CONTROL CIRCUIT OF THERMAL HEAD

PURPOSE:To improve print quality by charging a capacitor of a thermal head, synchronized to a latch signal, then controlling the discharge from the capacitor using a data signal to generate a printing energy control signal and driving the thermal head. CONSTITUTION:If LATCH is entered initially, SW1 is turned ON and SW2 and SW3 are turned OFF, so that a reference voltage Vo is charged on a capacitor Co. After charging, SW1 is turned OFF. Then DATA is 'L' during a DATA reading period, that is, the SW2 becomes turned ON, if data is printed in white, and the capacitor Co is electrically discharged at a time constant to be determined by tau = (R1 + R2) X Co. On the other hand, if DATA is 'H', that is, data is printed in black, SW2 becomes turned OFF and discharging the capacitor Co is temporarily suspended. In other words, if DATA is printed with a high printing rate (black proportion), the discharge is temporarily suspended so that Vout voltage is maintained high. If reading DATA is completed, the SW2 becomes turned OFF and the SW3 becomes turned ON until a next LATCH is entered. Further, the capacitor Co is electrically discharged by a time constant to be determined by tau = (R1 + R2) X Co.