Print Head Temperature Control Part

L6452

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Print Head Temperature Control Part

4.1

Introduction

For quality printing, it is necessary to know and control the temperature of the print head. Thus,

the latter has a built - in aluminium resistor, whose value changes slightly with the temperature.

The temperature determination is done by injecting a constant current in the resistor, and

measuring the voltage drop across it. Since high - end printers have two heads, it must also be

possible to switch quickly the measurement process from one to the other. The function is

foreseen to be integrated into the head driver, and is described hereafter.

4.2

Print Head Block Diagram (Figure 5.)

At first we have a constant current source, which can be disabled by an external pin (OnEnable)

or by a control register, described later. The value of the current can be programmed by an

external resistor, and is given by:

This current is injected either into the resistor of the head A (Ralu. A) or B (Ralu. B), depending

of the switch SW3. The resistors are grounded, and the voltage at their << hot >> side (Vx) is

re-entered via the pins VxA and VxB. Using separate pins from RxA and RxB permits to be

more flexible, and a filter can eventually be added as shown in the drawing.

The voltage Vx is amplified by A1 and A2, and then converted in a digital value. To be

compatible with the input range of the A/D converter, it is necessary to subtract an offset

voltage Voffset from Vx. Moreover, as the initial value of the aluminum resistor is very

imprecise. Voffset must be adjustable; this is done by means of a 5 bit - D/A converter, giving 32

different values. Finally, the voltage at the input of the A/D converter is:

or

the A/D and D/A converters. In this way, the system performance is independent from the

can be used for low consumption purposes. (The external sinked current has to be a DC

current).

The system is under control of a 10 bit register, CR. CR is accessed serially and has a

I

CCS

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