Driver ICs for Thermal Printheads

Driver ICs are ICs that drive and control electronic devices. Thermal printheads, which use heat to print, use driver ICs to control the heat generated by the heating elements (heaters) in the printhead. The driver IC causes the printhead to energize only the necessary heaters among the multiple heaters to print the corresponding characters or images. This article describes the operation of the driver IC by showing how the four signals (CLK/DI/LAT/STB) of the driver IC work together.

Driver ICs and Printing Operation

When printing with a thermal printhead, one line of data perpendicular to the paper feed direction is sent, one dot at a time, to the driver IC installed in the printhead. When all the data for one line has been sent, it is held in the driver IC and the corresponding heaters are energized to generate heat, which causes the thermal paper to color. When the power stops, the heaters are cooled and the next printing cycle begins. This operation is performed continuously for each line, and characters and images are output as the thermal paper passes over the heaters.

Driver IC Signals

The following describes the four signals that control the multiple heaters for printing.

1. Data In (DI) Signal
This is a digital signal used to input serial data of the print pattern to the driver IC. It sends information to the multiple heaters of the thermal printhead indicating whether to heat the corresponding pixels, thus determining the print pattern.

2. Clock (CLK) Signal
This signal is used to control the timing in electronic circuits and digital systems. In thermal printheads, it is used in conjunction with the DI signal to accurately transfer data etc. Specifically, when the voltage of the DI signal is high and the CLK signal changes from low to high, high is input to the thermal printhead. Conversely, if the CLK signal changes when the DI signal voltage is low, low is input to the thermal printhead.

3. Latch (LAT) Signal
This signal is used to temporarily hold externally supplied print data, which is one function of the driver IC. The LAT signal is generated after all serial data has been transferred; when the LAT signal rises, the serial data transferred to the shift register is ‘latched’ (finalized), is then applied to the heaters, and printing is executed.

The LAT signal is set to positive or negative logic by the printhead.

(Positive logic: active high, high voltage is logic value 1, low voltage is logic value 0;
negative logic: active low, high voltage is logic value 0, low voltage is logic value 1)

4. Strobe (STB) Signal
This is a trigger signal that applies current and drives only the heaters of the dots corresponding to the high data held in the latch register by the LAT signal. The pulse width of this signal determines the energization time for the heaters. By adjusting this energization time, it is possible to adjust the print density and quality. Although a large amount of electricity is required to drive the heaters, by using the STB signal to generate heat only for the necessary time with the appropriate timing, printing can be performed efficiently while reducing power consumption.

The STB signal is set to positive or negative logic by the printhead.

Printing Flow of Thermal Printhead Driver IC

1. Transferring data

   ：The DI signal (serial data) is sent to the shift register in synchronization with the CLK signal.

2. Latching data

   ：Data is latched and finalized by the LAT signal.

3. Driving heaters

   ：Heaters are driven by the STB signal based on the latched data.

4. Printing

   ：Heaters heat the paper to print.

5. Cooling

   ：Heaters are cooled, and then the next cycle begins.

【Printing Example】

【Timing Chart Example】

The thermal printhead driver IC thus controls serial data transfer and latching, as well as driving of the heaters, and the thermal printer performs these operations continuously to achieve fast and accurate printing.

Attention

Thermal printheads are precision electronic components that can be destroyed by mechanical or electrical shocks, so they should be handled with care.

Power On/Off
Follow the power on/off sequence below to avoid damaging the heaters.

For power on: Apply VDD first, then VH.

For power off: Turn off VH, then VDD.

If the above sequence is reversed, the heaters will be damaged and correct control will not be possible.

Maximum Energy Rating
When the STB signal is input, energy is applied to the heaters, causing them to heat up. If the applied energy exceeds a certain level, the heaters will be damaged, which will adversely affect the print quality and life of the thermal printhead. Therefore, the STB signal input time should not exceed the maximum energy rating.

The applied energy is the product of the power consumed by the heaters and the energization time of the STB.

Static Electricity Prevention
To prevent the destruction of thermal printheads due to static electricity, the operators handling the thermal printheads should always wear a grounded wristband. The work area should also be grounded to 10 Ω or less. Equipment should also be grounded, and conductive tools should be used.

For any inquiry on a thermal printhead driver IC, please contact us:
https://www.rohm.com/contactus