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Groups > sci.electronics.design > #445935
| From | Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> |
|---|---|
| Newsgroups | sci.electronics.design |
| Subject | Re: buck converter for negative voltages |
| Date | 2017-02-03 09:44 -0500 |
| Organization | A noiseless patient Spider |
| Message-ID | <o724r7$e2q$1@dont-email.me> (permalink) |
| References | <o6ti3m0312e@drn.newsguy.com> <3pk49ct21gjuhka45gqe30g5c7s7f48upg@4ax.com> <43f62388-eb31-41b0-89e4-72f861ac6411@googlegroups.com> <o701hv014n9@drn.newsguy.com> |
On 02/02/2017 02:33 PM, Winfield Hill wrote: > Lasse Langwadt Christensen wrote... >> >> Den onsdag den 1. februar 2017 kl. 23.27.44 UTC+1 skrev Winfield Hill: >>> Lasse Langwadt Christensen wrote... >>>> >>>> Den onsdag den 1. februar 2017 kl. skrev Winfield Hill: >>>>> John Larkin wrote... >>>>>> >>>>>> Winfield Hill wrote: >>>>>> >>>>>>> Suggestions for buck converter ICs for negative voltages? >>>>>>> Controller ICs (external MOSFETs) would be nice. >>>>>> >>>>>> I can't recall ever seeing a real negative-to-negative >>>>>> switcher! People sometimes hack a positive controller. >>>>> >>>>> LTC also suggests the positive boost-converter hack, >>>>> in design note DN1022. >>>> >>>> if you can figure out a way to flip the feedback or live with the >>>> output being relative to input, wouldn't any synchronous buck work? >>> >>> The feedback is inverted in a *boost* converter, one >>> just needs to translate it down to the negative rail. >> >> afaict it is the same issue, you need to make the feedback >> relative to ground instead of to the negative rail > > Actually, besides level-shifting the feedback to the IC's > FB pin, which is referenced to -Vin, it's necessary to > invert the sign of the feedback. That becomes clear when > comparing buck-converter duty-cycle formula Vout/Vin = D, > with a boost converter, Vout/Vin = 1/(1-D). Also, it's > obvious that when -Vout increases (more negative), FB > must increase (more positive). > > The inversion occurs in the level-shift transistor, Q1. > > . gnd ----+---------+--------+--------, > . ___|____ | _|_ | > . | | _|_ --- R1 > . | boost | /_\ | | > . | | | | | > . | SW |----+-#####--+---+------- -Vout > . | | B | > . | FB |----+----------C E--' > . |________| | Q1 pnp > . | R2 > . | | > . -Vin ---+---------+----- > > I sometimes put cap multipliers inside the feedback loop of a positive-supply switcher, with an RC highpass to make the loop stable (the way you do with a slow buffer or a big C load). (It helps the load regulation without wasting power on maintaining worst-case headroom all the time.) You can't do that when using a buck in inverting mode, for exactly this reason. There are shunt references that work upside down, e.g. the LM385 and LM4041-ADJ, but no switchers that I know of. Using one of those references instead of your PNP (with an RC highpass as above) would get rid of the V_BE error for fairly cheap. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
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buck converter for negative voltages Winfield Hill <hill@rowland.harvard.edu> - 2017-02-01 12:57 -0800
Re: buck converter for negative voltages John Larkin <jjlarkinxyxy@highlandtechnology.com> - 2017-02-01 13:35 -0800
Re: buck converter for negative voltages Winfield Hill <hill@rowland.harvard.edu> - 2017-02-01 14:02 -0800
Re: buck converter for negative voltages Lasse Langwadt Christensen <langwadt@fonz.dk> - 2017-02-01 14:16 -0800
Re: buck converter for negative voltages Winfield Hill <hill@rowland.harvard.edu> - 2017-02-01 14:27 -0800
Re: buck converter for negative voltages Lasse Langwadt Christensen <langwadt@fonz.dk> - 2017-02-01 14:39 -0800
Re: buck converter for negative voltages Winfield Hill <hill@rowland.harvard.edu> - 2017-02-02 11:33 -0800
Re: buck converter for negative voltages Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-02-03 09:44 -0500
Re: buck converter for negative voltages John Larkin <jjlarkinxyxy@highlandtechnology.com> - 2017-02-01 14:38 -0800
Re: buck converter for negative voltages Lasse Langwadt Christensen <langwadt@fonz.dk> - 2017-02-01 14:43 -0800
Re: buck converter for negative voltages John Larkin <jjlarkinxyxy@highlandtechnology.com> - 2017-02-01 15:35 -0800
Re: buck converter for negative voltages Winfield Hill <hill@rowland.harvard.edu> - 2017-02-01 18:25 -0800
Re: buck converter for negative voltages Winfield Hill <hill@rowland.harvard.edu> - 2017-02-01 13:36 -0800
Re: buck converter for negative voltages Wanderer <wanderer@dialup4less.com> - 2017-02-01 15:04 -0800
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