ONET1101L: Laser safety feature

Greetings,

I believe device is working as expected. That is by setting FLTEN=0 bias and modulation current goes back on.

Based on your comments, reason for fault occurrence, fault can occur if PD current goes over 150% of its set value. PD current is a function of PDRNG settings. You are using PDRNG[6:5]=0x00 - max pd current of 0.77mA. I am thinking perhaps reason for your fault condition is because bias current falls more than 50% of its set value. Using your present settings, to validate this idea, please change your BIASC[9:0] settings to a lower value such that a fault wouldn't occur. 

Regards, Nasser

I am unsure if I understand the answer.   According to the datasheet there are three situations that may trigger a Fault. I believe I understand how #1 works.  My bias current (42mA) is <50% of max bias current (96mA), and is thus far from triggering a fault. 

Regarding #2 my photo current (60uA) is way below max photo current of 770uA and should therefore not trigger a fault.

This leave me with #3 ("Bias control DAC drops in value by more than 50% in one step." ).  I can't find anywhere in the data sheet what this really means.  Who/what controls the "Bias control DAC" and may cause his value suddenly change by 50%?  (Our value of reg5 (bias current bit9-bit3 = 0x12).  . 

What does "one step" mean.  Is a step the same as time interval between when the DAC is updated.  If yes, what is the length of this interval?

Can you please check if this is the correct interpretation of criteria #3?  In my design the bias current is 42mA and a change of 50uA represents a change of -60dB.  This is smaller than the resolution of the DAC and I cannot see how this is detectable,  Moreover, if it is detectable, and the safety feature is designed to trigger on such small ripples it will be activated all the time and not be very useful.

My understanding of the description in the data sheet is that the bias current is controlled by a DAC which again is controlled by the value of BIASC (reg.4 and 5).  If my understanding is correct, in open loop mode where I_bias=98uA x BIASC, what can cause the DAC value to do a sudden drop of 50%?  

In APC mode such drops may happen due to failure in the monitoring circuit.  E.g. if the PD suddenly short circuits the PD current will go to max and the APC mechanism will compensate by turning off the bias current.  However, I can't see how this can represent an safety issue.

 If, on the other hand, the PD circuit opens, the APC will compensate by turning up the bias current and thereby increase the laser output power.  This may represent a safety issue.  My question is if the safety mechanism also will react on 50% increases, or just if the bias current drops 50% ? 

Your understanding is correct. In open loop bias current is mainly controlled through BIASC register times 98uA. I may have misunderstood your earlier comment, are we saying when you go from close to open loop you see bias current drop?

Based on my search, fault occurs on bias current drop. Also bias current drop is mainly like an alarm vs safety issue and this maybe when this is reported.

Regards, Nasser

Thanks for the clarification.  Just one more question, will the safety mechanism react on drops only and not on sudden increases?  The latter is the most concerning for a laser safety point of view.