On Saturday, January 20, 2018 at 2:55:52 PM UTC-5, Joerg wrote:> On 2018-01-20 11:42, dagmargoodboat@yahoo.com wrote: > > On Saturday, January 20, 2018 at 2:35:48 PM UTC-5, Joerg wrote: > >> On 2018-01-20 09:08, dagmargoodboat@yahoo.com wrote: > >>> On Friday, January 19, 2018 at 6:31:53 PM UTC-5, Joerg wrote: > >>>> Attention, highly non-political post. > >>>> > >>>> Here is a puzzler. This ST datasheet says on page 5 that the junction to > >>>> ambient temp rise is 55C/W for SO8 and 115C/W for SOT89: > >>>> > >>>> https://media.digikey.com/pdf/Data%20Sheets/ST%20Microelectronics%20PDFS/L79L.pdf > >>> > >>> Explained by two footnotes-- > >>> > >>> "(1) Our SO-8 package used for Voltage Regulators is modified internally > >>> to have pins 2, 3, 6 and 7 electrically communed to the die attach flag. > >>> This particular frame decreases the total thermal resistance of the package > >>> and increases its ability to dissipate power when an appropriate area of > >>> copper on the printed circuit board is available for heat-sinking. The > >>> external dimensions are the same as for the standard SO-8." > >>> > >>> > >>> "RthJA Thermal resistance junction-ambient (Max) 55 (1) 200 115 °C/W > >>> > >>> ==> Notes: (1) ***Considering 6 cm^2 of copper Board heat-sink.***" <=== > >>> > >> > >> That's fine but an apples to apples comparison will also provide 6cm^2 > >> for the SOT89. Nobody in their right mind would mount them without > >> serious copper area if heavily loaded. > >> > >> [...] > >> > >> -- > >> Regards, Joerg > >> > >> http://www.analogconsultants.com/ > > > > From ST's numbers, the SOT89 with the same heatsink would have > > RthJA of 50°C/W, 5°C/W better than the SO-8 package. > > > > It's hard to beat a solid copper tab. > > > > > I only see 115C/W for the SOT89 in table 2. > > -- > Regards, Joerg > > http://www.analogconsultants.com/Look at the difference between the a) SO-8's junction-to-case and (20°C/W) b) junction-to-ambient-with-6cm^2-heatsink. (55°C/W) This amounts to 35°C/W for the heatsink-to-ambient portion. Apply that to the SOT89, 15°C/W + 35°C/W = 50°C/W to ambient, with the same copper pour. (I'm assuming they're measuring to the leads rather than the actual epoxy case, otherwise their lower junction-to-case for the SOT89 wouldn't make sense--it's thicker than an SO-8.) Cheers, James Arthur
SO8 versus SOT89 junction to ambient?
Started by ●January 19, 2018
Reply by ●January 20, 20182018-01-20
Reply by ●January 21, 20182018-01-21
On 2018-01-20 17:07, dagmargoodboat@yahoo.com wrote:> On Saturday, January 20, 2018 at 2:55:52 PM UTC-5, Joerg wrote: >> On 2018-01-20 11:42, dagmargoodboat@yahoo.com wrote: >>> On Saturday, January 20, 2018 at 2:35:48 PM UTC-5, Joerg wrote: >>>> On 2018-01-20 09:08, dagmargoodboat@yahoo.com wrote: >>>>> On Friday, January 19, 2018 at 6:31:53 PM UTC-5, Joerg wrote: >>>>>> Attention, highly non-political post. >>>>>> >>>>>> Here is a puzzler. This ST datasheet says on page 5 that the junction to >>>>>> ambient temp rise is 55C/W for SO8 and 115C/W for SOT89: >>>>>> >>>>>> https://media.digikey.com/pdf/Data%20Sheets/ST%20Microelectronics%20PDFS/L79L.pdf >>>>> >>>>> Explained by two footnotes-- >>>>> >>>>> "(1) Our SO-8 package used for Voltage Regulators is modified internally >>>>> to have pins 2, 3, 6 and 7 electrically communed to the die attach flag. >>>>> This particular frame decreases the total thermal resistance of the package >>>>> and increases its ability to dissipate power when an appropriate area of >>>>> copper on the printed circuit board is available for heat-sinking. The >>>>> external dimensions are the same as for the standard SO-8." >>>>> >>>>> >>>>> "RthJA Thermal resistance junction-ambient (Max) 55 (1) 200 115 °C/W >>>>> >>>>> ==> Notes: (1) ***Considering 6 cm^2 of copper Board heat-sink.