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| When constant current isn't constant.... on LED driver. |
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| Zero999:
The LM317 shouldn't be used for such low current designs. Use the LM317L instead. It has a lower minimum operating current and even if it goes out of regulation, the maximum surge will be limited to 220mA, rather than the >2.2A permitted by the LM317. It also has a much smaller output transistor, so should respond more quickly. |
| David Hess:
Do not blame the LM317 operating as a current source as shown before measuring the actual surge current when coming out of saturation. |
| Zero999:
--- Quote from: David Hess on March 26, 2019, 06:09:54 pm ---Do not blame the LM317 operating as a current source as shown before measuring the actual surge current when coming out of saturation. --- End quote --- I agree. I just don't get why people often use the overkill LM317 in applications designed for a few tens of mA. I understand it might make sense for prototyping and to an extent hobbyist use, as stocking only one part is more convenient, than two, but for large production runs it makes more sense to use the smaller and cheaper LM317L. The lower minimum load and current limit are also advantageous, in lower power applications. |
| Yansi:
I don't understand why would one even come up with an N-MOS based current mirror solution. Hell, use a damn NPN with emitter resistance as an constant current sink! Just two simple NPNs driven directly by the logic circuitry, defining the reference voltage for the current sink. |
| David Hess:
--- Quote from: Zero999 on March 26, 2019, 11:04:09 pm ---I just don't get why people often use the overkill LM317 in applications designed for a few tens of mA. I understand it might make sense for prototyping and to an extent hobbyist use, as stocking only one part is more convenient, than two, but for large production runs it makes more sense to use the smaller and cheaper LM317L. The lower minimum load and current limit are also advantageous, in lower power applications. --- End quote --- I have had to use the TO-220 package 317 even at low currents sometimes to meet power dissipation requirements. A TO-220 package without any heat sinking is good for 2 watts versus 625 milliwatts for the TO-92 317. |
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