LM117 Internally Limited Power Dissipation

Brett Paterson

Other Parts Discussed in Thread: LM117

I am using the LM117 in a Constant Voltage regulator configuration. However, I am taking the device to what seems like the maximum Power Dissipation and the device changes in operating characteristics.

I am trying to understand what is happening when this occurs? I've heard a few people talk about the regulator switching to a constant current mode? Where can I get more information on this so that I can justify that my design meets the given requirements.

over 12 years ago

0 Vincenzo Pizzolante over 12 years ago

TI__Mastermind 25296 points

the LM117 is an LDO, meaning it is easy to estimate its power loss:

PD = ( VIN - VOUT ) x IOUT

as PD is known, you can estimate the junction temeprature of the device, based on ambient temeprature you are working and Rth-ja, available on datasheet:

Tj = Ta + Rth-ja x PD

if the "expected" Tj is too high for the poor device then the thermal protection will be engaged...

to look for the best solution to your needs, please use the Power Quick Search tool, at the link http://www.ti.com/lsds/ti/analog/powermanagement/power_portal.page or use Webench as well...

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LM117 Internally Limited Power Dissipation