I've been reading datasheets all night trying to figure out which LDO linear regular would give me the most current at 3.3V with no heatsink from a 2S LiFePO4 battery (assuming a max voltage range of 5V-8.4V) without overheating. The old LM1117 appears to be the best thing out there for use without a heatsink, unless I'm reading the datasheet wrong. Am I?
Junction-to-ambient thermal resistance appears to be the relevant specification. Some datasheets specify that they're giving this data without a heatsink (e.g. REG1117, pg. 5). Some datasheets specify that the data is for specific areas and weights of PCB copper that the regulator is soldered to as a heatsink (e.g. TPS7A45XX, pg. 25).
For the LM1117, the value 23.8°C/W is specified for the TO-220 packages in section 6.4 on pg. 4. There are no notes in this section specifying that heat sinking is required to achieve this value. There are notes to that effect for other thermal characteristics (e.g. sections 6.1 and 6.3), and there are notes to that effect for other packages (e.g. TO-252 and WSON), suggesting that this number for the TO-220 package might be without a heatsink.
However… given that it's less than half of the value for similar regulators where the datasheet is unambiguous that the data is for the no-heatsink case, I'm suspicious. Is this really the miraculous regulator I'm looking for (minus the whole tantalum capacitor requirement), one that'll give me twice as much current before overheating as anything else, or am I misreading the datasheet and/or missing something obvious?
All the regulators I've been looking at, for comparison:
- LM1117: 23.8°C/W
- TLV1117: 30°C/W (with the same ambiguity about heat sinking)
- LT1965: 3°C/W (for junction to case, case to ambient unspecified)
- LT1963A: 4°C/W (for junction to case, case to ambient unspecified)
- MIC5239: 50°C/W (also ambiguous about heat sinking)
- LD1117: 50°C/W
- LD1117A: 50°C/W
- REG1117: 65°C/W
- REG104: 65°C/W
- AZ1117C: 100°C/W
- AZ1117I: 100°C/W
- LDL1117: 120°C/W
- IFX1117: 164°C/W
Edit: I picked an assortment of TO-220 datasheets from Mouser and looked at junction-to-ambient values. Most manufacturers were pretty much always in the range 50-65°C/W. Texas Instruments was mostly in the range 20-30°C/W. I'm thinking that either TI has figured out a better way to make TO-220 packages that no-one else has, or they're measuring things differently.