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Pushing the limits of TDA1543 NOS DAC

For determining the maximum current, this graph of a typical BSP129 gives guidance:

BSS129.png


Bear in mind this is a typical plot and individual BSP129s may deviate significantly from it. The VGS=0 line indicates the maximum current available, which is about 130mA. To get for example 100mA the VGS is about -0.2V so divide those numbers (volts/current) to obtain the resistor value at the source terminal, which works out at 2R.
A single BSS129 won't handle over 130mA but the current source circuit (FET plus resistor) can be paralleled to go beyond the limits of a single transistor.
 
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If you don't limit the current in some way, a shunt reg can't work. So the current source keeps a shunt reg a practical solution and normally we attempt to minimize waste heat by setting the current source a safe margin above the current consumption of the load. There's no performance downside for running hot other than the excess heat and its consequences for reliability, there may even be a performance upside (lower output impedance) from running hotter.
 
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