PRINT HEAD TEMPERATURE CONTROL PART

Introduction

For quality printing, it is necessary to know and

control the temperature of the printhead. Thus,

the latter has a built - in aluminium resistor,

whose value changes slightly with the tempera-

ture. 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 pos-

sible to switch quikly the measurement process

from one to the other. The function is foreseen to

be integrated into the head driver, and is de-

scribed hereafter.

Print Head Block Diagram (fig. 4)

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:

=

⋅4

2

This current is injected either into the aluminium

resistor of the head A (Ralu. A) or B (Ralu. B), de-

pending of the switch SW3. The alu. resistors are

grounded, and the voltage at their << hot >> side

(Vx) is re-entered via the pins Vxa and Vxb. Us-

ing 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 compat-

ible 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 alu.

resistor is very unprecise. Voffset must be adjust-

able; this is done by means of a 5 bit - D/A con-

verter, giving 32 different values. Finally, the volt-

age at the input of the A/D converter is:

or

N = 0, 1, ...31

grated, and used for the current source and both

the A/D and D/A converters. In this way, the sys-

tem performance is independent from the preci-

Vref is also available on pin #45, and can be

used for low consumption purposes. (The exter-

nal 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 transpar-

ent latch, which can be used or not (by trying the

REF

VOLT

VREF

+

g1

A1

g2

A

VREF/2

VREF OUT

Va

D/A 5 BIT

A/B

ON/OFF

DA4

DA3

DA2

DA1

DA0

S2

S1

S0

D

VREF

CONTROL

REGISTER

LATCH 10 BIT

SHIFT REG. 10 BIT

CRLATCH

CRCLOCK

CRDATA

A

B

D

C

VREF

A2

Vx

SW1

SW2

SW3

CH0

+

-

CH1

CH2

CH3

CH4

CH5

ADDATA

ADCK

CONV START

A/D

INPUTS

CH0_BUF

REXT

ONENABLE

RXA, RXB

VXA, VXB

RALU B

RALU A

ANALOG GND

Note; the analog ground is separated from the digital ground of the remaining part of the driver

D97IN533B

HIGH-SIDE

CONSTANT

CURRENT

SOURCE

VOFFSET

Figure 4. Print Head Block Diagram

L6452

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