Rainbow Electronics DS1862 - Manual (Page 15)

DS1862
XFP Laser Control and Digital Diagnostic IC
____________________________________________________________________ 15
Detailed Description
The DS1862’s block diagram is described in detail
within the following sections and memory map/memory
description.
Automatic Power Control (APC)
The DS1862’s APC is accomplished by closed-loop
adjustment of the bias current (BIASSET) until the feed-
back current (BMD) from a photodiode matches the
value determined by the APC registers. The relation-
ship between the APC register and I
BMD
is given by:
I BMD = 5.859µA x APC C <7:0> +
(1.464µA x APC F <1:0>)
where APC C <7:0> is the 8-bit value in Table 04h, byte
84h that controls the coarse BMD current and
APC
F <1:0> is the 2-bit value that controls the fine BMD
current.
The BMD pin appears as a voltage source in series with
two resistors. The overall equivalent resistance of the
BMD input pin can be closely approximated by the plot
in Figure 8. The voltage that appears on the BMD pin,
assuming no external current load, is 1.2V if BMD is in
sink-current mode (SRC_SINK_B = 0) or 2.0V if BMD is
set to source current (SRC_SINK_B = 1). This allows the
photodiode to be referenced to either V
CC3 or GND.
When the control loop is at steady state, the BMD current
setting matches the current that is measured by the I
BMD
voltage across the internal resistance. During a transient
period, the DS1862 adjusts the current drive on the
BIASSET pin to bring the loop into steady state. The
DS1862 is designed to support loop gains of 1/20 to 10.
On power-up, the BMD current ramps up to the previ-
ously saved current setting in EEPROM APC registers.
While operating, the DS1862 monitors the BMD current.
If it begins to deviate from the desired (set) I
BMD value,
then, again, the current on the BIASSET pin is adjusted
to compensate.
Extinction Ratio Control
Look-Up Table (LUT)
The DS1862 uses a temperature indexed look-up table
(LUT) to control the extinction ratio. The MODSET pin is
capable of sinking current based on the 8-bit binary
value that is controlling it. The DS1862 also features a
user-configurable current range to increase extinction
ratio resolution. Five current ranges, as described in
Table 1, are available to control the current entering
MODSET.
I BMD
VOLTAGE
BMD
NOTE: V BMD IS
CONTROLLED BY THE
SRC_SINK BIT IN
TABLE 04h
I
BMD
(mA)
R
BMD
(Ω
)
1.50 1.25 0.25 0.50 0.75 1.00
BMD RESISTANCE vs. BMD SUPPLY CURRENT
470
489
508
527
546
565
584
600
0
0
V BMD
R BMD
Figure 8. Approximate Model of the BMD Input
Table 1. Selectable Current Ranges for
MODSET
LUT CURRENT RANGE
TABLE 04h, BYTE 86h<2:0>
CURRENT RANGE
(µA)
000 0 to 75
001 0 to 150
010 0 to 300
011 0 to 600
100 0 to 1200