Maximum Ratings and Standard Printing Conditions

Understanding standard printing conditions and maximum ratings is important for ensuring the long life and high performance of equipment with a thermal printhead (TPH). The standard printing conditions provide criteria for achieving the best printing results. The maximum ratings define the upper limits for the safe and efficient use of equipment and determine the characteristics of the printer. If these ratings are not satisfied, the characteristics of the product will be affected. Failure to meet these conditions and maximum ratings may result in the malfunction or premature failure of the print head. Printers should be designed according to the guidelines for various applications, such as printing receipts and labels, to ensure device durability and output quality.

Maximum ratings

The maximum ratings indicate the maximum values that can be set for a TPH product. These values depend on the product. When designing a printer, care should be taken not to exceed the maximum ratings of the TPH product, taking into consideration the selection of peripheral devices. If a TPH product is used at a value that exceeds the maximum rating, its characteristics may be degraded or it may be damaged. Even if the operating conditions for the product are set within the maximum ratings, if the product does not have sufficient characteristics for the target printer application, the selection of the TPH model or the setting values may be inappropriate. Please contact ROHM for further information.

The following is an example of the maximum ratings listed in ROHM’s TPH specifications.

Maximum ratings (at head ambient temperature of 25℃)

Printing power supply voltage（V_H）

The power supply voltage (V_H) is used to generate heat in the heating elements. The resistance value of the heating elements of the TPH is set to satisfy the allowable voltage and current for the driver ICs mounted on the TPH, even when the maximum power supply voltage is reached. Therefore, if the maximum rating is exceeded, the driver ICs will be subjected to a load that exceeds the allowable amount, which may cause damage to the TPH. The maximum ratings also apply to the voltage that is applied when the battery is fully charged or a voltage that fluctuates when printing is started or stopped. Ensure that the voltage applied to the TPH does not exceed the maximum rating during printer operation.

Applied energy（E₀）

Applied energy is expressed as the product of the power used to generate heat in the heating elements and the time during which the power is applied. The applied energy in the specifications indicates the energy consumed by the heating elements. The total energy supplied to the entire TPH is the sum of the above energy plus the energy consumed by the driver circuit and wiring. This energy depends on the printing conditions, even for a given TPH.
If the energy applied to the TPH exceeds the maximum rating, the heating elements may be damaged due to overheating. This may also degrade product characteristics and cause abnormal resistance values for the heating elements.

Substrate temperature

The substrate temperature is the temperature measured by the thermistor mounted on the TPH. Since the TPH has heating elements, the substrate temperature increases with product operation time. Therefore, set the operating conditions and heat dissipation characteristics (e.g., using a heat sink) so that the substrate temperature does not exceed the maximum rating.
If the substrate temperature exceeds the maximum rating, the mounted driver ICs or other devices may malfunction, resulting in product damage.

Logic power supply voltage, logic input voltage

The logic power supply voltage is used to drive the driver ICs mounted in the TPH. The logic input voltage is the voltage of the signals to be input to the driver IC. Logic signals other than the power supply voltage should be within the values specified according to the logic supply voltage. If the input signal voltage exceeds the specified value, the driver ICs may not operate properly and the product may malfunction or be damaged.

Standard printing conditions

The recommended standard printing conditions are given for the use of TPH products. The standard printing conditions depend on the TPH. When designing a printer, the operating conditions for the TPH should be set based on the standard printing conditions. If sufficient characteristics cannot be obtained for the target printer application, the selection of the TPH model or setting values may be inappropriate. Please contact ROHM for further information.

Three standard printing conditions, namely electrical conditions, mechanical conditions, and thermal paper, are explained below using examples.

(1) Electrical conditions

Electrical conditions for the standard printing conditions are explained using the ROHM TPH specifications.

　（at head ambient temperature of 25℃）

Power consumption(P_o)

Power consumption is the power consumed by the heating elements of a TPH
Power consumption values are calculated for products manufactured at the median (design target value).

Supply voltage(V_H)

The supply voltage is the voltage applied to the heating element circuit of a TPH. The energy applied to the heating elements depends on the printing conditions.

