On 07/14/2016 03:26 PM, bitrex wrote:> On 07/12/2016 03:00 PM, Jim Thompson wrote: >> On Tue, 12 Jul 2016 11:39:18 -0700, dplatt@coop.radagast.org (Dave >> Platt) wrote: >> >>> In article <Hbahz.1$Dh7.0@fx13.iad>, >>> bitrex <bitrex@de.lete.earthlink.net> wrote: >>> >>>>> Prototyping a little toy for the kiddos out of junk box parts... >>>>> >>>>> I have a IV-9 Numitron connected up on a PCB to a 74LS47 BCD to 7 >>>>> segment driver. uP is a 3.3 volt ATTiny (Adafruit "Trinket" devboard) >>>>> powered via an on-board LDO directly off a 3.7 V 150mAh lipo. 3.7 >>>>> volts >>>>> is boosted to around 5 to power the 74LS and Numitron segments via an >>>>> eBay special micro boost converter with shutdown pin. >>>>> >>>>> The test code right now simply outputs a BCD to the '47 inputs and >>>>> pulls >>>>> the shutdown pin on the boost high to turn it on. >>>>> >>>>> The problem is the uP starts up fine and all the output voltages on >>>>> the >>>>> 4 BCD pins looks good in isolation, but when the circuit is powered up >>>>> with the Numitron and '47 connected up the bench supply immediately >>>>> falls over into overcurrent protection at about 80 mA, way more >>>>> than say >>>>> two incandescent segments to form the numeral 1 should be drawing at 5 >>>>> volts, I believe. >>> >>> Are the ATTiny's pins 5-volt tolerant, when the chip is operated from >>> a 3.3-volt supply? >>> >>> Based on what I see in the 74LS74 data sheet, the flip-flop's input >>> pins are almost certainly going to be trying to pull the Tiny's >>> output pins up to around 4.3 volts. This may very well be turning on >>> some overvoltage-protection diodes in the Tiny. The 74LS74 >>> data sheet suggests that Iil is limited to around 3.2 mA per pin, >>> worst case, so 80 mA surprises me. >>> >>> ... Looking at the ATTiny85 data, it appears to me that it is *not* >>> tolerant in this way. Absolute maximum voltage on any pin is limited >>> to -0.5V, to Vcc+0.5V. So, you're overdriving it. >>> >>> You'll need a different sort of driver circuit to do this safely. >> >> Some of those old LS parts may draw significant crowbar currents as >> the supply is ramped up... choking the "eBay special". >> >> ...Jim Thompson >> > > I did some more experimenting this afternoon and I think something along > these lines is the cause of the issue. > > Had the driver and incandescent display in isolation running from the > bench supply at 3.7 volts. If I connect up the bench supply and then > turn the power supply on from the switch, the circuit latches up and the > supply goes into protection. > > If I allow the bench supply to start up, with the leads to the > protoboard unconnected, and then connect the leads, the display and > 74LS47 function normally.Basically, it seems the (admittedly rather small) converter can handle everything once everything is up and running, but it can't handle all the startup currents from the '47, Numitron, and itself at the same time without falling down. Needs soft start...
