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LTC5100 - APC mode

Could someone help to activate the APC mode on the LTC5100 VCSEL Driver.

In our application, the photodiode integrated into the VCSEL package delivers a photodiode current proportional to the emitted flux, between 20µA to 100µA. Measurement done with the User-ADC of the driver (register 0x18, selected signal IMD 011), in CCC mode, no modulation applied. IMD_rg 0 and 1 has been configured to cover the whole signal amplitude.

Selecting APC mode with register SYS_config 0x1B gives a fail, whatever the initial value of IMD. Diagnostic fault type is a Laser underpower default. Note that initial CCC mode is SYS_CONFIG 0x13. Only bit#3 has been changed between CCC and APC mode. MD polarity bit is set to 0.

We have tried to change the loop gain register to approximate small-signal loop gain A=0,5 (0x15), without success.

 In some cases, we have observed the regulation is starting over a short time (less than 1s) where the VCSEL produces a light flux, before to go into fault mode.

At this stage, we have no idea to explain the observed fault mode, and we don’t see how to configure the driver to accept the APC mode. Could you please give recommendations.

Thanks in advance.

 

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  • Greetings Oliver OE,

    There are 6 events that can trigger an alarm condition. For early on in the development cycle, it might be more helpful to first focus on basic functionality. After that is established, then go back and make sure the alarms are working properly and are not asserted during normal operation. This requires FLT_CONFIG bit 6= 0 (ref. Table 20). 

    Next, is the LTC5100 working OK in CCC mode? In other words, is the VCSEL emitting optical power as observed by external instruments? And can it be modulated? It's convenient and recommended to begin testing this way, because it doesn't require that the monitor diode be hooked up or  functional. It's a good way to validate the test setup, optical power and modulation, etc. 

    After CCC mode has proven successful, then is a good time to switch to the APC mode via SYS_CONFIG bit 3 (Table 8). Figure 3 shows the variables that contribute to Imd_set. The main one is Imd_nom, so that default value might need to be customized for your MD and MD settings. 

    Then after all this, we suggest you go back and configure the alarm register settings as desired for fault detection. 

    Hope that all helps. Kindly advise if any further questions or guidance needed.  -Bruce H. 

  • Hi Bruce,

    Thanks for your answer.

    We have already tested CCC mode, with a good behavior.

    Now we have got a diode monitored mode by disactivating LTC errors (0x4E0@0x13 when changing the set of parameters for APC mode, and reactivating (0x4E7@0x13). Is that a good method ?

    Regards,

    Oliver

  • Greetings Oliver OE,

    It seems you've found a good workaround for this problem. What might be happening is on Tx enable, the LTC5100 might be checking for faults too quickly, before the APC loop is stabilized, generating the Laser Underpower Fault. The LTC5100 should continue to continuously scan for faults that are enabled (the bottom 3 bits of FLT_CONFIG register). If any doubt, you could attempt to hardware-force one of the 3 faults at bottom of datasheet Table 20 and confirm that the LTC5100 does indeed detect the fault. Hope that helps.  -Bruce H. 

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  • Greetings Oliver OE,

    It seems you've found a good workaround for this problem. What might be happening is on Tx enable, the LTC5100 might be checking for faults too quickly, before the APC loop is stabilized, generating the Laser Underpower Fault. The LTC5100 should continue to continuously scan for faults that are enabled (the bottom 3 bits of FLT_CONFIG register). If any doubt, you could attempt to hardware-force one of the 3 faults at bottom of datasheet Table 20 and confirm that the LTC5100 does indeed detect the fault. Hope that helps.  -Bruce H. 

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