switching regulator with mcu
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From: legg on 22 Oct 2009 10:02 On Wed, 21 Oct 2009 10:40:10 +0300, "michael nikolaou" <michaelnikolaou_remove_me_(a)yahoo.com> wrote: >I need to make clear so we avoid confusions > >The device uses a 2.2" tft lcd . This thing has a backlight and no matter >what >draws 60 ma @ 3.3V .That i can't avoid . So with 60 ma Arm7 consumption >i end up to 120 ma. As described, you intend this 'small' device to dissipate almost 4W with a linear solution. You should assume a surface temperature rise of one degree for every milliwatt dissipated by a square centimeter of surface area for the package. You might want to recalculate permissible losses on this basis and reappraise some basic design/cost considerations that include your power source. >Now as i see whether i have a small switcher with huge inductor and >capacitor >so my board space is too large . >The other solution is super fast switcher that has everything small but >needs 2 euros >at least . >I imagine every engineer designing devices has faced this question . >I'm sure though that a simple cpu controlled switcher could cope with the >large dropout >from 24V to 4-5 volts and then a simple linear regulator could take you to >3.3V so you play >it safe.If things go wrong (cpu malfunction ) thermal shutdown from the >linear regulator would >reset the cpu and that would restart the system >As i see it a simple fet switch with cpu pwm control should satisfy that . >Any opinions or experience on that subject??? In any system with a self-regulated supply voltage, the first question is 'Who's on first?'. Don't turn this into an Abott and Costello routine. RL
From: Phil Hobbs on 22 Oct 2009 10:06 legg wrote: > On Wed, 21 Oct 2009 10:40:10 +0300, "michael nikolaou" > <michaelnikolaou_remove_me_(a)yahoo.com> wrote: > >> I need to make clear so we avoid confusions >> >> The device uses a 2.2" tft lcd . This thing has a backlight and no matter >> what >> draws 60 ma @ 3.3V .That i can't avoid . So with 60 ma Arm7 consumption >> i end up to 120 ma. > > As described, you intend this 'small' device to dissipate almost 4W > with a linear solution. You should assume a surface temperature rise > of one degree for every milliwatt dissipated by a square centimeter of > surface area for the package. You might want to recalculate > permissible losses on this basis and reappraise some basic design/cost > considerations that include your power source. > >> Now as i see whether i have a small switcher with huge inductor and >> capacitor >> so my board space is too large . >> The other solution is super fast switcher that has everything small but >> needs 2 euros >> at least . >> I imagine every engineer designing devices has faced this question . >> I'm sure though that a simple cpu controlled switcher could cope with the >> large dropout >>from 24V to 4-5 volts and then a simple linear regulator could take you to >> 3.3V so you play >> it safe.If things go wrong (cpu malfunction ) thermal shutdown from the >> linear regulator would >> reset the cpu and that would restart the system >> As i see it a simple fet switch with cpu pwm control should satisfy that . >> Any opinions or experience on that subject??? > > In any system with a self-regulated supply voltage, the first question > is 'Who's on first?'. Don't turn this into an Abott and Costello > routine. > > RL 1000 kelvin per watt in 1 cm**2? That's pretty pessimistic. Even SOT-23s do better than that. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: legg on 22 Oct 2009 10:41 On Thu, 22 Oct 2009 10:06:35 -0400, Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >legg wrote: >> On Wed, 21 Oct 2009 10:40:10 +0300, "michael nikolaou" >> <michaelnikolaou_remove_me_(a)yahoo.com> wrote: >> >>> I need to make clear so we avoid confusions >>> >>> The device uses a 2.2" tft lcd . This thing has a backlight and no matter >>> what >>> draws 60 ma @ 3.3V .That i can't avoid . So with 60 ma Arm7 consumption >>> i end up to 120 ma. >> >> As described, you intend this 'small' device to dissipate almost 4W >> with a linear solution. You should assume a surface temperature rise >> of one degree for every milliwatt dissipated by a square centimeter of >> surface area for the package. You might want to recalculate >> permissible losses on this basis and reappraise some basic design/cost >> considerations that include your power source. >> >>> Now as i see whether i have a small switcher with huge inductor and >>> capacitor >>> so my board space is too large . >>> The other solution is super fast switcher that has everything small but >>> needs 2 euros >>> at least . >>> I imagine every engineer designing devices has faced this question . >>> I'm sure though that a simple cpu controlled switcher could cope with the >>> large dropout >>>from 24V to 4-5 volts and then a simple linear regulator could take you to >>> 3.3V so you play >>> it safe.If things go wrong (cpu malfunction ) thermal shutdown from the >>> linear regulator would >>> reset the cpu and that would restart the system >>> As i see it a simple fet switch with cpu pwm control should satisfy that . >>> Any opinions or experience on that subject??? >> >> In any system with a self-regulated supply voltage, the first question >> is 'Who's on first?'