Driving 74LS from 3.3V uP
Started by ●July 12, 2016
Reply by ●July 14, 20162016-07-14
Reply by ●July 14, 20162016-07-14
On 07/13/2016 01:00 PM, John Larkin wrote:> On Wed, 13 Jul 2016 12:27:13 -0400, bitrex > <bitrex@de.lete.earthlink.net> wrote: > >> On 07/13/2016 11:57 AM, bloggs.fredbloggs.fred@gmail.com wrote: >>> On Wednesday, July 13, 2016 at 10:02:19 AM UTC-4, John Larkin wrote: >>>> On Tue, 12 Jul 2016 13:52:29 -0400, bitrex >>>> <bitrex@de.lete.earthlink.net> wrote: >>>> >>>>> Prototyping a little toy for the kiddos out of junk box parts... >>>>> >>>>> I have a IV-9 Numitron connected up on a PCB to a 74LS47 BCD to 7 >>>>> segment driver. uP is a 3.3 volt ATTiny (Adafruit "Trinket" devboard) >>>>> powered via an on-board LDO directly off a 3.7 V 150mAh lipo. 3.7 volts >>>>> is boosted to around 5 to power the 74LS and Numitron segments via an >>>>> eBay special micro boost converter with shutdown pin. >>>>> >>>>> The test code right now simply outputs a BCD to the '47 inputs and pulls >>>>> the shutdown pin on the boost high to turn it on. >>>>> >>>>> The problem is the uP starts up fine and all the output voltages on the >>>>> 4 BCD pins looks good in isolation, but when the circuit is powered up >>>>> with the Numitron and '47 connected up the bench supply immediately >>>>> falls over into overcurrent protection at about 80 mA, way more than say >>>>> two incandescent segments to form the numeral 1 should be drawing at 5 >>>>> volts, I believe. >>>>> >>>>> The boost has plenty enough oomph to power all the Numitron digits from >>>>> either the LiPo or the bench supply, I tested that in isolation. >>>>> >>>>> Tested two different '47s with the same result. Not sure exactly what's >>>>> going on here...could there be some ESD/clamp diode pathway that's being >>>>> driven that I don't know about? Or a startup sequencing issue? >>>> >>>> You are facing that eternal cosmic question: where the hell is the >>>> current going? >>> >>> 2x tubes at 14 segments with 20mA/segment gets you 14 x 5 x 20 = 1.4W which at 85% or less efficiency (Chinese special) = 1.65W or more from the LiPo or 1.65/3.5= 0.5A *average* with possible 1A peaks for the boost. Ckt is not the only thing hopelessly underpowered around here. >> >> It's only one tube. I tested the tube/converter circuit in isolation and >> it appears to light all the digit segment combinations of one tube OK > > All segments from a cold start? > >I realized that seems to be the problem. The admittedly rather wimpy boost and battery pack can handle everything if everything is sequenced slowly, but it can't handle all the sudden startup currents of the display, Numitron, and itself without falling over.
Reply by ●July 14, 20162016-07-14
On 07/14/2016 03:35 PM, bitrex wrote:> On 07/13/2016 01:00 PM, John Larkin wrote: >> On Wed, 13 Jul 2016 12:27:13 -0400, bitrex >> <bitrex@de.lete.earthlink.net> wrote: >> >>> On 07/13/2016 11:57 AM, bloggs.fredbloggs.fred@gmail.com wrote: >>>> On Wednesday, July 13, 2016 at 10:02:19 AM UTC-4, John Larkin wrote: >>>>> On Tue, 12 Jul 2016 13:52:29 -0400, bitrex >>>>> <bitrex@de.lete.earthlink.net> wrote: >>>>> >>>>>> Prototyping a little toy for the kiddos out of junk box parts... >>>>>> >>>>>> I have a IV-9 Numitron connected up on a PCB to a 74LS47 BCD to 7 >>>>>> segment driver. uP is a 3.3 volt ATTiny (Adafruit "Trinket" devboard) >>>>>> powered via an on-board LDO directly off a 3.7 V 150mAh lipo. 3.7 >>>>>> volts >>>>>> is boosted to around 5 to power the 74LS and Numitron segments via an >>>>>> eBay special micro boost converter with shutdown pin. >>>>>> >>>>>> The test code right now simply outputs a BCD to the '47 inputs and >>>>>> pulls >>>>>> the shutdown pin on the boost high to turn it on. >>>>>> >>>>>> The problem is the uP starts up fine and all the output voltages >>>>>> on the >>>>>> 4 BCD pins looks good in isolation, but when the circuit is >>>>>> powered up >>>>>> with the Numitron and '47 connected up the bench supply immediately >>>>>> falls over into overcurrent protection at about 80 mA, way more >>>>>> than say >>>>>> two incandescent segments to form the numeral 1 should be drawing >>>>>> at 5 >>>>>> volts, I believe. >>>>>> >>>>>> The boost has plenty enough oomph to power all the Numitron digits >>>>>> from >>>>>> either the LiPo or the bench supply, I tested that in isolation. >>>>>> >>>>>> Tested two different '47s with the same result. Not sure exactly >>>>>> what's >>>>>> going on here...could there be some ESD/clamp diode pathway that's >>>>>> being >>>>>> driven that I don't know about? Or a startup sequencing issue? >>>>> >>>>> You are facing that eternal cosmic question: where the hell is the >>>>> current going? >>>> >>>> 2x tubes at 14 segments with 20mA/segment gets you 14 x 5 x 20 = >>>> 1.4W which at 85% or less efficiency (Chinese special) = 1.65W or >>>> more from the LiPo or 1.65/3.5= 0.5A *average* with possible 1A >>>> peaks for the boost. Ckt is not the only thing hopelessly >>>> underpowered around here. >>> >>> It's only one tube. I tested the tube/converter circuit in isolation and >>> it appears to light all the digit segment combinations of one tube OK >> >> All segments from a cold start? >> >> > > I realized that seems to be the problem. The admittedly rather wimpy > boost and battery pack can handle everything if everything is sequenced > slowly, but it can't handle all the sudden startup currents of the > display, Numitron, and itself without falling over.I brought the boost voltage down from 5 volts to around 4.5. The LS still seems to work fine, and the segments are still plenty bright, but the current consumption is significantly less.
Reply by ●July 14, 20162016-07-14
On 07/14/2016 03:35 PM, bitrex wrote:> On 07/13/2016 01:00 PM, John Larkin wrote: >> On Wed, 13 Jul 2016 12:27:13 -0400, bitrex >> <bitrex@de.lete.earthlink.net> wrote: >> >>> On 07/13/2016 11:57 AM, bloggs.fredbloggs.fred@gmail.com wrote: >>>> On Wednesday, July 13, 2016 at 10:02:19 AM UTC-4, John Larkin wrote: >>>>> On Tue, 12 Jul 2016 13:52:29 -0400, bitrex >>>>> <bitrex@de.lete.earthlink.net> wrote: >>>>> >>>>>> Prototyping a little toy for the kiddos out of junk box parts... >>>>>> >>>>>> I have a IV-9 Numitron connected up on a PCB to a 74LS47 BCD to 7 >>>>>> segment driver. uP is a 3.3 volt ATTiny (Adafruit "Trinket" devboard) >>>>>> powered via an on-board LDO directly off a 3.7 V 150mAh lipo. 3.7 >>>>>> volts >>>>>> is boosted to around 5 to power the 74LS and Numitron segments via an >>>>>> eBay special micro boost converter with shutdown pin. >>>>>> >>>>>> The test code right now simply outputs a BCD to the '47 inputs and >>>>>> pulls >>>>>> the shutdown pin on the boost high to turn it on. >>>>>> >>>>>> The problem is the uP starts up fine and all the output voltages >>>>>> on the >>>>>> 4 BCD pins looks good in isolation, but when the circuit is >>>>>> powered up >>>>>> with the Numitron and '47 connected up the bench supply immediately >>>>>> falls over into overcurrent protection at about 80 mA, way more >>>>>> than say >>>>>> two incandescent segments to form the numeral 1 should be drawing >>>>>> at 5 >>>>>> volts, I believe. >>>>>> >>>>>> The boost has plenty enough oomph to power all the Numitron digits >>>>>> from >>>>>> either the LiPo or the bench supply, I tested that in isolation. >>>>>> >>>>>> Tested two different '47s with the same result. Not sure exactly >>>>>> what's >>>>>> going on here...could there be some ESD/clamp diode pathway that's >>>>>> being >>>>>> driven that I don't know about? Or a startup sequencing issue? >>>>> >>>>> You are facing that eternal cosmic question: where the hell is the >>>>> current going? >>>> >>>> 2x tubes at 14 segments with 20mA/segment gets you 14 x 5 x 20 = >>>> 1.4W which at 85% or less efficiency (Chinese special) = 1.65W or >>>> more from the LiPo or 1.65/3.5= 0.5A *average* with possible 1A >>>> peaks for the boost. Ckt is not the only thing hopelessly >>>> underpowered around here. >>> >>> It's only one tube. I tested the tube/converter circuit in isolation and >>> it appears to light all the digit segment combinations of one tube OK >> >> All segments from a cold start? >> >> > > I realized that seems to be the problem. The admittedly rather wimpy > boost and battery pack can handle everything if everything is sequenced > slowly, but it can't handle all the sudden startup currents of the > display, Numitron, and itself without falling over.Numitron and 74LS IC I should say
Reply by ●July 14, 20162016-07-14
On Thu, 14 Jul 2016 15:35:15 -0400, bitrex <bitrex@de.lete.earthlink.net> wrote:>On 07/13/2016 01:00 PM, John Larkin wrote: >> On Wed, 13 Jul 2016 12:27:13 -0400, bitrex >> <bitrex@de.lete.earthlink.net> wrote: >> >>> On 07/13/2016 11:57 AM, bloggs.fredbloggs.fred@gmail.com wrote: >>>> On Wednesday, July 13, 2016 at 10:02:19 AM UTC-4, John Larkin wrote: >>>>> On Tue, 12 Jul 2016 13:52:29 -0400, bitrex >>>>> <bitrex@de.lete.earthlink.net> wrote: >>>>> >>>>>> Prototyping a little toy for the kiddos out of junk box parts... >>>>>> >>>>>> I have a IV-9 Numitron connected up on a PCB to a 74LS47 BCD to 7 >>>>>> segment driver. uP is a 3.3 volt ATTiny (Adafruit "Trinket" devboard) >>>>>> powered via an on-board LDO directly off a 3.7 V 150mAh lipo. 3.7 volts >>>>>> is boosted to around 5 to power the 74LS and Numitron segments via an >>>>>> eBay special micro boost converter with shutdown pin. >>>>>> >>>>>> The test code right now simply outputs a BCD