Design Note 33 Issue 1 April 1996 Isolating High Side Switch for Thermal Printer SMPS R1 L1 +12V d.c 60µH 68R 2.5W L2 D1 1N5821 Vin Vsw Q1 ZTX 949 +24V d.c 1.5µH R3 22k LT1170 Vfb Vc GND C1 68µF CONTROL SIGNAL 5.0V Q2 ZTX107 R6 C4 100µF C5 390µF C6 100µF R4 1.2k 470R R5 10k GND C3 100µF R2 1k C2 1µF Figure 1 LT1170 Effected Boost Converter Circuit using a ZTX949 to Isolate Supply. This circuit has been designed for use in thermal printers, to isolate the output of a boost converter from the input supply. used with the original circuit to provide an isolated 24V 2A high efficiency power supply. This allows the print head to be powered down when not in use, so that the thermal paper is not discoloured - this can be a problem with this converter topology as even when the converter IC is disabled, a direct path is available through the boost inductor L1 and the Schottky diode D1. The low VCE(sat) of the ZTX949 (rated at a BVCEO of 30V, a continuous collector current of 4.5A and a pulsed current at 20A) enables the device to switch low duty cycle pulse trains up to 7A without a heatsink. The circuit modification includes a low VCE(sat), high current PNP transistor - the ZTX949, as a high side switch controlled via the ZTX107. These components are Si m il a r is o l a ti ng sw itches can b e configured using P-Channel MOSFETs, but due to their lower silicon efficiency these parts do not permit cost and space effective designs. DN33 - 1