example display pattern shows a 2 line, 20 character 5×7 dot matrix VFD with underline cursors. The electrodes consist of 20 grids and 2 x 36 anodes which totals 92 driver outputs. Since the driver has 96 outputs, 4 bits * are left unassigned and can be set High or Low.

4.3 Brightness Control

The brightness level (intensity of the display) can be adjusted using the enable control (EN) by applying an enable pulse with a minimum frequency of 100Hz in order to avoid display flicker. The duty factor of the enable pulse determines the the brightness level according to the formula:

Brightness = tEN/Tx100(%).

5.1 Number of Silicon Chips and Glass Package Size

There are 2 types of silicon chips available with dot pitches of 0.347mm and 0.308mm. Both of them have a 16 x 16 dot matrix configuration per chip. The minimum number of the chips per VFD tube is “ONE”, and the maximum is up to about 20 chips per single array or 40 chips for a dual array (2 rows). The relationship between the number of chips and the outer dimension of glass package is shown in the following table.

5.2 Multi line Chip Array

A minimimum space is required between arrays depending on the position of the bonding area. Please refer to the following table.

Condtions	No wire bonding area between line	One wire bonding area between line	Two wire bonding area between line
Minimum Gap

5.3 Combining with a Conventional VFD Display Pattern

The silicon chips ‘C’ can be combined with a conventional custom designed VFD pattern which can be controlled by a CIG (Chip in Glass) driver to maintain a low number of pin outs as shown in Figure 19.

5.4 Lead Pins

The standard lead pin of an active Matrix VFD is 6.0mm in length and 2.0mm pitch. The dimensions of standard lead pin is shown in Figure 20.

Custom lead pins are available upon request.

Spec. No.

The “Spec No.” shows the revision number for the specification sheet. Each documented specification carries an area of revision history in the upper right corner of the top page. The specifications of the standard models are subject to change without prior notice, so, please check if the revision number of your specification is the latest version before evaluating it.

Absolute Maximum Ratings

The “Absolute Maximum Ratings” refer to values that must not be exceeded in any event. Using a VFD in excess of the specified rating may lead to its permanent breakdown. Therefore, high reliability will be secured if special attention is paid to the design of a power supply circuit, possible fluctuations in the supply voltage, surrounding components, operating temperature, environment surges or spikes, etc.

Rating or Recommended Operating Conditions

The “Rating” or “Recommended Operating Conditions” represent the specification of a recommended operating condition that guarantees the operation of the VFD. It also serves as a test condition for which the product is subjected prior to shipment from the factory. Therefore, it should be noted that if a VFD is used in excess of the maximum or minimum rating specified here, its operation and quality will not generally be guaranteed even when it remains within the absolute maximum rating.

In addition, this rated value has been defined assuming the most standard service conditions by users. However, some may want to study the possibility of using the VFDs outside the rated value, prompted by a desired individual requirement. In such a case, please feel free to consult with us, as we may be willing to discuss the possibility of the proposed specifications.

Block Diagram

It shows how to connect the power supplies. The basic circuit used in factory inspection is the same as this one.

Electrical (and Optical) Characteristics

All of numbers specified in this section shows the characteristics when the Active Matrix VFDs are tested under the typical (TYP) operating conditions unless otherwise noted.

Timing Chart

It shows the AC characteristics and relationship of interface operations.

Serial Data Format

It shows the shift register and segment or grid assignment, and how to control these data to optimize grid scan.