ds1861 Maxim Integrated Products, Inc., ds1861 Datasheet - Page 12

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

Full Laser Control with Fault Management
The DS1861 APC is accomplished by adjusting the
bias current (I
from a photodiode matches the value determined by
the APC register. The relationship between the APC
register and I
where APC<4:0> is the numerical value determined by
the five least significant bits of the APC register, and
the gain value is determined by the upper three bits of
the APC register, as shown in the truth table below. See
Figure 7 for a graph of the BMD current-set point vs.
the APC register value.
12
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TX-D
TX-F
GND
SDA
SCL
V
I
CC
A
APC<7:5>
A
A
SET
2
1
0
000
001
010
011
1xx
BMD
= 1.525µA Gain (APC<4:0> + 32)
BIAS
is given by:
) until the feedback current (I
Detailed Description
V
Automatic Power Control
CC
I
WITH SETTINGS
2
C-COMPATIBLE
PROTECTION
USER EEPROM
PASSWORD
INTERFACE
(8 BYTES)
DS1861
GAIN
16
1
2
4
8
FAULT MONITOR
NONVOLATILE
QUICK TRIP
BMD
POWER CONTROL
RATIO CONTROL
DISTURBANCE
NONVOLATILE
NONVOLATILE
AUTOMATIC
AUTOMATIC
EXTINCTION
)
CURRENT
The BMD pin appears to the photodiode as a voltage
source (V
in parallel with a current source (I
setpoint. When the control loop is at its steady state
value, the voltage at the BMD pin (V
V
When an imbalance causes the feedback current to differ
from the set current, the difference in the currents causes
the DS1861 to adjust the BIASSET and the MODSET cur-
rents to new settings so the input current matches the
set current. During transient periods, the difference in
the I
pass through R
decrease in the V
sink and source current, which must be configured using
the DPOL bit in the control byte for proper operation.
This allows the photodiode to be referenced to either
V
Register Descriptions sections for more detail.
REF
CC
I
I
B
BMD
V
CC
SET
, and the BMD current will all be sourced by I
or GND. See the Memory Map and Detailed
I
I
I
I
DIST
BMD
BMD
B
SINK/SOURCE
and I
REF
CURRENT
I
SET
) with an output resistance equal to R
BMD
BMD
BMD
V
currents causes a small current to
REF
. This causes a slight increase or
R
Σ
voltage. The current source can
BMD
Functional Diagram
CURRENT
SENSE
SET
) equal to the I
BMD
) is equal to
MODSET
BMD
BIASSET
BMD
BMD
SET
.