3
MITSUBISHI
DIGITAL ASSP
M66512P/FP
LASER-DIODE DRIVER
OPERATION
1. Setting for Laser Drive Current
The M66512 has 3 built-in constant current sources, I
L1
, I
L2
,
and I
L3
. Each output current can be controlled independently.
The following (1) to (3) describe the method for how to set
I
L1
,I
L2
, and I
L3
.
(1)I
L1
setting method
The value of I
L1
is determined by the voltage on the V
L1
pin and the resistor (RC1) connected between the 1RC
pin and GND. The following equation is used for approxi-
mation.
V
L1
[V]
RC1 [k
]
provided that 0
≤
V
L1
≤
V
CC
–1.8V and I
L1(max.)
=60mA
(2)I
L2
setting method
The value of I
L2
is determined by the voltage on the V
L2
pin and the resistor (RC2) connected between the 2RC
pin and GND. The following equation is used for approxi-
mation.
V
L2
[V]
RC2 [k
]
provided that 0
≤
V
L2
≤
V
CC
–1.8V and I
L2(max.)
=30mA
(3)I
L3
setting method
The value of I
L3
is determined by the internal reference
voltage (V
ref
) and the resistor (RC3) connected between
the 3RC pin and GND. The following equation is used for
approximation.
V
ref
[V]
RC3 [k
]
provided that V
ref
=1.4V (typ.) and I
L3(max.)
=30mA
Note: Each of the above equations is a typical on for ob-
taining I
L1
–I
L3
. In practice values vary by a few per-
cent due to some reasons such as differences of
ICs from lot to lot and variations in operation tem-
peratures.
2. Switching Operation
The laser turns on if DATA =“L”. The laser drive current at that
moment is I
L1
+I
L2
+ I
L3
.
If DATA = “H” the laser is turned off, and the laser drive cur-
rent is almost zero irrespective of the values of I
L1
to I
L3
.
3. Use of the V
OFF
Input
When DATA = “H” the current flowing through the laser is zero
so the laser is turned off, but the internal current sources are
in operation.
When V
OFF
= “L” in contrast, the internal current sources are
turned off. Accordingly, it is possible to prevent an excessive
current from flowing through the laser by, for example, fixing
the V
OFF
input to “L” until V
CC
, after turned on, reaches
3.5V(typ.). (See the section dealing with internal reset.)
4. Laser Power Monitoring Operation
At the MO and CO pins, the M66512 outputs data obtained
by the monitoring photodiode (PD) contained in the laser, in
the sequence explained below.
(1)A current equal to the PD current generated by laser light
flows through the resistor (RM) connected between 1RM
and 2RM. Then, a potential difference (V
M
) proportional to
the output of laser light occurs at RM.
(2)The V
M
is output at the MO pin as an analog signal
through a buffer amp. At the same time, V
M
is compared
with the internal reference voltage V
ref
(1.4V typ.) by the
comparator, thence the result of the comparison is output
at the CO pin in TTL level.
I
L1
[mA] = 12
×
I
L2
[mA] = 6
×
I
L3
[mA] = 10
×
Condition
V
M
<V
ref
V
M
>V
ref
CO output
“L”
“H”
A Schematic Diagram of Monitor Circuits
LD
PD
RM
VM
Vref
+
_
_
+
MO output
CO output
Buffer amp
Comparator