Hi all,
I have a circuit running at 3.3V from a LM9036 voltage regulator. This is supplied from a 4-20mA loop (loop powered) from a 24V supply. For those of you unfamiliar with a 4-20mA circuit, there is a device (transmitter) which regulates the current from 4mA (0%) to 20mA (100%) as an analog output of a measured parameter (pressure, temperature, etc). Any indicator devices are in series in the same loop to measure the current and display the value and/or be powered by the 4-20mA signal. My board is powered by the 4-20mA signal (loop powered).
In effect my voltage reg is being supplied by a constant current supply. My circuit is designed to use less than 4mA (I think it's about 3.2mA). The rest of the current is sunk by a 5.2V zener in parallel with the voltage reg. The idea is that when the voltage reg is getting too much current it will raise its input voltage to resist it and then the zener will kick in.
The problem is that although the voltage reg is a 3.3V one it often sits at 2.6-2.7V. If i run the supply up to 18-20mA then it will go to 3.3V and stay there even if I come back down to 4mA. My circuit still works at the lower voltage but I feel its going to bite me later on and either die or become unreliable. Plus I would like to understand why so I can build better circuits in the future.
I'm guessing it's because the voltage reg is being starved or it's in an unstable state. On the DSO the voltage is stable and it starts up ok (quick rise, slight overshoot then settles). I have the minimum recommended capacitance but adding more doesn't help. The LM9036 datasheet says the ESR should be between 0.1 and 8Ω but the capacitor datasheet doesn't have ESR only tanδ. Tanδ is 0.2 at 120Hz. I worked out the ESR was about 6.7Ω at 1kHz but I'm not confident in that value. I'm not even sure if it's relevant.
Anyway, has anyone come across this sort of power supply issue before? I couldn't find anything on any of the forums or the web in general.
Thanks for reading,
Stuart
I have a circuit running at 3.3V from a LM9036 voltage regulator. This is supplied from a 4-20mA loop (loop powered) from a 24V supply. For those of you unfamiliar with a 4-20mA circuit, there is a device (transmitter) which regulates the current from 4mA (0%) to 20mA (100%) as an analog output of a measured parameter (pressure, temperature, etc). Any indicator devices are in series in the same loop to measure the current and display the value and/or be powered by the 4-20mA signal. My board is powered by the 4-20mA signal (loop powered).
In effect my voltage reg is being supplied by a constant current supply. My circuit is designed to use less than 4mA (I think it's about 3.2mA). The rest of the current is sunk by a 5.2V zener in parallel with the voltage reg. The idea is that when the voltage reg is getting too much current it will raise its input voltage to resist it and then the zener will kick in.
The problem is that although the voltage reg is a 3.3V one it often sits at 2.6-2.7V. If i run the supply up to 18-20mA then it will go to 3.3V and stay there even if I come back down to 4mA. My circuit still works at the lower voltage but I feel its going to bite me later on and either die or become unreliable. Plus I would like to understand why so I can build better circuits in the future.
I'm guessing it's because the voltage reg is being starved or it's in an unstable state. On the DSO the voltage is stable and it starts up ok (quick rise, slight overshoot then settles). I have the minimum recommended capacitance but adding more doesn't help. The LM9036 datasheet says the ESR should be between 0.1 and 8Ω but the capacitor datasheet doesn't have ESR only tanδ. Tanδ is 0.2 at 120Hz. I worked out the ESR was about 6.7Ω at 1kHz but I'm not confident in that value. I'm not even sure if it's relevant.
Anyway, has anyone come across this sort of power supply issue before? I couldn't find anything on any of the forums or the web in general.
Thanks for reading,
Stuart