LDO specs
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From: a7yvm109gf5d1 on 16 May 2010 10:42 I'm not understanding a line in a datasheet. ST Micro LD291500 This monkey here http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=497-3468-2-ND Datasheet: http://www.st.com/stonline/books/pdf/docs/9614.pdf I want a linear post regulator for a 6V output switcher to get me a 5V supply at 1A max output current. Factoring in all the worst case bla bla bla, I need the regulator to work with less than 1V dropout. Seems simple enough. But on page 10 they give the dropout spec: 0.7V max with 1.5A out (I'll never get to 1.5A) But note 2 says Dropout voltage is defined as the input-to-output differential when the output voltage drops to 99 % of its nominal value with VO + 1 V applied to VI. OK, if you're applying VO+1V to VI, that's a 1V droput, no? So what does this mean? Will the part supply me a nominal 5V output at 1A throughout temp and 5.9V at the input? What does dropout mean here? 0.7v max or 1V or what? I feel dumb asking this, I guess I shouldn't have eaten that Big Mac in London ten years ago.
From: TTman on 16 May 2010 10:53 <a7yvm109gf5d1(a)netzero.com> wrote in message news:5e904b1d-3a4a-4abe-a782-b2fcc2487442(a)y12g2000vbg.googlegroups.com... > I'm not understanding a line in a datasheet. ST Micro LD291500 > > This monkey here > > http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=497-3468-2-ND > > Datasheet: > > http://www.st.com/stonline/books/pdf/docs/9614.pdf > > I want a linear post regulator for a 6V output switcher to get me a 5V > supply at 1A max output current. > > Factoring in all the worst case bla bla bla, I need the regulator to > work with less than 1V dropout. Seems simple enough. > > But on page 10 they give the dropout spec: > > 0.7V max with 1.5A out (I'll never get to 1.5A) > > But note 2 says > > Dropout voltage is defined as the input-to-output differential when > the output voltage drops to 99 % of its nominal value with VO + 1 V > applied to VI. > > OK, if you're applying VO+1V to VI, that's a 1V droput, no? > > So what does this mean? Will the part supply me a nominal 5V output at > 1A throughout temp and 5.9V at the input? > > What does dropout mean here? 0.7v max or 1V or what? > > I feel dumb asking this, I guess I shouldn't have eaten that Big Mac > in London ten years ago. Look at page 12, fig. 8. At 1A, Vd <0.3V . Theoretically, you need a minimum of 5.3V input to get your 1 amp at 5 volts.
From: Joerg on 16 May 2010 10:56 a7yvm109gf5d1(a)netzero.com wrote: > I'm not understanding a line in a datasheet. ST Micro LD291500 > > This monkey here > > http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=497-3468-2-ND > > Datasheet: > > http://www.st.com/stonline/books/pdf/docs/9614.pdf > > I want a linear post regulator for a 6V output switcher to get me a 5V > supply at 1A max output current. > > Factoring in all the worst case bla bla bla, I need the regulator to > work with less than 1V dropout. Seems simple enough. > > But on page 10 they give the dropout spec: > > 0.7V max with 1.5A out (I'll never get to 1.5A) > > But note 2 says > > Dropout voltage is defined as the input-to-output differential when > the output voltage drops to 99 % of its nominal value with VO + 1 V > applied to VI. > > OK, if you're applying VO+1V to VI, that's a 1V droput, no? > Yup. > So what does this mean? Will the part supply me a nominal 5V output at > 1A throughout temp and 5.9V at the input? > It will. As long as it doesn't oscillate, that is. Personally I do not trust LDOs much. > What does dropout mean here? 0.7v max or 1V or what? > It means that it's guaranteed not to be below 4.95V out (meaning 99% of 5V) when the input is at 5.7V at 1.5A load. Plus minus its own output voltage tolerance band, of course. > I feel dumb asking this, I guess I shouldn't have eaten that Big Mac > in London ten years ago. :-) -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: a7yvm109gf5d1 on 16 May 2010 12:05 On May 16, 9:56 am, Joerg <inva...(a)invalid.invalid> wrote: > a7yvm109gf...(a)netzero.com wrote: > > I'm not understanding a line in a datasheet. ST Micro LD291500 > > > This monkey here > > >http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=497-.... > > > Datasheet: > > >http://www.st.com/stonline/books/pdf/docs/9614.pdf > > > I want a linear post regulator for a 6V output switcher to get me a 5V > > supply at 1A max output current. > > > Factoring in all the worst case bla bla bla, I need the regulator to > > work with less than 1V dropout. Seems simple enough. > > > But on page 10 they give the dropout spec: > > > 0.7V max with 1.5A out (I'll never get to 1.5A) > > > But note 2 says > > > Dropout voltage is defined as the input-to-output differential when > > the output voltage drops to 99 % of its nominal value with VO + 1 V > > applied to VI. > > > OK, if you're applying VO+1V to VI, that's a 1V droput, no? > > Yup. > > > So what does this mean? Will the part supply me a nominal 5V output at > > 1A throughout temp and 5.9V at the input? > > It will. > > As long as it doesn't oscillate, that is. Personally I do not trust LDOs > much. Well that's the next thing. That graph sure is weird. Do the hatch lines mean "not here" or 'here"? > > > What does dropout mean here? 0.7v max or 1V or what? > > It means that it's guaranteed not to be below 4.95V out (meaning 99% of > 5V) when the input is at 5.7V at 1.5A load. Plus minus its own output > voltage tolerance band, of course. That's not how note 2 reads to me... Why would you specify a VI=VO+1V when specifying a dropout voltage?
From: John Larkin on 16 May 2010 13:33
On Sun, 16 May 2010 09:05:30 -0700 (PDT), a7yvm109gf5d1(a)netzero.com wrote: >On May 16, 9:56�am, Joerg <inva...(a)invalid.invalid> wrote: >> a7yvm109gf...(a)netzero.com wrote: >> > I'm not understanding a line in a datasheet. ST Micro LD291500 >> >> > This monkey here >> >> >http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=497-... >> >> > Datasheet: >> >> >http://www.st.com/stonline/books/pdf/docs/9614.pdf >> >> > I want a linear post regulator for a 6V output switcher to get me a 5V >> > supply at 1A max output current. >> >> > Factoring in all the worst case bla bla bla, I need the regulator to >> > work with less than 1V dropout. Seems simple enough. >> >> > But on page 10 they give the dropout spec: >> >> > 0.7V max with 1.5A out (I'll never get to 1.5A) >> >> > But note 2 says >> >> > Dropout voltage is defined as the input-to-output differential when >> > the output voltage drops to 99 % of its nominal value with VO + 1 V >> > applied to VI. >> >> > OK, if you're applying VO+1V to VI, that's a 1V droput, no? >> >> Yup. >> >> > So what does this mean? Will the part supply me a nominal 5V output at >> > 1A throughout temp and 5.9V at the input? >> >> It will. >> >> As long as it doesn't oscillate, that is. Personally I do not trust LDOs >> much. > >Well that's the next thing. That graph sure is weird. Do the hatch >lines mean "not here" or 'here"? That graph makes little or no sense. The 0,0 point is no cap at all, the 12,12 point is a cap with 12 ohms ESR. It's not clear if the hatched area extrapolates to, say infinite C with infinite ESR, which is again no cap. So no cap is both unstable and stable, whatever tha hatch means. The only application schematics show polarized caps. I hate most LDO specs. Most of them go out of their way to avoid a useful discussion of stability. I'd use another part that addresses stability better. John |