Dc operation, Modulation control, Bias and mod output during p – Rainbow Electronics DS1875 User Manual
Page 17: Bias and mod output during power-up, Figure 1. power-up timing (ben is a long burst), Ds1875, Pon triplexer and sfp controller
DS1875
DC Operation
When using autodetect mode or closed-loop mode,
BEN should be equal to V
CC
or long burst. In open-loop
mode, BEN should be ground or any burst length.
Modulation Control
The MOD output is an 8-bit scaleable voltage output
that interfaces with the MAX3643’s VMSET input. An
external resistor to ground from the MAX3643’s
MODSET pin sets the maximum current that the voltage
at the VMSET input can produce for a given output
range. This resistor value should be chosen to produce
the maximum modulation current the laser type requires
over temperature. Then the MOD output’s scaling is
used to calibrate the full-scale (FS) modulation output
to a particular laser’s requirements. This allows the
application to take full advantage of the MOD output’s
resolution. The modulation LUT can be programmed in
2°C increments over the -40°C to +102°C range.
Ranging of the MOD DAC is possible by programming
a single byte in Table 02h, Register 8Bh.
BIAS and MOD Output During Power-Up
On power-up the modulation and bias outputs remain off
until V
CC
is above V
POA
, a temperature conversion has
been completed, and, if the V
CC
ADC alarm is enabled,
a V
CC
conversion above the customer-defined V
CC
low
alarm level must clear the V
CC
low alarm (t
INIT
). Once all
these conditions (t
INIT
) are satisfied, the MOD output is
enabled with the value determined by the temperature
conversion and the modulation LUT (Table 04h).
When the MOD output is enabled, the BIAS output is
turned on to a value equal to the temperature-indexed
value in the BIAS LUT (Table 08h). Next, the APC inte-
grator is enabled, and single LSB steps are taken to
tightly control the average power.
If a fault is detected and TX-D is toggled to re-enable
the outputs, the DS1875 powers up following a similar
sequence to an initial power-up. The only difference is
that the DS1875 already determined the present tem-
perature, so the t
INIT
time is not required for the
DS1875 to recall the APC and MOD set points from
EEPROM.
1
2
3
4
5
6
7
8
9
10
11
12
13
t
INIT
V
POA
V
MOD
I
BIAS
V
CC
BIAS
SAMPLE
APC INTEGRATOR ON
BIAS LUT
VALUE
Figure 1. Power-Up Timing (BEN is a Long Burst)
PON Triplexer and SFP Controller
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