4-9

Both analog and digital data is input to control the action of

the dual laser driver. Address codes and input data are

described in the Table 1 Address Map. Digital data is

normally entered first, followed by the analog data via the

multiplexed analog bus which updates the S/H stored

voltage levels. Four bit digital addresses to pins A3, A2, A1

and A0 are decoded to define the programming functions

for data input. It is important to note that Address 15 is a

reset for Addresses 9 - 14 and should be performed as the

first step in programming because there is no power-on-

reset on-chip.

Diagnostic Output Data

For the purpose of monitoring on-chip signals, the

multiplexed bus can be used to output signals (at the DIAG

pin) via an analog diagnostic amplifier. This mode has the

capability to monitor the multiplexed output of four (0-5V)

analog signals for each laser channel. Note that the

diagnostic information at the VLOW (Amplifier A3) output is

to 5V range. To minimize noise problems the monitoring

function is normally performed during the laser scan for only

one signal per scan. In addition, there are 3 test modes

which allows the bus to present analog signals for testing the

performance of the eight S/H circuits.

Thermo-Electric-Cooler (TEC) Circuitry

Figure 4 shows the Thermal Electric Cooler (TEC) drive

circuit with an external reference resistance, a thermo-

resistor to sense temperature plus feedback components for

stable drive. There is a thermo-resistor reference input on

the chip for monitoring the laser's temperature via a 5-10k

Ω

thermo-resistor which must be mounted near the laser. A

reference voltage on the external reference resistor is

established by a current from a stable bias source. This

current is mirrored to the thermo-resistor (one for each laser

driver system). A comparator senses the voltage across the

reference resistor versus the voltage across the thermo-

resistor and drives the gate of the TEC FET driver. As such,

the TEC circuit senses the thermo-resistor input as a

measure of the laser temperature and the TEC drive is

adjusted to maintain a stable Laser temperature slightly

below the ambient temperature. An external Power FET is

needed to provide the high TEC driver currents. An out of

range temperature output for each laser is also provided.

Laser Protection Diodes

Another feature included on the chip is two high current ESD

diodes which, in the printer system, are used to protect the

Laser Diodes from ESD damage. Another component of Laser

protection in printer systems are relays to disconnect the

Lasers when in a non-operating mode. For this purpose, a

single relay driver is included.

Over Current Flag Output (OC1, OC2)

Over-current detection is also included on-chip. The circuit of

Figure 3 shows the over current detection circuit. For each

laser source current driver, the over current monitor compares

the S/H input of amplifier A2 to the output of amplifier A1. If

the output voltage of A1 exceeds the input of A2 by 40%, then

an over current state exists and the OC output will go high.

Invert Option

An INVERT input reconfigures the device such that the

constant current source can be high side instead of the

normal low side. This provides functionality for driving laser

diodes in a common cathode configuration as opposed to

the normal common anode configuration. The INVERT must

be low (or open with the internal pull down) for the Figure 1

circuit.

Reset Action

The RESETB (active low) controls three things:

(1) The TEC driver is turned off.

(2) The Shunt driver is turned on to turn off the laser.

(3) The Constant current driver is turned off.

+

-

46

1.9V

Q2

47

48

TEC

+

-

+

-

CURRENT

MIRROR

TRES2

+

TECREFR

TECGDR2

TECFB2

TECREF2

+

FIGURE 4. TEC (THERMAL ELECTRIC COOLER) CIRCUIT WITH REFERENCE RESISTOR AND THERMAL RESISTOR SENSOR OF

LASER TEMPERATURE

REFERENCE

RESISTOR

THERMALLY

LASER2

THERMO-

RESISTOR

(TEMPERATURE SENSOR)

45

44

12

COUPLED

COMPONENTS

43

OT2

O.T.

U.T.

TEMP.

COMP.

HIP9022