In this system, 12 printheads, each having 128 jets, were used to print a page-width image on a web at speeds of up to 8 feet per second. The pulse requirements were extremely demanding, requiring that the pulse amplitude and slew rates were programmably controlled to 1%. To make it even more difficult, the printhead capacitance could vary dynamically from zero to 50,000 pf, and the printhead was located at the end of a long cable; this produced a ringing pulse at the printhead with a shape that varied from pulse to pulse.

In order to achieve constant pulse shape at the far end of the cable, we designed the pulse generator to supply a band-limited pulse, and we also designed a custom low-inductance cable to drive the printhead. This enabled us to keep the lowest resonance of the head-cable system above the cutoff frequency of the pulse, yielding a constant-shape pulse under all conditions.

Because of the large currents (60A) and dI/dT ( >100A/microsecond), the grounding and power distribution systems were carefully designed to avoid ringing and crosstalk between the 12 channels, with full consideration of the PCB trace impedances.

Foldback limiting was used to insure that the output transistors would be safe under all fault conditions, including a shorted output. The design of the foldback technique was guided by an analysis of the instantaneous junction temperatures, and its performance was proven experimentally.