***" <=== >>>>> >>>> >>>> That's fine but an apples to apples comparison will also provide 6cm^2 >>>> for the SOT89. Nobody in their right mind would mount them without >>>> serious copper area if heavily loaded. >>>> >>>> [...] >>>> >>>> -- >>>> Regards, Joerg >>>> >>>> http://www.analogconsultants.com/ >>> >>> From ST's numbers, the SOT89 with the same heatsink would have >>> RthJA of 50°C/W, 5°C/W better than the SO-8 package. >>> >>> It's hard to beat a solid copper tab. >>> >> >> >> I only see 115C/W for the SOT89 in table 2. >> >> -- >> Regards, Joerg >> >> http://www.analogconsultants.com/ > > Look at the difference between the > a) SO-8's junction-to-case and (20°C/W) > b) junction-to-ambient-with-6cm^2-heatsink. (55°C/W) > > This amounts to 35°C/W for the heatsink-to-ambient portion. > > Apply that to the SOT89, 15°C/W + 35°C/W = 50°C/W to ambient, with the > same copper pour. >That's what we don't know and what a datasheet should specify.> (I'm assuming they're measuring to the leads rather than the actual > epoxy case, otherwise their lower junction-to-case for the SOT89 > wouldn't make sense--it's thicker than an SO-8.) >Still, that's assumptions and I can't design on assumptions but John has hard data so that's good. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●January 21, 20182018-01-21
On Sat, 20 Jan 2018 16:20:31 -0800, Joerg <news@analogconsultants.com> wrote:>On 2018-01-20 16:16, John Larkin wrote: >> On Fri, 19 Jan 2018 15:57:08 -0800, Joerg <news@analogconsultants.com> >> wrote: >> >>> On 2018-01-19 15:45, John Larkin wrote: > >[...] > > >>>> I've dissipated 3 watts from a SOT89 soldered to a copper pour. >>>> >>>> https://www.dropbox.com/s/rkg3jkzvqutl822/DSC01850.JPG?raw=1 >>>> >>>> https://www.dropbox.com/s/iy5gyxjvgi14sf1/DSC01857.JPG?raw=1 >>>> >>>> Thermal vias could make this better. >>>> >>> >>> Good words of wisdom. I'll stay with SOT89 then. That is what I used >>> before and while they do become toasty it seems SO8 sans pad won't be >>> better. >> >> How much power do you need to dump? >> > >Not sure yet but probably 500mW or a little more per device. Things >shouldn't get too hot to touch though. It is also a sort of hi-rel >application where the product lifetime should be several decades.Half a watt should be easy from a SOT89. A little topside copper patch maybe. If you don't have much room, via down to a copper pour on the bottom and/or an inner layer. An isolated inner layer pour will spread heat and dump heat into adjacent planes. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●January 21, 20182018-01-21
On Sun, 21 Jan 2018 07:33:24 -0800, Joerg <news@analogconsultants.com> wrote:>On 2018-01-20 17:07, dagmargoodboat@yahoo.com wrote: >> On Saturday, January 20, 2018 at 2:55:52 PM UTC-5, Joerg wrote: >>> On 2018-01-20 11:42, dagmargoodboat@yahoo.com wrote: >>>> On Saturday, January 20, 2018 at 2:35:48 PM UTC-5, Joerg wrote: >>>>> On 2018-01-20 09:08, dagmargoodboat@yahoo.com wrote: >>>>>> On Friday, January 19, 2018 at 6:31:53 PM UTC-5, Joerg wrote: >>>>>>> Attention, highly non-political post. >>>>>>> >>>>>>> Here is a puzzler. This ST datasheet says on page 5 that the junction to >>>>>>> ambient temp rise is 55C/W for