Just bumping the thread in the hope of getting some hints as to the shunt regulator current setting and capacity (my full question on previous page).
For determining the maximum current, this graph of a typical BSP129 gives guidance:
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.
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.
Thank you abraxalito. So connecting gate straight to source will max out the regulator's current at around 130mA.
Am I correct in assuming that we limit the current just to avoid unnecessary waste (heat and power) - or is there a performance downside to running the reg hot?
Am I correct in assuming that we limit the current just to avoid unnecessary waste (heat and power) - or is there a performance downside to running the reg hot?
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.