ADN2870ACPZ Analog Devices Inc, ADN2870ACPZ Datasheet - Page 11

IC,Laser Diode/LED Driver,LLCC,24PIN,PLASTIC

ADN2870ACPZ

Manufacturer Part Number
ADN2870ACPZ
Description
IC,Laser Diode/LED Driver,LLCC,24PIN,PLASTIC
Manufacturer
Analog Devices Inc
Type
Laser Diode Driver (Fiber Optic)r
Datasheet

Specifications of ADN2870ACPZ

Data Rate
3.3Gbps
Number Of Channels
1
Voltage - Supply
3 V ~ 3.6 V
Current - Supply
30mA
Current - Modulation
90mA
Current - Bias
100mA
Operating Temperature
-40°C ~ 85°C
Package / Case
24-VFQFN, CSP Exposed Pad
Mounting Type
Surface Mount
Lead Free Status / RoHS Status
Lead free / RoHS Compliant

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THEORY OF OPERATION
Laser diodes have a current-in to light-out transfer function, as
shown in Figure 23. Two key characteristics of this transfer
function are the threshold current, Ith, and slope in the linear
region beyond the threshold current, referred to as slope
efficiency, LI.
DUAL-LOOP CONTROL
Typically, laser threshold current and slope efficiency are both
functions of temperature. For FP and DFB type lasers, the
threshold current increases and the slope efficiency decreases
with increasing temperature. In addition, these parameters vary
as the laser ages. To maintain a constant optical average power
and a constant optical extinction ratio over temperature and
laser lifetime, it is necessary to vary the applied electrical bias
current and modulation current to compensate for the laser
changing LI characteristics.
Single-loop compensation schemes use the average monitor
photodiode current to measure and maintain the average
optical output power over temperature and laser aging. The
ADN2870 is a dual-loop device, implementing both this
primary average power control loop and a secondary control
loop, which maintains a constant optical extinction ratio. The
dual-loop control of the average power and extinction ratio
implemented in the ADN2870 can be used successfully both
with lasers that maintain good linearity of LI transfer character-
istics over temperature and with those that exhibit increasing
nonlinearity of the LI characteristics over temperature.
Dual Loop
The ADN2870 uses a proprietary patented method to control
both average power and extinction ratio. The ADN2870 is
constantly sending a test signal on the modulation current
signal and reading the resulting change in the monitor photo-
diode (MPD) current as a means of detecting the slope of the
laser in real time. This information is used in a servo to control
the ER of the laser, which is done in a time-multiplexed manner
at a low frequency, typically 80 Hz. Figure 24 shows the dual-
loop control implementation on the ADN2870.
P
P1
AV
P
0
Figure 23. Laser Transfer Function
ER =
P
AV
=
P1
P
P1 + P
0
2
0
Ith
ΔI
CURRENT
ΔP
LI =
ΔP
ΔI
Rev. A | Page 11 of 20
A dual loop is made up of an average power control loop
(APCL) and the extinction ratio control loop (ERCL), which are
separated into two time states. During Time Φ1, the APC loop
is operating, and during Time Φ2, the ER loop is operating.
Average Power Control Loop
The APCL compensates for changes in Ith and LI by varying
I
I
a problem because the APCL must be low frequency; the APCL
must respond to the average current from the MPD. The APCL
compares I
the bias current is increased until I
Conversely, if the I
Modulation Control Loop
The ERCL measures the slope efficiency, LI, of the LD and
changes I
arily increased by ΔI
fixed ratio of 50:1, but during startup, this ratio is increased to
decrease settling time.
During ERCL, switching in ΔI
in average optical power, ΔP
during ERCL, and the increase is kept small enough so as not to
disturb the optical eye. When ΔI
circuit, an equal current, I
resistor. The user sets the value of IEX; this is the ERSET setpoint.
If ΔI
and increases I
is equal to I
settings are stored on the hold capacitors, PAVCAP and ERCAP.
The ERCL is constantly measuring the actual LI curve; there-
fore, it compensates for the effects of temperature and for
changes in the LI curve due to laser aging. Therefore, the laser
can be calibrated once at 25°C and can then automatically
control the laser over temperature. This eliminates expensive
and time consuming temperature calibration of the laser.
ERSET
BIAS
MPD
Figure 24. Dual-Loop Control of Average Power and Extinction Ratio
. APC control is performed by measuring MPD current,
. This current is bandwidth limited by the MPD. This is not
MPD
is too small, the control loop knows that LI has decreased
MPD
INPUT
MPD
EX
MPD
Φ
. The previous time state values of the bias and mod
as LI changes. During the ERCL, I
2
I
EX
MPD
× R
PAVSET
I
PA
and, therefore, ΔI
PAVSET
MPD
V
MOD
1.2V
BGAP
increases, I
to the BGAP voltage, V
. The ratio between I
Gm
EX
AV
, is switched into the PAVSET
. However, the APC loop is disabled
MOD
OPTICAL COUPLING
Φ
Φ
MOD
1
2
BIAS
causes a temporary increase
MPD
MOD
BIAS
MOD
SHA
SHA
is switched into the laser
is decreased.
× R
accordingly until ΔI
PAVSET
CURRENT
BIAS
MPD
CURRENT
SWITCH
BGAP
MPD
equals V
SPEED
MOD
and ΔI
HIGH
ADN2870
. If I
is tempor-
100
MPD
MOD
BGAP
V
falls,
MPD
is a
CC
.
2
Φ
2

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