I try to construct a shut down pin or a feature which can set the output of LM317 or LM78XX with negative voltage that is created through a transformerless buck converter.
A buck converter having a positive input voltage can be convert its output to a negative output voltage without a transformer. But will it differ from a negative voltage created through a transformer center tap.
Can I hook the negative voltage from the buck converter to the resistor divider bottom leg . Some designs use current buffer to induce current in milliamp range to the resistor divider network. Why or for which conditions is this necessary?
I am trying to construct a shut down switch for lm317 so that its output is pulled to 0V. In shut down state, the load on the lm317 output can be as low as 0.1 ohm. So i try to find an optimal solution so that the output stays lower than 10mV when shut down pin is enabled. (if my load decreases down to 0.1 ohm when lm317 is in shut down state and the lm317 output stays below 10mV my leakage current will be less than 100mA which is acceptable in my design) I send a schematic and simulation.
I face a problem that my optocouplers output transistor has a VCESAT of 0.1-0.2V (according to datasheet). So I tried to compensate it by increasing the negative voltage from -1.25V to -1.5 and adding a compensation resistor R4 (120ohm). When shut down pin is disabled. lm317 will face a load of 0.1ohm and drive itself to its thermal equilibrium. By attached heatsink it delivers a maximum current of 1.5A. I use the lm317 as a current limited regulator. The current limitation is through thermal limit of lm317.
The lm317 works as current source. It will be operated at its thermal equilibrium. So that it can not go beyond 1.5A. With precision heatsink, it is stabilized at 1.56A at 0.2ohm. It functions as a current source between loads 0.2ohms and 0.3 ohms. at the final stage, I plan to omit 0.2 ohm and decrease this value to 0.05 ohm, so that precision heatsink defines the maximum current lm317 can deliver. It is 1.56A with a large heatsink. (I tested it , it does not go under 1.56A after 24 hours at 45 degree ambient. This is a scenario that will not happen in the operating enviroment.)