Print cycle(SLT)

The print cycle, or scanning line time (SLT), is the cycle of driving the heating elements in the main scanning direction. The cycle is defined as the time required to drive strobes sequentially to complete the printing of one line.

Energy consumption

The applied energy listed in the specifications is the energy consumed by the heating elements. Applied energy is expressed as the product of the power used to generate heat in the heating elements and the time during which the power is applied (recording pulse width). The total energy supplied to the entire TPH is the sum of the above energy plus the energy consumed by the driver circuit and wiring. This energy depends on the printing conditions, even for a given TPH.

The recording pulse width indicates the energization time required to drive the heating elements. This time is usually given by a strobe signal. For TPHs with built-in thermal history control, it is given by the start signal.

The standard recording pulse width depends on the environmental temperature because the amount of heat required for color development recording depends on the environmental temperature. In addition, continuous use of the product will cause the substrate temperature to fluctuate, so consider setting the recording pulse width according to the substrate temperature as well. By setting the recording pulse width according to the environmental temperature, stable print quality can be expected.

Depending on the thermal paper (print media) used, the optimization of the recording pulse width should be evaluated. The coloring sensitivity of thermal paper depends on its storage characteristics and conditions. By setting the recording pulse width according to the coloring sensitivity, stable print quality can be expected. If it is difficult to set the appropriate print quality, adjust the print cycle and other conditions.

Current consumption(I_o)

Current consumption is the instantaneous maximum current that flows during a printing operation. Its value depends on the printing conditions. To ensure stable print quality, it is necessary to secure the required power supply. The effective value of current consumption should be controlled not to exceed the current capacity of the TPH. The effective current value can be calculated from the energization time and recording cycle of the operating conditions. However, the current capacity of the cable is not included, so please also check the components you plan to use.

Number of divisions

The number of divisions is calculated by dividing the number of heating elements in one line by the number of dots to be applied simultaneously. It indicates the degree of division of heating elements in one line when they are energized multiple times.

The number of simultaneously applied dots is used to determine print quality. A higher number of dots applied simultaneously may increase the variation in print density.

(2) Mechanical conditions

Mechanical conditions for the standard printing conditions are explained using the ROHM TPH specifications.

（Head ambient temperature of 25℃）

Platen pressure

Platen pressure refers to the pressure exerted by the platen roller on the heating element of a TPH. A thermally reactive material such as thermal paper or thermal transfer ribbon is placed between the platen roller and the TPH. The recommended platen pressure depends on the product specifications. A load should be applied over the entire printing width. It is recommended that the load be applied uniformly in the main scanning direction.

Set the platen pressure to obtain optimum print quality according to the printer’s application. The characteristics of ROHM products have been verified within the set values of standard printing conditions.

Platen hardness

Platen hardness refers to the hardness of the rubber material of the platen roller. A higher value indicates greater hardness. Platen hardness determines the local stress on the heating elements and the stress load area on the platen roller. The hardness selection should be made in accordance with the printing media used.

Platen diameter

Platen diameter refers to the diameter of the platen roller. The platen diameter depends on the product specifications.
The maximum platen diameter should satisfy the specified diameter value. If the maximum platen diameter value is exceeded, the platen roller may come in contact with the driver IC protective resin, which may affect print quality or cause product defects.

Sub-scanning direction feed pitch

The sub-scanning direction feed pitch refers to the feed pitch of thermal paper or thermal transfer ribbon in the transport direction. Normally, the feed pitch in the sub-scanning direction is the same value as the resolution of the heating elements in the main scanning direction of the TPH. Some products have a heat-concentrated structure that makes the heating element smaller than the dot size, thereby improving coloring efficiency.

(3) Thermal paper

ROHM specifications recommend the following thermal papers for standard printing conditions.

 PD150R (Oji Imaging Media) or equivalent

Standard thermal paper is set assuming printer applications where TPH products are used. When using paper other than the recommended paper, adjust the printing conditions to obtain appropriate print quality. Printer setting conditions may also affect the TPH product characteristics. Evaluate the TPH product thoroughly, including whether the TPH characteristics are satisfactory.

A wide variety of thermal paper and other printing media are available. It is necessary to set the printing conditions according to the characteristics of the printing media and to consider the effects of the printing conditions on the product characteristics.