LM317: LDO Circuit design guide

Hi Shelton,

Shelton Li said:

Do we have any design guide for below circuit with LM317 solution?

We do not have further design documentation for this schematic, the diode is a generic diode selection to protect the device and allow reverse current to flow in a fault condition through the capacitance from ADJ to GND. The resistors either set the output voltage, or set the current drive for the NPN and PNP transistors, and the transistors themselves are not the only transistors that could work in this circuit, they are just the ones that were tested and used as the reference design. The electrical principles can be used with a multitude of PNP/NPN combinations.

Shelton Li said:

Customer want to know the power loss of LM317 and TIP73 and to see whether below circuit is suitable for the demand voltage and current or not?

At the customers current condition of 700mA, the power loss of the LM317 will be ~ 0.7*(20-15) ~ 3.5W of power. The device itself is capable of this except thermals may become a concern. In this setup you allow for additional current to flow through the BJT branch which lessens the dissipation on the LDO itself. For specific values of dissipation they would be dependent upon the resistor selection that determines the current they select and the transistors that are chosen for the application. For the LDO it would be 5* the current that it would be sourcing, for a rough estimate of a 1-1 circuit, that would be 1.75W of power on the LDO, for the TIP73 BJT, it would be a near identical power dissipation as almost all the current would be flowing through it and it's Collector and Emitter are connected to VI and VO as well which would be once again a rough estimate of 1.75W of power.

Please let me know if further clarification is needed, or this did not answer your question

Regards,

John