ds1861 Maxim Integrated Products, Inc., ds1861 Datasheet - Page 15

• Manufacturer Part Number: ds1861
• Description: Ds1861 Full Laser Control With Fault Management
• Manufacturer: Maxim Integrated Products, Inc.

Figure 11 shows the extinction ratios (expressed in
decibels) as a function of N for several A
Note the actual extinction ratio value is determined by
the ratio A
varies due to voltage, temperature, or aging effects, it
becomes an additional error source when determining
the overall extinction ratio variance. The DS1861 is
meant to be used with a monolithic laser driver, where
A
changes in the ratio of A
The N value itself is determined by the extinction ratio
register. There are two components of the ER register
used to calculate N. The range select bit (RSEL), which
selects the high (RSEL = 1) or low (RSEL = 0) range of
N, is the MSB of the register. The lower seven bits of
the register (ER<6:0>) determine the value of N within
the selected range. The value of N is given by:
The ER<6:0> value should be programmed to values
between 28 and 104 (decimal), regardless if the high or
low range is selected, to ensure that the N value
remains accurate and constant as intended. These lim-
its allow the N values of the high and low range to over-
lap to ensure that all N values can be attained, but
prevents potential errors that can be caused by using
the extremes of each RSEL range. Figure 12 shows the
N values as a function of the ER register setting.
The DS1861 uses a tiered slew-rate control system that
adjusts the DAC update rate and the number of LSBs it
increases/decreases per update cycle when the control
loop is seeking to converge to its steady state value. For
the APC loop, it makes its decision on the required con-
vergence rate based on the percent error between the
present BMD current and the BMD current-set point.
The modulation current slew rate is adjusted based
solely on the difference between its present code and
the code-set point that is determined by the AERC cir-
cuitry. Both update rates are designed to prevent any
overshoot during large set-point changes in excess of
10%, which assumes (see the following) the ratio of the
laser diode’s threshold current (I
power current (I
exhibit any overshoot when using the DS1861.
M
and A
N
D
M
=
/A





are generated on the same chip, so
Full Laser Control with Fault Management
The Convergence Algorithm and
ER
ER
D
APC
<
in addition to N. If the A
<
6 0
6 0
) is below 7:1. Most systems do not
:
:
2
> +
> +
M
32
to A
32
when RSEL
D
when RSEL
Overshoot Control
are minimal.
TH
____________________________________________________________________
) to the average
M
/A
=
=
1
D
0
M
ratios.
/A
D
ratio
Figure 11. Typical Extinction Ratios vs. N Codes
25
20
15
10
25
20
15
10
25
20
15
10
5
0
5
0
5
0
16
16
16
RSEL = 1
EXTINCTION RATIO (AM/AD = 0.8)
EXTINCTION RATIO (AM/AD = 0.4)
EXTINCTION RATIO (AM/AD = 1.2)
32
32
32
RSEL = 0
ER SETTING (DECIMAL)
ER SETTING (DECIMAL)
ER SETTING (DECIMAL)
vs. ER SETTING
vs. ER SETTING
vs. ER SETTING
48
48
48
RSEL = 1
RSEL = 0
RSEL = 0
64
64
64
80
80
80
96
96
96
112
112
112
15