Post by thread:[EE]: Low voltage boost regulator

part 1 5212 bytes content-type:text/plain; (decoded 7bit) I had a need to produce 10-15V at 20mA or so from a single AA battery, or maybe two at most. This is significant power for an AA cell, so efficiency is important. This means it would be desirable to have a FET switching the inductor, but that is impossible to do efficiently on just 1V. I also wanted a PIC in there to do a few smart things in dealing with the battery and the load. Of course a PIC won't run at 1V either, so the strategy was to have a dumb circuit create 4-5V or so with minimal current draw, which provides just enough power to do the smart things with the real (and much higher power) load. To this end, I came up with a circuit that uses only cheap off the shelf parts, and can run a 12F629 on its internal oscillator from less than 1V. The output voltage is high enough to allow switching the FET gate for the high power boost regulator directly from a PIC pin. The remaining smarts related to the high power load are implemented in the PIC firmware, and aren't relevant in this discussion. I thought the dumb low voltage boost regulator might be useful in general, so I put the schematic at http://www.embedinc.com/pic/lvboost.pdf. I know this looks like a lot of parts, but if you look carefully you will see they are all of the cheap jellybean variety. The most expensive component is L1, which is available from Coilcraft (RFB0810-151) for $.64 in single quantities. The total parts cost is about $1.75 if buying hobby quantities from Jameco, obviously less for real manufacturing volume. I have also attached two plots that show the performance with 1Kohm and 470ohm loads as a function of input voltage. A bare 12F629 running from its internal 4MHz oscillator never presents less than a 1Kohm load worst case over the full temperature and voltage range. The typical at room temperature is *much* higher. The efficiency is about 2/3 over the desired range. That's a bit low for switching power supply standards, but not bad at all when you consider that the input voltage is only a little more than the voltage drop of a silicon junction. Also, most of the battery power will go to the real load, so the 50% extra required to run the PIC isn't all that significant. This regulator is also well behaved in that the efficiency stays near its maximum over wide input voltage and load ranges. With no load, input current is under 1.5mA at .85 to 1.0V input with over 4.5V output. THEORY OF OPERATION The rest of this message discusses how this circuit works with some suggestions how to customize it for particular applications. Q1, Q2, and Q6 form a low power free running oscillator. Cap C3 charges up thru R3 until Q2 turns on. This turns on Q6, which turns on Q1, which discharges C3 to start the cycle all over again. C4 and R4 provide positive feedback so that Q6 switches on and off a little more abruptly, although that is a minor effect with these particular values. Q3, Q7, and Q4 amplify the pulse from Q6 and sharpen its edges. C6 provides some more positive feedback which sharpens the edges of the pulse a little more. By the time the pulse reaches Q4, it can sink substantial collector current when the pulse is on. This charges up L1 during the pulse, and causes the energy in L1 to dump via D1 into C5 when the pulse ends and Q4 is abruptly turned off. R13, R14, and Q5 provide the output voltage regulation. R13 and R14 are a voltage divider to turn on Q5 when the output reaches the desired value. When Q5 is on, further pulses are inhibited by holding C3 discharged. The B-E voltage drop of Q5 is used as the reference voltage to regulate the output. This is not very accurate and dependent on temperature, but good enough for this particular purpose. The pulses are about 20uS long with these values. When Q5 stays off, the switching frequency is about 20KHz. As the output voltage rises and becomes regulated, the time between pulses is stretched. With no load, the switching frequency drops to a few Hertz. You can see this working in both plots. Initially the input current and output voltage rise sharply as the input voltage is increased. In this region, the output voltage is below the desired threshold and pulse occur at the maximum rate. The time between pulses stretches out once the output voltage becomes regulated. At this point, the output voltage flattens out and the input current starts to go down at the same time. The peak of the input current curve therefore identifies the start of output voltage regulation. An important attribute of this circuit is that almost all active components are off between pulses. All current during that time flows thru R3 and R13, which in this case are both 100Kohm resistors. R13, R14, and Q5 are a very cheap but low quality regulation circuit. The output voltage could be made considerably stiffer and more accurate with a more elaborate circuit using a real voltage reference. The pulse duration can be changed by adjusting R4 and C4, and the free run frequency is largely a function of R3 and C3. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details. part 2 8086 bytes content-type:image/gif; (decode) part 3 8212 bytes content-type:image/gif; (decode) part 4 2 bytes -

At 03:38 PM 6/18/03 -0400, you wrote: >I thought the dumb low voltage boost regulator might be useful in general, >so I put the schematic at http://www.embedinc.com/pic/lvboost.pdf. My guess is that in version 1.1, the anode of D1 connects to the collector of Q4. But that's just a guess. Tom M. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

> My guess is that in version 1.1, the anode of D1 connects to the > collector of Q4. But that's just a guess. Oops, good guess. I breadboarded it right but drew it wrong. The schematic at http://www.embedinc.com/pic/lvboost.pdf has been updated. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

At 05:27 PM 6/18/2003 -0400, you wrote: > > My guess is that in version 1.1, the anode of D1 connects to the > > collector of Q4. But that's just a guess. > >Oops, good guess. I breadboarded it right but drew it wrong. The >schematic at http://www.embedinc.com/pic/lvboost.pdf has been updated. I've done that before. The brain sees (and wires) the right circuit even if the eyes don't. ;-) Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" spam_OUTspeffTakeThisOuTinterlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

You have another current loss when you are up to voltage... When Q5 turns on (at reg) it will pull the lower end of R3 to ground. R3 is now between +Vin and ground. Bob Ammerman RAm Systems -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservKILLspam@spam@mitvma.mit.edu with SET PICList DIGEST in the body

Olin Lathrop wrote: > I had a need to produce 10-15V at 20mA or so from a > single AA battery, or maybe two at most. This is > significant power for an AA cell, so efficiency is > important. This means it would be desirable to have > a FET switching the inductor, but that is impossible > to do efficiently on just 1V... LEDs on your cap for hiking again? Perhaps LM2623 would be better? Mike. AN-1250: Inductive Based Switching Regulator Circuits Provide High Efficiency White LED Drives AN-1258: LM2623 Boost Converter - A Simple Technology Twist Produces the Industry's Most Versatile Supply From http://www1.national.com/ds.cgi/LM/LM2623.pdf Ratio Adaptive Gated Oscillator Based, DC DC Boost Converter-Low Vin, Low Ripple, General Purpose. ---------------------------------------------------------- The LM2623 is a high efficiency, general purpose, step-up DC-DC switching regulator for battery-powered and low input voltage systems. It accepts an input voltage between 0.8 and 14 volts and converts it into a regulated output voltage. The LM2623 can be used to generate up to 14 volts using an inductor and almost any positive voltage using a transformer. It has an internal .17§Ù N-Channel MOSFET power switch. Efficiencies up to 90% are achievable with the LM2623. In order to adapt to a number of applications, the LM2623 allows the designer to vary the output voltage, the operating frequency (300kHz to 2 MHz) and duty cycle (17% to 90%) to optimize the part¡¯s performance. The selected values can be fixed or can vary with battery voltage or input to output voltage ratio. Using these techniques according to the design guidelines may improve efficiency or reduce parts cost or both. The LM2623 is a gated oscillator and uses a very simple, on/off regulation mode to produce good efficiency and stable operation over a wide operating range. It normally regulates by skipping switching cycles when it reaches the regulation limit (Pulse Frequency Modulation). In some applications, with the proper selection of external feedback components, it can also regulate cycle to cycle, similar to Pulse Width Modulated parts. It does this by adjusting the off time between switching cycles, while maintaining continuous current through the coil. This cycle to cycle type of PFM results in very low ripple on the output voltage and/or minimum sized output capacitors, similar to PWM parts. This mode also typically maximizes efficiency. ------------------------------ -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

>I had a need to produce 10-15V at 20mA or so from a single AA battery, or >maybe two at most. This is significant power for an AA cell, so >efficiency is important. This means it would be desirable to have a FET >switching the inductor, but that is impossible to do efficiently on just >1V. I also wanted a PIC in there to do a few smart things in dealing with >the battery and the load. Have you looked at the TrenchFET's from Vishay Siliconix? The particular one I am looking at is the Si5447DC, which is a P channel one, but they also come as N channel. These are 1.8V rated, and looking at the transfer curve will be well able to self oscillate a coil with a feedback winding at 1.5V. The knee of the transfer curve is about 1.1V. Output characteristics show a drain current of about 1.5A at Vds of 0.5V with Vgs = 1.5V, before it starts to go round the corner towards zero current. At this rate it probably would not work to the 0.9V you show on your circuit, but may be an alternative worth looking at. The device comes in an "8 pin" 1208 package which seems to be getting popular these days. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservKILLspam.....mitvma.mit.edu with SET PICList DIGEST in the body

> You have another current loss when you are up to voltage... > > When Q5 turns on (at reg) it will pull the lower end of R3 to ground. > R3 is now between +Vin and ground. Right. As I said, "All current during that time flows thru R3 and R13, ..." ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email EraseMElistservspam_OUTTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

> Have you looked at the TrenchFET's from Vishay Siliconix? The > particular one I am looking at is the Si5447DC, which is a P channel > one, but they also come as N channel. These are 1.8V rated, and looking > at the transfer curve will be well able to self oscillate a coil with a > feedback winding at 1.5V. The knee of the transfer curve is about 1.1V. > Output characteristics show a drain current of about 1.5A at Vds of > 0.5V with Vgs = 1.5V, before it starts to go round the corner towards > zero current. > > At this rate it probably would not work to the 0.9V you show on your > circuit, but may be an alternative worth looking at. The device comes > in an "8 pin" 1208 package which seems to be getting popular these days. I was not aware of these devices, but I was hoping to run this from a single NiMH cell down to empty. That's about 800mV, so I didn't quite achieve that, but the closer the better. The real power will be switched by a FET driven from the 5V created by this low voltage booster. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body

Oops, sorry! ----- Original Message ----- From: "Olin Lathrop" <@spam@olin_piclistKILLspamEMBEDINC.COM> To: <KILLspamPICLISTKILLspamMITVMA.MIT.EDU> Sent: Thursday, June 19, 2003 8:32 AM Subject: Re: [EE]: Low voltage boost regulator {Quote hidden}> > You have another current loss when you are up to voltage... > > > > When Q5 turns on (at reg) it will pull the lower end of R3 to ground. > > R3 is now between +Vin and ground. > > Right. As I said, "All current during that time flows thru R3 and R13, > ..." > > > ***************************************************************** > Embed Inc, embedded system specialists in Littleton Massachusetts > (978) 742-9014, http://www.embedinc.com > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email RemoveMElistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spamBeGonelistservspamBeGonemitvma.mit.edu with SET PICList DIGEST in the body

Olin wrote... >I thought the dumb low voltage boost regulator might be useful in general, >so I put the schematic at http://www.embedinc.com/pic/lvboost.pdf. Nice circuit: a "vampire" converter that can suck all the blood out of a single-cell battery while producing a regulated 5V output, using nothing but cheap jelly-bean components. This has beaucoup uses. Thanks for posting it. DD -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads