Keywords: margining capability, dc-dc converters, D/A converters, DACs, current-output DACs
VOUTMAX = 2.16VFirst, determine the necessary relationship between RTOP and RBOTTOM that yields the nominal output (VOUTNOM) when IDS4404 = 0A:
VOUTNOM = 1.8V
VOUTMIN = 1.44V
Solving for RTOP,
For our example,
The current (IDS4404) required to make VOUT increase to VOUTMAX is derived by summing currents at the FB node:
This equation can be simplified by solving Equation 1 for RBOTTOM and substituting, which yields:
In terms of margin percentage:
where margin = 0.2, to implement ±20% margining in this case. Before you can use this relationship to calculate RTOP and RBOTTOM, you must select the full-scale current IFS.
To calculate RFS, use the formula in the DS4404 datasheet plus the VREF value also found in that datasheet:
Finally, for completeness we determine the DS4404 output current as a function of register setting: IOUT(register setting) = step size × register setting.
Comparing RTOP and RBOTTOM for the two cases (IFS = 0.5mA vs. 2.0mA), you can see that IFS = 0.5mA is more attractive because the resistances are higher.
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