M02061 Datasheet, PDF (30/37 Pages) M/A-COM Technology Solutions, Inc. – 3.3 or 5 Volt Laser Driver

Functional Description
3.3.10
Monitor Outputs
To facilitate complying with laser safety and DDMI¹ requirements, output monitors are provided for transmit power
(TxPwr_MON) bias (BIAS_MON) and modulation current (MOD_MON).
These outputs will source current proportional to the emitted optical power (TxPwr_MON) the bias current (BIAS_MON)
and modulation current (MOD_MON). These pins should be terminated with a resistor to ground that sets the desired
full-scale voltage (not to exceed V_CC3-1V). Using a monitor polarity selection (MON_POL) these monitors can be set
to sink current instead of source current. They will then need to be terminated with a resistor to V_CC3 and the
induced voltage should not exceed 2.5V.
If the outputs of these monitors are not needed, MON_POL, TxPwr_MON, BIAS_MON, and MOD_MON can all be left
floating and the chip current consumption will be reduced by the value of the monitor currents.
3.4
Laser Eye Safety
Using this laser driver in the manner described herein does not ensure that the resulting laser transmitter complies
with established standards such as IEC 825. Users must take the necessary precautions to ensure that eye safety
and other applicable standards are met. Note that determining and implementing the level of fault tolerance
required by the applications that this part is going into is the responsibility of the transmitter designer and
manufacturer since the application of this device cannot be controlled by Mindspeed.
3.4.1
Safety Circuitry
On the M02061-12 with DISDLY on pin 8, SCB is internally bonded to ground so SCB is always in a logic low state.
When SCB is high the OUTP, BIASout and SVCC outputs will not be disabled when FAIL asserts (FAIL goes high).
The outputs are only disabled by making DIS high.
The FAIL output will also ignore much of the safety sensing circuitry when SCB is high. However, it will monitor the
state of the window comparators at pin APC_SET. The bias current is controlled to nominally maintain the voltage at
APC_SET to 1.3V. The threshold levels at the window comparators around APC_SET are specified by the parameters
V_BH and V_BL as shown in the table below. This provides the same level of eye safety protection as our previous
generation of laser drivers. The current sourced out of pin APC_SET is equal to the current into pin I_PIN sourced from
the laser monitor photo diode. If the laser is emitting excess power this will be reflected in the I_PIN current and the
voltage at APC_SET will go high and the FAIL pin will assert. If I_PIN is not connected to the laser monitor photo diode
then the voltage at APC_SET will fall and FAIL will assert.
When SCB is low, safety circuitry in the M02061 will disable the modulation and bias current and assert the FAIL
output immediately upon detecting a fault condition. In addition, the supply voltage that sources the laser (SV_CC or
an external switch controlled by SHDWN_OUT) will immediately go open circuit and prevent any current from passing
through the laser.
Fault conditions checked by the M02061 include shorts to ground or V_CC of all pins which can increase the laser
modulation or bias current.
For an initialization sequence to be successful, all the fault detection monitors must signal that the chip is “healthy”.
When DIS goes low, pins are checked for shorts to ground or V_CC and a FAIL condition is latched if there is a fault.
If the state of the pins is OK, a one-shot at the reset pin begins a countdown which will latch a FAIL condition if the
bias current has not stabilized to an acceptable level during the one-shot time. The one-shot can be extended with
an external capacitor connected from the RESET pin to ground.
The one-shot¹ width is approximately:
1.The one-shot is actually comprised of an oscillator and 10-bit counter.
02061-DSH-001-G
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