The adjustment range of VAPD is inherently limited by the high voltage supply (VPHV) provided to the ADL5317. VAPD output can only be varied within a range of VPHV through VSET.
For instance:
- With a 75 V VPHV supply, VAPD can be adjusted from 40 V to 73.5 V.
- With a 30 V VPHV supply, the range is limited to 6 V (the minimum allowed) to 28.5 V.
If VSET is adjusted such that VAPD reaches its lower clamp voltage, the output will no longer respond to further decreases in VSET. In this condition, the internal mirror circuitry may continue to function, but the VAPD bias voltage becomes fixed and unresponsive to incremental VSET changes.