Attempting to use an LDL1117S12R_SOT223 as a current regulator

[gbugh]

I'm attempting to use an LDL1117S12R_SOT223 as a current regulator as is shown in application notes for an LM1117 and many similar regulators but it is not working.  I have tried it with and without a 4.7uF capacitor across the current setting resistor but it does not work either way. I have R set to 3.4 ohms and with a load drawing 10mA I am dropping about 2.245V across the regulator and it has a 200mV down sloping sawtooth wave on the output. Should the LDL1117S12R_SOT223 work as a current regulator? If yes, can anyone please suggest the simplest circuit for current regulation? I don't need any precision. Thanks, Gbugh

[Peter BENSCH]

The LD1117 can certainly be used as a constant current source, but you must observe a few boundary conditions:

• LD1117S12TR regulates the voltage between its GND and VOUT pins to 1.2V - at least it tries to do so
• at 3.4ohms this would result in a current of I = 1.2V/3.4ohms = 353mA

However, if your load only allows 10mA to flow, the 1117 will try to increase the current, which is only possible with a correspondingly high input voltage.

• What voltage are you measuring across the 3.4ohms resistor?
• What are your environmental parameters, i.e. input voltage, load (resistive load with which value, LED chain with which data, etc.)?

Regards
/Peter

[gbugh]

350mA was my target current limit so at least I got that correct. I think I had about 6V or 6.3V supply voltage. I had very little voltage across the resistor, about 0.034V. I guess I was expecting the LDL1117 to stay saturated with less than 300mV across it until I had enough load to reach 350mA.  I tried cranking the supply voltage up to 15V but I still had much higher than 300mV across the LDL1117. I didn't measure it again but I had a status LED I put across the LDL1117 and it stays on with more than 1.8V so I must have had at least that much voltage drop across it.  

Is there a way to keep constant current of about 350mA and keep the voltage drop across the LDL1117 very low at the same time until I reach the current limit of 350mA?

[Peter BENSCH]

You are probably confusing a constant current source with a current limiter or an Intelligent Power Switch. I still don't know what you actually want to achieve and what the current is intended for?

[gbugh]

I had assumed that a constant current source connected to a load would act like a current limiter and drop very little voltage until it reached the amount of current it was set for.  I'm trying to achieve very little loss of voltage to the load and limit the current to no more than 350mA. I am also trying to do it with as few of parts as possible and the limit value does not need to be precise. I am also using PTC resettable fuses but they don't respond fast enough. I have a DC power distribution circuit that sends power to various subsystems of a radio transceiver for teaching students.  Students can redesign any subsystem without having to re-do one big circuit board.  In the process of testing improvements to any particular subsystem, it is more likely students burn up something with from bad design or solder bridges. Maybe I can use a FET or depletion mode MOSFET instead of the LDL1117.

[Peter BENSCH]

No, for such cases it makes more sense to use a Smart High Side Switch, which is available with various current limits, e.g. IPS161HF (limit at 0.6A) or IPS160HF (limit at 2.4A).

Good luck!

Regards
/Peter