Next-generation inkjet printhead drive electronics

Hewlett-Packard Journal, June, 1997 by Huston W. Rice

By integrating the functions of four ICs into one new custom IC and then moving all the electronics related to the pens up to the carriage with the pens, significant savings were realized. A simple, low-contact-count, inexpensive flexible cable is used to connect the carriage to the main printed circuit assembly.

The project team for the HP DeskJet 850C printer developed the many elements of the printing system in parallel. In particular, the print cartridges (called pens) were new designs, along with the electronics that control them. As a result of the pens being new designs, their drive and control requirements were not completely defined, but were changing during the development program. The result of this was a system that worked well electrically, but was not fully optimized from a cost standpoint.

In particular, two aspects of the pen drive system presented opportunities for significant cost reduction. First, the flexible cable connecting the carriage and pens to the main printed circuit assembly was very elaborate and fairly expensive. Second, the electronics that control the pens were implemented in four different analog ICs, three of them custom ASICs.

With the advantage of being able to look back on the now well-defined system needs, a new approach was selected. By integrating the functions of the four ICs into one new custom IC and then moving all the electronics related to the pens up to the carriage with the pens, significant savings were realized. The new, highly integrated ASIC is less expensive to purchase and to assemble into the product. Since the signals are restricted to digital data and raw power, a simple, low-contact-count, inexpensive flexible cable is used to connect the carriage to the main printed circuit assembly.

For this design approach to be successful in the HP DeskJet 820C, several issues had to be overcome. For the greatest benefit, all of the electronics associated with the the pens had to be contained on the carriage printed circuit assembly. Would it all fit? Because of a tight schedule and limited mechanical engineering staffing, no mechanical changes could be made to the carriage assembly to make more room. An additional mechanical constraint was that no components could be placed on the bottom half of the printed circuit board, which was needed for the connectors for the pens. To get the circuits to fit, all the analog IC functions had to be integrated into a single ASIC. Could all the different functions--power control, digital I/O, sensitive analog-to-digital measurements, power drivers--be integrated into a single device? If all the analog functions from four ICs were integrated into one IC, would there be thermal overheating issues in the IC? Would there be problems with radiated electromagnetic emissions from the digital interface to the carriage over a simple unshielded flexible cable?

To provide an aspect of excitement to the program, once this approach was chosen, there was no easy alternative to fall back upon if the above issues could not be dealt with. If this design failed, the whole HP DeskJet 820C printer program would be put in jeopardy.

Carriage Electronics Implementation

A key architecture change was made in the pen drive and control electronics in the HP DeskJet 820C compared to the DeskJet 850C. The power supply for the pens was modified in two ways. First, two independent dc-to-dc converters are used to supply power to the black and color pens in the DeskJet 850C. In the DeskJet 820C, a single pen power supply is used to drive both the black and color pens. Second, the control topology of the dc-to-dc converter was changed, as explained later in this article. The DeskJet 850C design requires seven large capacitors, two inductors, two power FETs, two power diodes, and several small discrete resistors and capacitors. All of this was replaced with two capacitors, one inductor, one power FET, one power diode, and one power resistor. This eliminates not only the need for several square inches of printed circuit board space that was not available on the DeskJet 820C carriage printed circuit board, but also the cost of the unneeded components.

The two pens in the product (black and color) must be driven at different voltages, and the DeskJet 820C design now only has one power supply, which is shared between the pens. This forced a change in the way printing is done. In the DeskJet 850C, both pens can be driven at any time, allowing maximum flexibility in how the printed image can be formed, and therefore maximum speed. In the DeskJet 820C, printing with the black and color pens alternates. For instance, black may be printed from right to left and then color from left to right. This difference costs a little in print speed for some color documents, but was key in enabling all the electronics to fit on the carriage printed circuit assembly.

Several techniques were used to integrate all the pen electronics onto the carriage for the HP DeskJet 820C. Beyond the power supply changes, the next most important step was designing a mixed-signal analog/digital/power ASIC that integrates all the functions required to drive and control the pens. The general strategy was to integrate all the relatively small-signal electronic functions into one ASIC to minimize the the total component count. This both minimizes the cost and uses the minimum printed circuit board area on the very small carriage printed circuit assembly. However, to keep the ASIC silicon die area under control and to minimize the total power dissipated by the ASIC, several key components are not integrated. The power FET and diode for the dc-to-dc converter, both very large devices (from a silicon area point of view) are implemented as discrete devices externally. Two linear regulators are also implemented with off-the-shelf discrete devices to keep their power dissipation out of the ASIC package. Beyond these parts and some discrete capacitors and inductors that cannot be integrated, everything else is internal to the ASIC.