Datasheet

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HIGH-SPEED DATA PATH
BIAS AND CONTROL
BANDGAP VOLTAGE AND BIAS GENERATION
AUTOMATIC POWER CONTROL
I
PD
[A] + P
AVG
[W] ò
MON
[AńW]
(1)
R
APCSET
[W] +
4.69 V
I
PD
[A]
+
4.69 V
P
AVG
[W] ò
MON
[AńW]
(2)
I
BIASMAX
[A] +
343 V
R
BIASMAX
[W]
(3)
V
MONB
[V] +
R
MONB
[W] I
BIAS
[A]
68
(4)
V
MONP
[V] + R
MONP
[W] I
PD
[A]
(5)
ONET4211LD
SLLS688A – NOVEMBER 2005 – REVISED SEPTEMBER 2007
The high-speed data path consists of an input buffer stage and a current modulator.
The input buffer stage takes CML-compatible differential signals. It provides on-chip, 50- Ω termination to VCC.
AC-coupling may be used at the DIN+ and DIN- inputs.
The laser diode current modulator consists mainly of two common-emitter output transistors and the required
driver circuitry. According to the input data stream, the modulation current is sunk at the MOD+ or the MOD – pin.
Modulation current setting is performed by means of the modulation current generator block, which is supervised
by the control circuit block.
The laser diode can be either ac- or dc-coupled. In either case, the maximum modulation current is 85 mA. The
modulation output is optimized for driving a 20- Ω load.
For optimum power efficiency, the laser driver does not provide any on-chip back-termination.
The bias-and-control circuitry consists of the bandgap voltage and bias generation block, the bias current
generator, the automatic power control block, and the supervising control circuitry.
The bandgap voltage reference provides the process- and temperature-independent reference voltages needed
to set bias current, modulation current, and photodiode reference current. Additionally, this block provides the
biasing for all internal circuits.
The ONET4211LD laser driver incorporates an APC loop to compensate for the changes in laser threshold
current over temperature and lifetime. The internal APC is enabled when resistors are connected to the IBMAX
and APCSET pins. A back-facet photodiode mounted in the laser package is used to detect the average laser
output power. The photodiode current I
PD
that is proportional to the average laser power can be calculated by
using the laser-to-monitor transfer ratio, ρ
MON
and the average power, P
AVG
:
In closed-loop operation, the APC modifies the laser diode bias current by comparing I
PD
with a reference current
I
APCSET
and generates a bias compensation current. I
PD
can be programmed by selecting the external resistor
R
APCSET
according to:
The bias compensation current subtracts from the maximum bias current to maintain the monitor photodiode
current. The maximum bias current is programmed by the resistor connected to IBMAX:
An external pin, MONB, is provided as a bias current monitor output. A fraction of the bias current (1/68) is
mirrored and develops a voltage drop across an external resistor to ground, R
MONB
. The voltage at MONB is
given as:
If the voltage at MONB is greater than the programmed threshold, a fault mode occurs.
MONP is also provided as a photocurrent monitor output. The photodiode current, I
PD
, is mirrored and develops a
voltage across an external resistor to ground, R
MONP
. The voltage at MONP is given as:
If the voltage at MONP is greater than the programmed threshold, a fault mode occurs.
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