to the '47 inputs and pulls >>>>>> the shutdown pin on the boost high to turn it on. >>>>>> >>>>>> The problem is the uP starts up fine and all the output voltages on the >>>>>> 4 BCD pins looks good in isolation, but when the circuit is powered up >>>>>> with the Numitron and '47 connected up the bench supply immediately >>>>>> falls over into overcurrent protection at about 80 mA, way more than say >>>>>> two incandescent segments to form the numeral 1 should be drawing at 5 >>>>>> volts, I believe. >>>>>> >>>>>> The boost has plenty enough oomph to power all the Numitron digits from >>>>>> either the LiPo or the bench supply, I tested that in isolation. >>>>>> >>>>>> Tested two different '47s with the same result. Not sure exactly what's >>>>>> going on here...could there be some ESD/clamp diode pathway that's being >>>>>> driven that I don't know about? Or a startup sequencing issue? >>>>> >>>>> You are facing that eternal cosmic question: where the hell is the >>>>> current going? >>>> >>>> 2x tubes at 14 segments with 20mA/segment gets you 14 x 5 x 20 = 1.4W which at 85% or less efficiency (Chinese special) = 1.65W or more from the LiPo or 1.65/3.5= 0.5A *average* with possible 1A peaks for the boost. Ckt is not the only thing hopelessly underpowered around here. >>> >>> It's only one tube. I tested the tube/converter circuit in isolation and >>> it appears to light all the digit segment combinations of one tube OK >> >> All segments from a cold start? >> >> > >I realized that seems to be the problem. The admittedly rather wimpy >boost and battery pack can handle everything if everything is sequenced >slowly, but it can't handle all the sudden startup currents of the >display, Numitron, and itself without falling over.Seven cold filaments could pull a lot of current. You could do software tricks to soften the turn-ons, but the display might look silly. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●July 14, 20162016-07-14
On 07/14/2016 04:27 PM, John Larkin wrote:> On Thu, 14 Jul 2016 15:35:15 -0400, bitrex > <bitrex@de.lete.earthlink.net> wrote: > >> On 07/13/2016 01:00 PM, John Larkin wrote: >>> On Wed, 13 Jul 2016 12:27:13 -0400, bitrex >>> <bitrex@de.lete.earthlink.net> wrote: >>> >>>> On 07/13/2016 11:57 AM, bloggs.fredbloggs.fred@gmail.com wrote: >>>>> On Wednesday, July 13, 2016 at 10:02:19 AM UTC-4, John Larkin wrote: >>>>>> On Tue, 12 Jul 2016 13:52:29 -0400, bitrex >>>>>> <bitrex@de.lete.earthlink.net> wrote: >>>>>> >>>>>>> Prototyping a little toy for the kiddos out of junk box parts... >>>>>>> >>>>>>> I have a IV-9 Numitron connected up on a PCB to a 74LS47 BCD to 7 >>>>>>> segment driver. uP is a 3.3 volt ATTiny (Adafruit "Trinket" devboard) >>>>>>> powered via an on-board LDO directly off a 3.7 V 150mAh lipo. 3.7 volts >>>>>>> is boosted to around 5 to power the 74LS and Numitron segments via an >>>>>>> eBay special micro boost converter with shutdown pin. >>>>>>> >>>>>>> The test code right now simply outputs a BCD to the '47 inputs and pulls >>>>>>> the shutdown pin on the boost high to turn it on. >>>>>>> >>>>>>> The problem is the uP starts up fine and all the output voltages on the >>>>>>> 4 BCD pins looks good in isolation, but when the circuit is powered up >>>>>>> with the Numitron and '47 connected up the bench supply immediately >>>>>>> falls over into overcurrent protection at about 80 mA, way more than say >>>>>>> two incandescent segments to form the numeral 1 should be drawing at 5 >>>>>>> volts, I believe. >>>>>>> >>>>>>> The boost has plenty enough oomph to power all the Numitron digits from >>>>>>> either the LiPo or the bench supply, I tested that in isolation. >>>>>>> >>>>>>> Tested two different '47s with the same result. Not sure exactly what's >>>>>>> going on here...could there be some ESD/clamp diode