I am working on a project (NiMH battery charger) which requires a constant current of 1.4A. But no matter what design I look at, there seems to be a lot of heat generated. This project is a portable device so size (and therefore heatsink area) is at a premium.
An LM317T configured as a CC source requires at least 2v dropout on the regulator itself, with another 1.25v on the program-current resistor. 3.25V @ 1.4A is over 4.5 watts.
A PNP configured as a CC source can be made to have a bit lower dissipation, but there is still quite a bit of heat.
The circuit needs to be powered from a 5-6v source. The current source has to be on the rail side of the circuit (not the ground side - hence the PNP transistor requirement).
And even if I can overcome the high dropout issue, the load resistance can fluctuate enough to where heat is still a problem.
When I looked at the LM317T datasheet, I saw some application hints for low-cost (and size) switching voltage regulators. Can a similar scheme be used to create a switching current source?
An LM317T configured as a CC source requires at least 2v dropout on the regulator itself, with another 1.25v on the program-current resistor. 3.25V @ 1.4A is over 4.5 watts.
A PNP configured as a CC source can be made to have a bit lower dissipation, but there is still quite a bit of heat.
The circuit needs to be powered from a 5-6v source. The current source has to be on the rail side of the circuit (not the ground side - hence the PNP transistor requirement).
And even if I can overcome the high dropout issue, the load resistance can fluctuate enough to where heat is still a problem.
When I looked at the LM317T datasheet, I saw some application hints for low-cost (and size) switching voltage regulators. Can a similar scheme be used to create a switching current source?
Last edited: