an96-2 ? 1998 semtech corp. HEATSINK selection for low dropout regulators march 9, 1998 652 mitchell road newbury park ca 91320 1 tel:805-498-2111 fax:805-498-3804 web:http ://www.semtech.com introduction a crucial part of the overall design in a system incorpo- rating a linear regulator is dissipating the heat that is generated in the regulator. this application note gives advice on choosing a HEATSINK and suitable part num- bers from leading manufacturers. selecting a HEATSINK (1) 1. on the chart for the device that you are planning to use, cross reference the maximum current required and the systems maximum ambient temperature. (2) 2. read the required HEATSINK thermal impedance (sink- to-ambient) off the chart. if your plotted point is between two lines, use the lower thermal impedance value. 3. look up your required thermal impedance value in table 1: suggested HEATSINK selection, ensuring that you use the column relevant to the air flow in your appli- cation. if the air flow in your application is between two of the values, then the lower flow rate column must be used. from this table you can obtain suggested part numbers for three different manufacturers. notes 1. these charts apply to HEATSINKing to-220 regulators converting from 5v to 3.3v only. please refer to the next column for information on how to calculate the HEATSINK requirements for other applications. 2. note that if the device is mounted with no insulation, the HEATSINK w ill be smaller, less hardware will be used, and therefore the cost w ill be lower! remember that in this case, the HEATSINK w ill be at the same potential as the output of the regulator. in cases with very high power dissipation, using insulation may not be an op- tion. in extreme cases, consider using one of semtechs switching solutions, or the ez1900 load balance con- troller to enable the use of two smaller HEATSINKs. detailed calculations for q s-a the required thermal impedance (sink-to-ambient) for any application is defined by the following equation:- where: q s-a = thermal impedance sink-to-ambient (c/w) t j = maximum junction temperature (c) (obtained from the device data sheet) t a = maximum ambient temperature (c) q j-c = thermal impedance junction-to-case (c/w) (obtained from the device data sheet) q c-s = thermal impedance case-to-sink (c/w) (look up in your thermal management hardware suppliers data - for the to-220 package, typi- cally 1.25c/w using silic one pads, and 0.5c/w using thermal grease with no insulation ) p = power dissipated in the device (w) (where p = i out(max) (v in(max) -v out(min) ) for worst case calculation) s c c j a j a s p t t - - - q - q - - = q
an96-2 ? 1998 semtech corp. HEATSINK selection for low dropout regulators march 9, 1998 652 mitchell road newbury park ca 91320 2 HEATSINK thermal impedance (sink-to-ambient) vs. ambient temperature and output current device mounted using thermal grease with no insulation on unfinished aluminum ( q c-s = 0.5 o c/w) HEATSINK for ez1086ct 0 0.5 1 1.5 2 2.5 25 45 65 85 105 125 ambient temperature ( o c) output current (a) a = 50 c/w b = 20 c/w c = 17.5 c/w d = 15 c/w HEATSINK thermal impedance (sink-to-ambient) vs. ambient temperature and output current device mounted using a silicone pad ( q c-s = 1.25 o c/w) 0 0.5 1 1.5 2 2.5 25 45 65 85 105 125 ambient temperature ( o c) output current (a) a = 50 c/w b = 20 c/w c = 17.5 c/w d = 15 c/w note: these charts valid for 5v to 3.3v conversion only a: no HEATSINK b c d a: no HEATSINK b c d
an96-2 ? 1998 semtech corp. HEATSINK selection for low dropout regulators march 9, 1998 652 mitchell road newbury park ca 91320 3 HEATSINK thermal impedance (sink-to-ambient) vs. ambient temperature and output current device mounted using thermal grease with no insulation on unfinished aluminum ( q c-s = 0.5 o c/w) HEATSINK for ez1085ct and ez1587ct 0 0.5 1 1.5 2 2.5 3 3.5 25 45 65 85 105 125 ambient temperature ( o c) output current (a) a = 20 c/w b = 17.5 c/w c = 15 c/w d = 12.5 c/w e = 10 c/w f = 7.5 c/w HEATSINK thermal impedance (sink-to-ambient) vs. ambient temperature and output current device mounted using a silicone pad ( q c-s = 1.25 o c/w) 0 0.5 1 1.5 2 2.5 3 3.5 25 45 65 85 105 125 ambient temperature ( o c) output current (a) a = 20 c/w b = 17.5 c/w c = 15 c/w d = 12.5 c/w e = 10 c/w f = 7.5 c/w g = 5 c/w note: these charts valid for 5v to 3.3v conversion only a b c d e f a b c d e f g
an96-2 ? 1998 semtech corp. HEATSINK selection for low dropout regulators march 9, 1998 652 mitchell road newbury park ca 91320 4 HEATSINK thermal impedance (sink-to-ambient) vs. ambient temperature and output current device mounted using thermal grease with no insulation on unfinished aluminum ( q c-s = 0.5 o c/w) HEATSINK for ez1084ct and ez1087ct 0 1 2 3 4 5 6 25 45 65 85 105 125 ambient temperature ( o c) output current (a) a = 15 c/w b = 12.5 c/w c = 10 c/w d = 7.5 c/w e = 5 c/w f = 2.5 c/w HEATSINK thermal impedance (sink-to-ambient) vs. ambient temperature and output current device mounted using a silicone pad ( q c-s = 1.25 o c/w) 0 1 2 3 4 5 6 25 45 65 85 105 125 ambient temperature ( o c) output current (a) a = 15 c/w b = 12.5 c/w c = 10 c/w d = 7.5 c/w e = 5 c/w f = 2.5 c/w note: these charts valid for 5v to 3.3v conversion only a b c d e f a b c d e f
an96-2 ? 1998 semtech corp. HEATSINK selection for low dropout regulators march 9, 1998 652 mitchell road newbury park ca 91320 5 HEATSINK thermal impedance (sink-to-ambient) vs. ambient temperature and output current device mounted using thermal grease with no insulation on unfinished aluminum ( q c-s = 0.5 o c/w) HEATSINK for ez1083ct 0 1 2 3 4 5 6 7 8 25 45 65 85 105 125 ambient temperature ( o c) output current (a) a = 12.5 c/w b = 10 c/w c = 7.5 c/w d = 5 c/w e = 2.5 c/w f = 1 c/w HEATSINK thermal impedance (sink-to-ambient) vs. ambient temperature and output current device mounted using a silicone pad ( q c-s = 1.25 o c/w) 0 1 2 3 4 5 6 7 8 25 45 65 85 105 125 ambient temperature ( o c) output current (a) a = 12.5 c/w b = 10 c/w c = 7.5 c/w d = 5 c/w e = 2.5 c/w f = 1 c/w note: these charts valid for 5v to 3.3v conversion only a a b c d e f b c d e f
an96-2 ? 1998 semtech corp. HEATSINK selection for low dropout regulators march 9, 1998 652 mitchell road newbury park ca 91320 6 HEATSINK thermal impedance (sink-to-ambient) vs. ambient temperature and output current device mounted using thermal grease with no insulation on unfinished aluminum ( q c-s = 0.5 o c/w) HEATSINK for ez1082ct 0 1 2 3 4 5 6 7 8 9 10 11 25 45 65 85 105 125 ambient temperature ( o c) output current (a) a = 12.5 c/w b = 10 c/w c = 7.5 c/w d = 5 c/w e = 2.5 c/w f = 1 c/w HEATSINK thermal impedance (sink-to-ambient) vs. ambient temperature and output current device mounted using a silicone pad ( q c-s = 1.25 o c/w) 0 1 2 3 4 5 6 7 8 9 10 11 25 45 65 85 105 125 ambient temperature ( o c) output current (a) a = 12.5 c/w b = 10 c/w c = 7.5 c/w d = 5 c/w e = 2.5 c/w f = 1 c/w g = 0.5 c/w note: these charts valid for 5v to 3.3v conversion only a b c d e f g a b c d e f
an96-2 ? 1998 semtech corp. HEATSINK selection for low dropout regulators march 9, 1998 652 mitchell road newbury park ca 91320 7 HEATSINK q s-a (c/w) forced air c ooling mfr. (1) none (convect ion only) 100 linear feet per m inute 800 linear feet per m inute 20 a 574802 (2) 574802 (2) 574802 (2) t 6038b (2) 6038b (2) 6038b (2) w 297-v2-80b (2) 297-v2-80b (2) 297-v2-80b (2) 17.5 a 575002 574802 (2) 574802 (2) t 6021pb 6038b (2) 6038b (2) w 286-ab 297-v2-80b (2) 297-v2-80b (2) 15 a 575002 574802 (2) 574802 (2) t 6021pb 6021pb 6038b (2) w 286-ab 297-v2-80b (2) 297-v2-80b (2) 12.5 a 513102 575002 574802 (2) t 6099b 6021pb 6038b (2) w 627-15abp 286-ab 297-v2-80b (2) 10 a 513202 575002 574802 (2) t 6100b 6021pb 6038b (2) w 627-20abp 286-ab 297-v2-80b (2) 7.5 a 532602 513302 574802 (2) t 6298b 6101b 6038b (2) w 657-15abp 627-25abp 297-v2-80b (2) 5 a 532702 513302 574802 (2) t 6299b 6101b 6021pb w 657-20abp 627-25abp 286-ab 2.5 a not applicable 532802 513002 t 6400b 6098b w 677-25abp 627-10abp 1 a not applicable 532802 t 6300b w 657-25abp 0.5 specialized solutions only table 1: suggested HEATSINK selection notes: 1. manufacturer: a = aavid engineering, inc. t = thermalloy, inc. w = wakefield engineering 2. no additional hardware required for these HEATSINKs (other than insulators/thermal grease as needed)
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