pathway that's being >>>>>>> driven that I don't know about? Or a startup sequencing issue? >>>>>> >>>>>> You are facing that eternal cosmic question: where the hell is the >>>>>> current going? >>>>> >>>>> 2x tubes at 14 segments with 20mA/segment gets you 14 x 5 x 20 = 1.4W which at 85% or less efficiency (Chinese special) = 1.65W or more from the LiPo or 1.65/3.5= 0.5A *average* with possible 1A peaks for the boost. Ckt is not the only thing hopelessly underpowered around here. >>>> >>>> It's only one tube. I tested the tube/converter circuit in isolation and >>>> it appears to light all the digit segment combinations of one tube OK >>> >>> All segments from a cold start? >>> >>> >> >> I realized that seems to be the problem. The admittedly rather wimpy >> boost and battery pack can handle everything if everything is sequenced >> slowly, but it can't handle all the sudden startup currents of the >> display, Numitron, and itself without falling over. > > Seven cold filaments could pull a lot of current. You could do > software tricks to soften the turn-ons, but the display might look > silly. > >I'm going to give preheating the filaments via PWMing the "shutdown" pin of the boost module a try, though I'll have to check the datasheet to see what the delay from a high level applied to that pin to output power good is. If that's not plausible I can try PWMing the "ripple blanking" input to the '47.
Reply by ●July 15, 20162016-07-15
On Thursday, 14 July 2016 21:27:16 UTC+1, John Larkin wrote:> On Thu, 14 Jul 2016 15:35:15 -0400, bitrex > <bitrex@de.lete.earthlink.net> wrote: > >On 07/13/2016 01:00 PM, John Larkin wrote: > >> On Wed, 13 Jul 2016 12:27:13 -0400, bitrex > >> <bitrex@de.lete.earthlink.net> wrote: > >>> On 07/13/2016 11:57 AM, bloggs.fredbloggs.fred@gmail.com wrote: > >>>> On Wednesday, July 13, 2016 at 10:02:19 AM UTC-4, John Larkin wrote: > >>>>> On Tue, 12 Jul 2016 13:52:29 -0400, bitrex > >>>>> <bitrex@de.lete.earthlink.net> wrote: > >>>>> > >>>>>> Prototyping a little toy for the kiddos out of junk box parts... > >>>>>> > >>>>>> I have a IV-9 Numitron connected up on a PCB to a 74LS47 BCD to 7 > >>>>>> segment driver. uP is a 3.3 volt ATTiny (Adafruit "Trinket" devboard) > >>>>>> powered via an on-board LDO directly off a 3.7 V 150mAh lipo. 3.7 volts > >>>>>> is boosted to around 5 to power the 74LS and Numitron segments via an > >>>>>> eBay special micro boost converter with shutdown pin. > >>>>>> > >>>>>> The test code right now simply outputs a BCD to the '47 inputs and pulls > >>>>>> the shutdown pin on the boost high to turn it on. > >>>>>> > >>>>>> The problem is the uP starts up fine and all the output voltages on the > >>>>>> 4 BCD pins looks good in isolation, but when the circuit is powered up > >>>>>> with the Numitron and '47 connected up the bench supply immediately > >>>>>> falls over into overcurrent protection at about 80 mA, way more than say > >>>>>> two incandescent segments to form the numeral 1 should be drawing at 5 > >>>>>> volts, I believe. > >>>>>> > >>>>>> The boost has plenty enough oomph to power all the Numitron digits from > >>>>>> either the LiPo or the bench supply, I tested that in isolation. > >>>>>> > >>>>>> Tested two different '47s with the same result. Not sure exactly what's > >>>>>> going on here...could there be some ESD/clamp diode pathway that's being > >>>>>> driven that I don't know about? Or a startup sequencing issue? > >>>>> > >>>>> You are facing that eternal cosmic question: where the hell is the > >>>>> current going? > >>>> > >>>> 2x tubes at 14 segments with 20mA/segment gets you 14 x 5 x 20 = 1.4W which at 85% or less efficiency (Chinese special) = 1.65W or more from the LiPo or 1.65/3.5= 0.5A *average* with possible 1A peaks for the boost. Ckt is not the only thing hopelessly underpowered around here. > >>> > >>> It's only one tube. I tested the tube/converter circuit in isolation and > >>> it appears to light all the digit segment combinations of one tube OK > >> > >> All segments from a cold start? > >> > >> > > > >I realized that seems to be the problem. The admittedly rather wimpy > >boost and battery pack can handle everything if everything is sequenced > >slowly, but it can't handle all the sudden startup currents of the > >display, Numitron, and itself without falling over. > > Seven cold filaments could pull a lot of current. You could do > software tricks to soften the turn-ons, but the display might look > silly.It could only look funny for a fraction of a second. You could prewarm the filaments by turning each on for its own time slice. Then turn on 2 at a time etc. It can all be done so quick the eye barely notices. NT
Reply by ●July 15, 20162016-07-15
On 14/07/2016 21:51, bitrex wrote:> On 07/14/2016 04:27 PM, John Larkin wrote: >> >> Seven cold filaments could pull a lot of current. You could do >> software tricks to soften the turn-ons, but the display might look >> silly. >> >> > > I'm going to give preheating the filaments via PWMing the "shutdown" pin > of the boost module a try, though I'll have to check the datasheet to > see what the delay from a high level applied to that pin to output power > good is. > > If that's not plausible I can try PWMing the "ripple blanking" input to > the '47.Could be easier just to add resistors across LS47 outputs to preheat, depending on ease of changing hardware vs software I suppose. piglet
Reply by ●July 15, 20162016-07-15
On 07/15/2016 05:54 AM, piglet wrote:> On 14/07/2016 21:51, bitrex wrote: >> On 07/14/2016 04:27 PM, John Larkin wrote: >>> >>> Seven cold filaments could pull a lot of current. You could do >>> software tricks to soften the turn-ons, but the display might look >>> silly. >>> >>> >> >> I'm going to give preheating the filaments via PWMing the "shutdown" pin >> of the boost module a try, though I'll have to check the datasheet to >> see what the delay from a high level applied to that pin to output power >> good is. >> >> If that's not plausible I can try PWMing the "ripple blanking" input to >> the '47. > > Could be easier just to add resistors across LS47 outputs to preheat, > depending on ease of changing hardware vs software I suppose. > > piglet >I'm hoping to get a few PCBs of the circuit I'm working on made if it turns out alright, so I'd like to keep everything as compact as possible and do it in software if I can, rather than add a resistor network to the board. In the final design I'd like to ditch the LS chip entirely and simply use a uP of the same architecture with enough pins to handle all the segments directly. Many AVR chips are rated to sink 200mA to ground total through all pins, which surprised me a bit...though this circuit would push that limit and perhaps make me feel a little uncomfortable. I'd also like to dynamically adjust the voltage to the segments depending on how many are lit to increase battery life via PWM, but that may not be possible if the boost's enable pin can't handle being PWMed, so I may have to use a driver of some type. The cost/space advantage of using one 20 pin SOIC uC vs an 8 pin SOIC uC and a 16 pin SOIC is not gigantic.
Reply by ●July 15, 20162016-07-15
>I'd also like to dynamically adjust the voltage to the segments >depending on how many are lit to increase battery life via PWM, but that >may not be possible if the boost's enable pin can't handle being PWMed, >so I may have to use a driver of some type.You can use tristate outputs to adjust the boost's voltage by switching feedback resistors in and out. I've sometimes used PLDs as analogue elements to switch between photodiode segments. Cheers Phil Hobbs
