4-8

Circuit Block Descriptions

Laser Drive Circuitry

In Figure 3, the gate of the external current source Power

FET, Q2 is driven via the Operational Transconductance

Amplifier (OTA), A2 on the IC. The voltage on the current

monitored by a X10 gain of the feedback amplifier, A1. The

stability of the current loop is established with an external

0.1

µF capacitor to ground at the gate of the Power FET. The

sampled voltage range is 0 to 0.5V when the proper value of

OTA, A2 compares the X10 gain signal toa0to5Vreference

signal from an on-chip Sample and Hold (First S/H) circuit.

voltage reference signal.

The S/H reference for the Laser Drive Current current is

updated with other multiplexed S/H circuits from a serial bus

and an off-chip D/A converter. Laser constant current is fully

controllable by the multiplex analog S/H bus, allowing accurate

calibration of the laser output and corrections as the laser ages.

In Figure 1, the laser drive current from Q2 is digitally switched

to either flow through or is shunted around the laser diode by

switching the external Shunt Power FET, Q3 on or off. The gate

of the Shunt FET is switched between two voltages (Upper and

Lower) which are provide by 2nd and 3rd S/H circuits. These

Shunt FET gate drive levels are fully programmable via the

multiplexed analog S/H bus. By adjusting these levels to

account for the laser power supply, the Shunt FET threshold

and channel resistance; minimum Shunt FET gate drive power

levels can be established. The Upper and Lower gate voltage

driver circuits are two high current OTA amplifiers with two filter

capacitors. The upper voltage is programmable in the 0V to

accomplished by an optional on-chip voltage inverter circuit.

The input to amplifier A3 is either direct from the S/H input or

inverted by amplifier A5.

The maximum laser on-off switching speeds are dependent

on the selection of Shunt FETs. A Harris dual

complementary MOSFET, RF3V49092 or RF3S49092 has

been designed specifically for this application. With the

constant current set at 0.8A, a typical laser switching speed

of 20ns has been measured.

Thermal Compensation

A 4th S/H circuit is used to set the amplitude of an optional

thermal compensation signal which can be used to

modulate the constant laser current source as a two pole

filtered effect of the laser on-off data. This feature may be

disabled when it is not required. This circuit is designed to

compensate for the temperature variations in the laser as

the laser is turned on and off. The bypass capacitors at the

Thermal Comparator (CTCx-10K, CTCx-27K) represent the

respective poles for the filter.

A laser cools after it has been off for a period of time and is

more efficient when it is turned-on. Compensation for the

increased efficiency is made by slightly reducing the

current level of the constant current source FET. The level

will be reduced by a programmable amount of 0 to 5% of

full scale. The programmable amount is fixed by the level of

compensation to S/H addresses 7 and 8 (see Table 1). The

percent of modulation (change) in drive current is

calculated as follows:

For example, if we control the Laser Drive current with 2V

programmed with address 1 and 2 for the First S/H’s, given

If 2V is programmed to addresses 7 and 8 as Thermal

Mod.% = (2/2) x 5% = 5%.

In Figure 3, the correction is applied from the output of the

Thermal Compensation circuit (where the current is

2V/20k

Ω = 0.1mA) to the input of amplifier A2. The 0.1mA is

forced into the 1k

Ω resistor (and the low Z output of A1) to

increase the voltage at the inverting input of A2 by 0.1V or 5%

Input Data

I

DL

V

IN DL

()

R

S

10

×

()

-------------------------

=

(EQ. 1)

Modulation

°°⁄

V

IN TC

()

V

IN DL

()

---------------------

5

°°⁄

×

=

(EQ. 2)

FIGURE 3. LASER CONSTANT CURRENT SOURCE DRIVER

WITH OVER CURRENT DETECTION AND

THERMAL COMPENSATION

S/H SYS

+

-

X10

CC2

XTEN2+

XTEN2-

53

54

55

0.25

Ω

A1

A2

Q2

1k

Ω

O. C.

COMP.

+

-

49

0.1

µF

12k

Ω

30k

Ω

51

VOLT. TO

CURRENT

52

VOLT. TO

CURRENT

OC2

CTC2-27K

CTC2-10K

THERMAL

COMP.

10k

Ω

27k

Ω

20k

Ω

0.1

µF

0.02

µF

HIP9022