SO8 and 115C/W for SOT89: >>>>>>> >>>>>>> https://media.digikey.com/pdf/Data%20Sheets/ST%20Microelectronics%20PDFS/L79L.pdf >>>>>> >>>>>> Explained by two footnotes-- >>>>>> >>>>>> "(1) Our SO-8 package used for Voltage Regulators is modified internally >>>>>> to have pins 2, 3, 6 and 7 electrically communed to the die attach flag. >>>>>> This particular frame decreases the total thermal resistance of the package >>>>>> and increases its ability to dissipate power when an appropriate area of >>>>>> copper on the printed circuit board is available for heat-sinking. The >>>>>> external dimensions are the same as for the standard SO-8." >>>>>> >>>>>> >>>>>> "RthJA Thermal resistance junction-ambient (Max) 55 (1) 200 115 �C/W >>>>>> >>>>>> ==> Notes: (1) ***Considering 6 cm^2 of copper Board heat-sink.***" <=== >>>>>> >>>>> >>>>> That's fine but an apples to apples comparison will also provide 6cm^2 >>>>> for the SOT89. Nobody in their right mind would mount them without >>>>> serious copper area if heavily loaded. >>>>> >>>>> [...] >>>>> >>>>> -- >>>>> Regards, Joerg >>>>> >>>>> http://www.analogconsultants.com/ >>>> >>>> From ST's numbers, the SOT89 with the same heatsink would have >>>> RthJA of 50�C/W, 5�C/W better than the SO-8 package. >>>> >>>> It's hard to beat a solid copper tab. >>>> >>> >>> >>> I only see 115C/W for the SOT89 in table 2. >>> >>> -- >>> Regards, Joerg >>> >>> http://www.analogconsultants.com/ >> >> Look at the difference between the >> a) SO-8's junction-to-case and (20�C/W) >> b) junction-to-ambient-with-6cm^2-heatsink. (55�C/W) >> >> This amounts to 35�C/W for the heatsink-to-ambient portion. >> >> Apply that to the SOT89, 15�C/W + 35�C/W = 50�C/W to ambient, with the >> same copper pour. >> > >That's what we don't know and what a datasheet should specify.Data sheet thermals are generally not to be trusted. When in doubt, experiment. An 0603 resistor will safely dump half a watt, if you do it right. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●January 21, 20182018-01-21
On 2018-01-21 09:44, John Larkin wrote:> On Sat, 20 Jan 2018 16:20:31 -0800, Joerg <news@analogconsultants.com> > wrote: > >> On 2018-01-20 16:16, John Larkin wrote: >>> On Fri, 19 Jan 2018 15:57:08 -0800, Joerg <news@analogconsultants.com> >>> wrote: >>> >>>> On 2018-01-19 15:45, John Larkin wrote: >> >> [...] >> >> >>>>> I've dissipated 3 watts from a SOT89 soldered to a copper pour. >>>>> >>>>> https://www.dropbox.com/s/rkg3jkzvqutl822/DSC01850.JPG?raw=1 >>>>> >>>>> https://www.dropbox.com/s/iy5gyxjvgi14sf1/DSC01857.JPG?raw=1 >>>>> >>>>> Thermal vias could make this better. >>>>> >>>> >>>> Good words of wisdom. I'll stay with SOT89 then. That is what I used >>>> before and while they do become toasty it seems SO8 sans pad won't be >>>> better. >>> >>> How much power do you need to dump? >>> >> >> Not sure yet but probably 500mW or a little more per device. Things >> shouldn't get too hot to touch though. It is also a sort of hi-rel >> application where the product lifetime should be several decades. > > Half a watt should be easy from a SOT89. A little topside copper patch > maybe. If you don't have much room, via down to a copper pour on the > bottom and/or an inner layer. An isolated inner layer pour will spread > heat and dump heat into adjacent planes. >No space at all, this board will look like the inside of a Tokyo subway during rush hour. After the pushers have squeezed the people in. So it'll have to be a few vias without thermal reliefs to a copper plane inside the board. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●January 21, 20182018-01-21