. Don't turn this into an Abott and Costello >> routine. >> >> RL > >1000 kelvin per watt in 1 cm**2? That's pretty pessimistic. Even >SOT-23s do better than that. > >Cheers > >Phil Hobbs An SOT23 is not, in itself, the package that claims the Rthja rating; it counts on conduction through the leadframe to larger external surface areas (standard coupon patterns) and produces junction/surface limits greatly exceeding those permissible to commercial external surface touch or hold limits. This simple rule of thumb for average surface temperature rise works at room temperature for packages between matchbook and breadbox sizes, without forced convection, to an accuracy of 20%. This is not manufacturer's published bumph; it is not optimism or pessimism; it is the result of many physical demonstrations over the years, repeated usually for the benefit of midding-to-highly educated and highly over-paid doubting Thomases. It is also demonstrated in the free magnetics and capacitor application software provided by mfrs such as Siemens, Magnetics Inc and Cornell Dubilier - with pedigrees that reach back into the ancient past of (gasp) print literature. Crunch the numbers back yourself on whatever packages you currently are familiar with. Any software package (no matter how expensive) has been mis-programmed or mis-applied, if it predicts otherwise. Ball park compliance is a good two-second paper and pencil check of the integrity of a convective simulation. RL
From: John Devereux on 22 Oct 2009 10:58 legg <legg(a)nospam.magma.ca> writes: > On Thu, 22 Oct 2009 10:06:35 -0400, Phil Hobbs > <pcdhSpamMeSenseless(a)electrooptical.net> wrote: > >>legg wrote: [...] >>> As described, you intend this 'small' device to dissipate almost 4W >>> with a linear solution. You should assume a surface temperature rise >>> of one degree for every milliwatt dissipated by a square centimeter of >>> surface area for the package. You might want to recalculate >>> permissible losses on this basis and reappraise some basic design/cost >>> considerations that include your power source. [...] >> >>1000 kelvin per watt in 1 cm**2? That's pretty pessimistic. Even >>SOT-23s do better than that. >> > > An SOT23 is not, in itself, the package that claims the Rthja rating; > it counts on conduction through the leadframe to larger external > surface areas (standard coupon patterns) and produces junction/surface > limits greatly exceeding those permissible to commercial external > surface touch or hold limits. > > This simple rule of thumb for average surface temperature rise works > at room temperature for packages between matchbook and breadbox sizes, > without forced convection, to an accuracy of 20%. > > This is not manufacturer's published bumph; it is not optimism or > pessimism; it is the result of many physical demonstrations over the > years, repeated usually for the benefit of midding-to-highly educated > and highly over-paid doubting Thomases. > > It is also demonstrated in the free magnetics and capacitor > application software provided by mfrs such as Siemens, Magnetics Inc > and Cornell Dubilier - with pedigrees that reach back into the ancient > past of (gasp) print literature. Crunch the numbers back yourself on > whatever packages you currently are familiar with. Any software > package (no matter how expensive) has been mis-programmed or > mis-applied, if it predicts otherwise. Ball park compliance is a good > two-second paper and pencil check of the integrity of a convective > simulation. > > RL That's an interesting and useful "rule of thumb", thanks. -- John Devereux
From: John Devereux on 22 Oct 2009 13:49
legg <legg(a)nospam.magma.ca> writes: >On Thu, 22 Oct 2009 10:06:35 -0400, Phil Hobbs ><pcdhSpamMeSenseless(a)electrooptical.net> wrote: >> As described, you intend this 'small' device to dissipate almost 4W >> with a linear solution. You should assume a surface temperature rise >> of one degree for every milliwatt dissipated by a square centimeter of >> surface area for the package. You might want to recalculate >> permissible losses on this basis and reappraise some basic design/cost >> considerations that include your power source. >> [...] >> >>1000 kelvin per watt in 1 cm**2? That's pretty pessimistic. Even >>SOT-23s do better than that. >> >>Cheers >> >>Phil Hobbs > > An SOT23 is not, in itself, the package that claims the Rthja rating; > it counts on conduction through the leadframe to larger external > surface areas (standard coupon patterns) and produces junction/surface > limits greatly exceeding those permissible to commercial external > surface touch or hold limits. > > This simple rule of thumb for average surface temperature rise works > at room temperature for packages between matchbook and breadbox sizes, > without forced convection, to an accuracy of 20%. > > This is not manufacturer's published bumph; it is not optimism or > pessimism; it is the result of many physical demonstrations over the > years, repeated usually for the benefit of midding-to-highly educated > and highly over-paid doubting Thomases. > > It is also demonstrated in the free magnetics and capacitor > application software provided by mfrs such as Siemens, Magnetics Inc > and Cornell Dubilier - with pedigrees that reach back into the ancient > past of (gasp) print literature. Crunch the numbers back yourself on > whatever packages you currently are familiar with. Any software > package (no matter how expensive) has been mis-programmed or > mis-applied, if it predicts otherwise. Ball park compliance is a good > two-second paper and pencil check of the integrity of a convective > simulation. That's an interesting and useful "rule of thumb", thanks. -- John Devereux |