Need a little design help on my latest project
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From: Charlie E. on 1 Mar 2010 16:57 On Mon, 01 Mar 2010 12:54:27 -0800, Joerg <invalid(a)invalid.invalid> wrote: >Charlie E. wrote: >> On Mon, 01 Mar 2010 11:12:58 -0800, Joerg <invalid(a)invalid.invalid> >> wrote: >> >>> Charlie E. wrote: >>>> On Mon, 01 Mar 2010 10:38:45 -0800, Joerg <invalid(a)invalid.invalid> >>>> wrote: >>>> >>>>> Charlie E. wrote: >>>> <snip original problem...> >>>> >>>>>> Joerg, >>>>>> Thanks for the advice. Yes, I had been concentrating so hard on the >>>>>> amplifiers I never really considered the LEDs. In my mind, they would >>>>>> just 'work' and I could then adjust accordingly. Didn't realize that >>>>>> they would vary that much. Will have to look at maybe adding a higher >>>>>> voltage, and go with the constant current drives for them. This does >>>>>> need to be pretty accurate! >>>>>> >>>>> Where is your VCC coming from? Regulator? If so, what's the minimum >>>>> voltage going into that regulator? If it is a battery that won't drop >>>>> below about 4.5V and has low load ripple (low source resistance, added >>>>> capacitors) fixing this part of the circuit would become fairly simple. >>>> Hi Joerge, >>>> I only have two AA batteries, so voltage is only about 2.5-3.1 volts. >>>> That was why I added in the power supply, to try and stabilize that >>>> voltage. Most of the parts were pretty power tolerant, but I figured >>>> (somewhat correctly) that the LEDs would be pretty voltage sensitive. >>>> >>> That will require switch mode conversion, no other choice. >>> >>> >>>> What do you think of this idea? Take an LED driver chip, like an >>>> LM3519 to do the voltage step up and current control, and then three >>>> fets to switch that current to each of the LEDs. Means a chip, a >>>> small inductor and schottkey, a couple of caps, and three fets. >>>> Shouldn't take up too much board space or budget... >>>> >>> Nope, it ain't quite that easy. It doesn't have an external sense >>> resistor and, consequently, the "accuracy" to which it holds the current >>> is really horrid. Look at the Iout versus Vin, that's just not good >>> enough. If you want to use a chip (or three) you need to find one with >>> at least and external Rsense. >>> >>> It is usually easier and less expensive to boost that voltage from the >>> two AA cells to 5V and add the analog current source circuits I >>> mentioned in my other post (one per LED section). The PIC could be >>> supplied directly from the AA cell if it's happy with 2.5V. >> >> You are probably right. One problem is that the opamps for the >> phototransistor (ambient light sensor) should also probably stay on >> the battery, to prevent overdriving the ADC inputs for the PIC. There >> are the other LED drivers with an external current sense resistor, so >> will look into the tradeoffs involved... >> > >The opamps can be handled with series resistors and, if necessary, BAV99 >double-diodes. But if you have suitable opamps you might as well run >them off the PIC rail. > >I'd really consider just one li'l boost converter that makes 5V. Tons of >those available. Then current sources for a clean control. Actually, just found the MCP1252 series - switchable 3.3 or 5.0 boost converters (charge pump) that don't need inductors! Two of these will be cheaper than the inductor based solution I have now, and give me both rails regulated... Charlie
From: Joerg on 1 Mar 2010 17:02 Charlie E. wrote: > On Mon, 01 Mar 2010 12:54:27 -0800, Joerg <invalid(a)invalid.invalid> > wrote: > >> Charlie E. wrote: >>> On Mon, 01 Mar 2010 11:12:58 -0800, Joerg <invalid(a)invalid.invalid> >>> wrote: >>> >>>> Charlie E. wrote: >>>>> On Mon, 01 Mar 2010 10:38:45 -0800, Joerg <invalid(a)invalid.invalid> >>>>> wrote: >>>>> >>>>>> Charlie E. wrote: >>>>> <snip original problem...> >>>>> >>>>>>> Joerg, >>>>>>> Thanks for the advice. Yes, I had been concentrating so hard on the >>>>>>> amplifiers I never really considered the LEDs. In my mind, they would >>>>>>> just 'work' and I could then adjust accordingly. Didn't realize that >>>>>>> they would vary that much. Will have to look at maybe adding a higher >>>>>>> voltage, and go with the constant current drives for them. This does >>>>>>> need to be pretty accurate! >>>>>>> >>>>>> Where is your VCC coming from? Regulator? If so, what's the minimum >>>>>> voltage going into that regulator? If it is a battery that won't drop >>>>>> below about 4.5V and has low load ripple (low source resistance, added >>>>>> capacitors) fixing this part of the circuit would become fairly simple. >>>>> Hi Joerge, >>>>> I only have two AA batteries, so voltage is only about 2.5-3.1 volts. >>>>> That was why I added in the power supply, to try and stabilize that >>>>> voltage. Most of the parts were pretty power tolerant, but I figured >>>>> (somewhat correctly) that the LEDs would be pretty voltage sensitive. >>>>> >>>> That will require switch mode conversion, no other choice. >>>> >>>> >>>>> What do you think of this idea? Take an LED driver chip, like an >>>>> LM3519 to do the voltage step up and current control, and then three >>>>> fets to switch that current to each of the LEDs. Means a chip, a >>>>> small inductor and schottkey, a couple of caps, and three fets. >>>>> Shouldn't take up too much board space or budget... >>>>> >>>> Nope, it ain't quite that easy. It doesn't have an external sense >>>> resistor and, consequently, the "accuracy" to which it holds the current >>>> is really horrid. Look at the Iout versus Vin, that's just not good >>>> enough. If you want to use a chip (or three) you need to find one with >>>> at least and external Rsense. >>>> >>>> It is usually easier and less expensive to boost that voltage from the >>>> two AA cells to 5V and add the analog current source circuits I >>>> mentioned in my other post (one per LED section). The PIC could be >>>> supplied directly from the AA cell if it's happy with 2.5V. >>> You are probably right. One problem is that the opamps for the >>> phototransistor (ambient light sensor) should also probably stay on >>> the battery, to prevent overdriving the ADC inputs for the PIC. There >>> are the other LED drivers with an external current sense resistor, so >>> will look into the tradeoffs involved... >>> >> The opamps can be handled with series resistors and, if necessary, BAV99 >> double-diodes. But if you have suitable opamps you might as well run >> them off the PIC rail. >> >> I'd really consider just one li'l boost converter that makes 5V. Tons of >> those available. Then current sources for a clean control. > > Actually, just found the MCP1252 series - switchable 3.3 or 5.0 boost > converters (charge pump) that don't need inductors! Two of these will > be cheaper than the inductor based solution I have now, and give me > both rails regulated... > Careful, the 5.0V version doesn't have any oomph below 3V input and AFAIK it quits once Vbat drops below 2.7V. I'd use a real boost converter that can comfortably cover your battery voltage range all the way to the end of discharge. That one little inductor isn't going to kill ya :-) -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: langwadt on 1 Mar 2010 17:05 On 1 Mar., 20:12, Joerg <inva...(a)invalid.invalid> wrote: > Charlie E. wrote: > > On Mon, 01 Mar 2010 10:38:45 -0800, Joerg <inva...(a)invalid.invalid> > > wrote: > > >> Charlie E. wrote: > > <snip original problem...> > > >>> Joerg, > >>> Thanks for the advice. Yes, I had been concentrating so hard on the > >>> amplifiers I never really considered the LEDs. In my mind, they would > >>> just 'work' and I could then adjust accordingly. Didn't realize that > >>> they would vary that much. Will have to look at maybe adding a higher > >>> voltage, and go with the constant current drives for them. This does > >>> need to be pretty accurate! > > >> Where is your VCC coming from? Regulator? If so, what's the minimum > >> voltage going into that regulator? If it is a battery that won't drop > >> below about 4.5V and has low load ripple (low source resistance, added > >> capacitors) fixing this part of the circuit would become fairly simple.. > > > Hi Joerge, > > I only have two AA batteries, so voltage is only about 2.5-3.1 volts. > > That was why I added in the power supply, to try and stabilize that > > voltage. Most of the parts were pretty power tolerant, but I figured > > (somewhat correctly) that the LEDs would be pretty voltage sensitive. > > That will require switch mode conversion, no other choice. > > > What do you think of this idea? Take an LED driver chip, like an > > LM3519 to do the voltage step up and current control, and then three > > fets to switch that current to each of the LEDs. Means a chip, a > > small inductor and schottkey, a couple of caps, and three fets. > > Shouldn't take up too much board space or budget... > > Nope, it ain't quite that easy. It doesn't have an external sense > resistor and, consequently, the "accuracy" to which it holds the current > is really horrid. Look at the Iout versus Vin, that's just not good > enough. If you want to use a chip (or three) you need to find one with > at least and external Rsense. > > It is usually easier and less expensive to boost that voltage from the > two AA cells to 5V and add the analog current source circuits I > mentioned in my other post (one per LED section). The PIC could be > supplied directly from the AA cell if it's happy with 2.5V. > something like: http://www.national.com/ds/LM/LM2705.pdf could be used either as constant current or as high voltage supply I think you could have three npns floating on top of a shared sense resistor to do the switching between leds. -Lasse
From: Charlie E. on 1 Mar 2010 17:09 On Mon, 1 Mar 2010 12:57:49 -0800 (PST), whit3rd <whit3rd(a)gmail.com> wrote: >On Mar 1, 8:54�am, Charlie E. <edmond...(a)ieee.org> wrote: > >> �Sensor is simply an RGB LED and a >> phototransistor, buffered by some amps, and then digitized by a PIC24. > >Your digitizing scheme is not what you really need; the small current >can easily be integrated onto a capacitor, and a comparator input >with one of the nice timer sections can give a single-slope >digitization of the noisy input. Using instead the DAC section, your >noise bandwidth is quite high. > >Current-input D/A conversion is easy, and voltage conversion then >voltage DAC has no real benefit here. Hmmmm... interesting idea. So, I just feed the emitter of the phototransistor directly to a cap, and use a comparator input to determine when it has reached a reference voltage. I probably need to hit it before and after with a ground to clear the cap for the next sample... Charlie
From: Jan Panteltje on 1 Mar 2010 17:10
On a sunny day (Mon, 01 Mar 2010 08:54:53 -0800) it happened Charlie E. <edmondson(a)ieee.org> wrote in <54rno5h38v0fq45a1i4gbkqfl2qc6no2gh(a)4ax.com>: >Hi Guys, I would likely do this very differently and only use one PIC and no other stuff. How about: Red on Read level1 Red off Read level2 Subtract level1 from level2, this is the red component independent of the environment light strength and changes. Repeat for green and blue. |