I've got a 24V incoming supply, and a need for a number of supplies (+12, +5, -5, and +3.3). The +/- 5V supplies are for a bit of analog circuitry on a largely digital board, so they have low current requirements. My plan for the +5V supply is to just hang a 78xx05 off the +12V. +12V and +3.3V are going to come from switchers off of +24V. Anyone have any suggestions for a clever way to get -(something) from one of the positive supplies? I'm thinking that if I can get some negative voltage for cheap from one of the existing switchers that I can then regulate it to -5V with a 79xx05. I expect I'll end up using a charge pump -- but suggestions are welcome. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Clever ways to get +/- volts from MC34063
Started by ●January 25, 2012
Reply by ●January 25, 20122012-01-25
On Wed, 25 Jan 2012 11:44:25 -0600, Tim Wescott <tim@seemywebsite.com> wrote:>I've got a 24V incoming supply, and a need for a number of supplies (+12, >+5, -5, and +3.3). The +/- 5V supplies are for a bit of analog circuitry >on a largely digital board, so they have low current requirements. My >plan for the +5V supply is to just hang a 78xx05 off the +12V. +12V and >+3.3V are going to come from switchers off of +24V. > >Anyone have any suggestions for a clever way to get -(something) from one >of the positive supplies? I'm thinking that if I can get some negative >voltage for cheap from one of the existing switchers that I can then >regulate it to -5V with a 79xx05. I expect I'll end up using a charge >pump -- but suggestions are welcome.You can charge pump off the switcher node of one of the positive switchers to get negative volts, essentially a classic "voltage doubler" capacitor-diode thing. It will make roughly -24, which is a lot to regulate to -5. A small series resistor is a good idea... some switchers don't like this being done to them. But if it's low current, you may as well buy a standard charge-pump voltage inverter chip and run that off +5. The LTC "LTM" module series can be used to make -5 from +24. Not real cheap, but easy. Or build a Cuk switcher. Another possibility: buy one of those cheap SIP dc/dc converters, $4 roughly. John
Reply by ●January 25, 20122012-01-25
Tim Wescott wrote:> I've got a 24V incoming supply, and a need for a number of supplies (+12, > +5, -5, and +3.3). The +/- 5V supplies are for a bit of analog circuitry > on a largely digital board, so they have low current requirements. My > plan for the +5V supply is to just hang a 78xx05 off the +12V. +12V and > +3.3V are going to come from switchers off of +24V. > > Anyone have any suggestions for a clever way to get -(something) from one > of the positive supplies? I'm thinking that if I can get some negative > voltage for cheap from one of the existing switchers that I can then > regulate it to -5V with a 79xx05. I expect I'll end up using a charge > pump -- but suggestions are welcome. >If you run a buck to make +5V you can use a dual inductor instead of a single, plus another diode -> There's your -5V. Dual inductors have become cheap catalog items these days. As for the model, the LTSpice group on Yahoo has some. I think the best one was from Helmut. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●January 25, 20122012-01-25
On Wed, 25 Jan 2012 11:44:25 -0600, Tim Wescott <tim@seemywebsite.com> wrote:>I've got a 24V incoming supply, and a need for a number of supplies (+12, >+5, -5, and +3.3). The +/- 5V supplies are for a bit of analog circuitry >on a largely digital board, so they have low current requirements. My >plan for the +5V supply is to just hang a 78xx05 off the +12V. +12V and >+3.3V are going to come from switchers off of +24V. > >Anyone have any suggestions for a clever way to get -(something) from one >of the positive supplies? I'm thinking that if I can get some negative >voltage for cheap from one of the existing switchers that I can then >regulate it to -5V with a 79xx05. I expect I'll end up using a charge >pump -- but suggestions are welcome.I've used another winding on the inductor, but it worries me a bit.
Reply by ●January 25, 20122012-01-25
On Wed, 25 Jan 2012 11:44:25 -0600, Tim Wescott <tim@seemywebsite.com> wrote:>I've got a 24V incoming supply, and a need for a number of supplies (+12, >+5, -5, and +3.3). The +/- 5V supplies are for a bit of analog circuitry >on a largely digital board, so they have low current requirements. My >plan for the +5V supply is to just hang a 78xx05 off the +12V. +12V and >+3.3V are going to come from switchers off of +24V. > >Anyone have any suggestions for a clever way to get -(something) from one >of the positive supplies? I'm thinking that if I can get some negative >voltage for cheap from one of the existing switchers that I can then >regulate it to -5V with a 79xx05. I expect I'll end up using a charge >pump -- but suggestions are welcome.Here's how I did a +5V/-12V switcher at GenRad ~1980... http://www.analog-innovations.com/SED/OldStyleBuckSwitcherWithAddedNegativeOutput.pdf I don't know if modern switcher chips will allow for such shenanigans :-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●January 25, 20122012-01-25
Spehro Pefhany wrote:> On Wed, 25 Jan 2012 11:44:25 -0600, Tim Wescott <tim@seemywebsite.com> > wrote: > >> I've got a 24V incoming supply, and a need for a number of supplies (+12, >> +5, -5, and +3.3). The +/- 5V supplies are for a bit of analog circuitry >> on a largely digital board, so they have low current requirements. My >> plan for the +5V supply is to just hang a 78xx05 off the +12V. +12V and >> +3.3V are going to come from switchers off of +24V. >> >> Anyone have any suggestions for a clever way to get -(something) from one >> of the positive supplies? I'm thinking that if I can get some negative >> voltage for cheap from one of the existing switchers that I can then >> regulate it to -5V with a 79xx05. I expect I'll end up using a charge >> pump -- but suggestions are welcome. > > I've used another winding on the inductor, but it worries me a bit. >Why? Me no worry :-) It does require the converter to keep running though. If the positive supply is not loaded much the negative can drop off. But with analog stuff that usually isn't a concern. On sync bucks one can force many chips to remain in sync-mode even under light loads and not let them go into burst mode. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●January 25, 20122012-01-25
Tim Wescott a écrit :> I've got a 24V incoming supply, and a need for a number of supplies (+12, > +5, -5, and +3.3). The +/- 5V supplies are for a bit of analog circuitry > on a largely digital board, so they have low current requirements. My > plan for the +5V supply is to just hang a 78xx05 off the +12V. +12V and > +3.3V are going to come from switchers off of +24V. > > Anyone have any suggestions for a clever way to get -(something) from one > of the positive supplies? I'm thinking that if I can get some negative > voltage for cheap from one of the existing switchers that I can then > regulate it to -5V with a 79xx05. I expect I'll end up using a charge > pump -- but suggestions are welcome. >I guess the +12V switcher is a buck. Just use a double winding coil in lieu of a simple inductor, and use the secondary so that it copies the buck output voltage at your negative output (energy taken back from the buck output bypass cap). If your main buck is *not* a synchronous rectification one, then you even have the buck free wheeling diode drop making for your negative branch diode drop. -- Thanks, Fred.
Reply by ●January 25, 20122012-01-25
Joerg <invalid@invalid.invalid> writes:> Tim Wescott wrote: >> I've got a 24V incoming supply, and a need for a number of supplies (+12, >> +5, -5, and +3.3). The +/- 5V supplies are for a bit of analog circuitry >> on a largely digital board, so they have low current requirements. My >> plan for the +5V supply is to just hang a 78xx05 off the +12V. +12V and >> +3.3V are going to come from switchers off of +24V. >> >> Anyone have any suggestions for a clever way to get -(something) from one >> of the positive supplies? I'm thinking that if I can get some negative >> voltage for cheap from one of the existing switchers that I can then >> regulate it to -5V with a 79xx05. I expect I'll end up using a charge >> pump -- but suggestions are welcome. >> > > If you run a buck to make +5V you can use a dual inductor instead of a > single, plus another diode -> There's your -5V. Dual inductors have > become cheap catalog items these days.NICE that seems a general and very useful technique. I have used those dual inductors for flyback but never thought of them used like that. [...] -- John Devereux
Reply by ●January 25, 20122012-01-25
John Devereux wrote:> Joerg <invalid@invalid.invalid> writes: > >> Tim Wescott wrote: >>> I've got a 24V incoming supply, and a need for a number of supplies (+12, >>> +5, -5, and +3.3). The +/- 5V supplies are for a bit of analog circuitry >>> on a largely digital board, so they have low current requirements. My >>> plan for the +5V supply is to just hang a 78xx05 off the +12V. +12V and >>> +3.3V are going to come from switchers off of +24V. >>> >>> Anyone have any suggestions for a clever way to get -(something) from one >>> of the positive supplies? I'm thinking that if I can get some negative >>> voltage for cheap from one of the existing switchers that I can then >>> regulate it to -5V with a 79xx05. I expect I'll end up using a charge >>> pump -- but suggestions are welcome. >>> >> If you run a buck to make +5V you can use a dual inductor instead of a >> single, plus another diode -> There's your -5V. Dual inductors have >> become cheap catalog items these days. > > NICE that seems a general and very useful technique. > > I have used those dual inductors for flyback but never thought of them > used like that. >Shhht! If was meant to be a secret. Oh wait ... If you want it to be extra cool, there's lots of amps flowing or it has to be tracking more precisely in voltage then you can use a FET instead of the diode and control that from your sync buck chip. This cuts out the diode losses. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●January 25, 20122012-01-25
On Jan 26, 6:44=A0am, Tim Wescott <t...@seemywebsite.com> wrote:> I've got a 24V incoming supply, and a need for a number of supplies (+12, > +5, -5, and +3.3). =A0The +/- 5V supplies are for a bit of analog circuit=ry> on a largely digital board, so they have low current requirements. =A0My > plan for the +5V supply is to just hang a 78xx05 off the +12V. =A0+12V an=d> +3.3V are going to come from switchers off of +24V. > > Anyone have any suggestions for a clever way to get -(something) from one > of the positive supplies? =A0I'm thinking that if I can get some negative > voltage for cheap from one of the existing switchers that I can then > regulate it to -5V with a 79xx05. =A0I expect I'll end up using a charge > pump -- but suggestions are welcome. > > -- > My liberal friends think I'm a conservative kook. > My conservative friends think I'm a liberal kook. > Why am I not happy that they have found common ground? > > Tim Wescott, Communications, Control, Circuits & Softwarehttp://www.wesco=ttdesign.com Unless you are looking for really low ripple then buck/boost is the modern way to go for everything. Far more efficient - up in the 95% odd. Hardy