On Sunday, January 21, 2018 at 10:33:30 AM UTC-5, Joerg wrote:> On 2018-01-20 17:07, dagmargoodboat@yahoo.com wrote: > > On Saturday, January 20, 2018 at 2:55:52 PM UTC-5, Joerg wrote: > >> On 2018-01-20 11:42, dagmargoodboat@yahoo.com wrote: > >>> On Saturday, January 20, 2018 at 2:35:48 PM UTC-5, Joerg wrote: > >>>> On 2018-01-20 09:08, dagmargoodboat@yahoo.com wrote: > >>>>> On Friday, January 19, 2018 at 6:31:53 PM UTC-5, Joerg wrote: > >>>>>> Attention, highly non-political post. > >>>>>> > >>>>>> Here is a puzzler. This ST datasheet says on page 5 that the junction to > >>>>>> ambient temp rise is 55C/W for SO8 and 115C/W for SOT89: > >>>>>> > >>>>>> https://media.digikey.com/pdf/Data%20Sheets/ST%20Microelectronics%20PDFS/L79L.pdf > >>>>> > >>>>> Explained by two footnotes-- > >>>>> > >>>>> "(1) Our SO-8 package used for Voltage Regulators is modified internally > >>>>> to have pins 2, 3, 6 and 7 electrically communed to the die attach flag. > >>>>> This particular frame decreases the total thermal resistance of the package > >>>>> and increases its ability to dissipate power when an appropriate area of > >>>>> copper on the printed circuit board is available for heat-sinking. The > >>>>> external dimensions are the same as for the standard SO-8." > >>>>> > >>>>> > >>>>> "RthJA Thermal resistance junction-ambient (Max) 55 (1) 200 115 °C/W > >>>>> > >>>>> ==> Notes: (1) ***Considering 6 cm^2 of copper Board heat-sink.***" <=== > >>>>> > >>>> > >>>> That's fine but an apples to apples comparison will also provide 6cm^2 > >>>> for the SOT89. Nobody in their right mind would mount them without > >>>> serious copper area if heavily loaded. > >>>> > >>>> [...] > >>>> > >>>> -- > >>>> Regards, Joerg > >>>> > >>>> http://www.analogconsultants.com/ > >>> > >>> From ST's numbers, the SOT89 with the same heatsink would have > >>> RthJA of 50°C/W, 5°C/W better than the SO-8 package. > >>> > >>> It's hard to beat a solid copper tab. > >>> > >> > >> > >> I only see 115C/W for the SOT89 in table 2. > >> > >> -- > >> Regards, Joerg > >> > >> http://www.analogconsultants.com/ > > > > Look at the difference between the > > a) SO-8's junction-to-case and (20°C/W) > > b) junction-to-ambient-with-6cm^2-heatsink. (55°C/W) > > > > This amounts to 35°C/W for the heatsink-to-ambient portion. > > > > Apply that to the SOT89, 15°C/W + 35°C/W = 50°C/W to ambient, with the > > same copper pour. > > > > That's what we don't know and what a datasheet should specify. > > > > (I'm assuming they're measuring to the leads rather than the actual > > epoxy case, otherwise their lower junction-to-case for the SOT89 > > wouldn't make sense--it's thicker than an SO-8.) > > > > Still, that's assumptions and I can't design on assumptions but John has > hard data so that's good.1. John doesn't have data on your specific SO-8, but the manufacturer has provided specifications for it, in actual application, no assumptions required. But it's moot--both sources indicate that the SOT89 wins. 2. There are many application notes for estimating the thermal resistance of a 6cm^2 copper pour--that's not top secret. This reference reports 40°C/W for an SOT223 with a topside 1 inch^2 2oz pour: http://www.onsemi.com/pub/Collateral/AN-1028.pdf.pdf 3. There are outstanding SO-8 packages built on giant copper slab thermal pads. I've used some. But your example here isn't of that construction. Cheers, James Arthur
