On 06/16/2016 04:31 AM, Peter Heitzer wrote:> bitrex <bitrex@de.lete.earthlink.net> wrote: >> Is there a CMOS-type substitute for the 74LS47 with "open collector" >> type outputs suitable for sinking around 20mA? > Although kind of overkill. Perhaps a small µC. The AVRs for example > can sink about 20 mA/pin with a total of 200 mA for VCC and GND. > Of course the outputs are not open collector. If you need OC and more > than one digit to drive a combination of µC and one OC driver can be > the solution with the least components. The µC could also do PWM and > multiplexing. >The other issue with using the uC pins to drive the display directly is that there aren't enough hardware PWM channels to dim every segment common anode. If the display I had to use was common cathode I could simply implement dimming by using an external pass transistor on the cathode and PWMing that to ground and driving the segments with a "high" logic voltage, but unfortunately I don't have that option here, so I think I'd have to implement some scheme using a filter and regulator on the supply and manipulating its reference, but it would likely have to be an LDO, which would probably cost around the same as an appropriate driver IC in a SOIC-16.
74LS47 substitution
Started by ●June 15, 2016
Reply by ●June 16, 20162016-06-16
Reply by ●June 16, 20162016-06-16
On 06/16/2016 06:29 AM, bitrex wrote:> On 06/16/2016 04:31 AM, Peter Heitzer wrote: >> bitrex <bitrex@de.lete.earthlink.net> wrote: >>> Is there a CMOS-type substitute for the 74LS47 with "open collector" >>> type outputs suitable for sinking around 20mA? >> Although kind of overkill. Perhaps a small µC. The AVRs for example >> can sink about 20 mA/pin with a total of 200 mA for VCC and GND. >> Of course the outputs are not open collector. If you need OC and more >> than one digit to drive a combination of µC and one OC driver can be >> the solution with the least components. The µC could also do PWM and >> multiplexing. >> > > The other issue with using the uC pins to drive the display directly is > that there aren't enough hardware PWM channels to dim every segment > common anode. If the display I had to use was common cathode I could > simply implement dimming by using an external pass transistor on the > cathode and PWMing that to ground and driving the segments with a "high" > logic voltage, but unfortunately I don't have that option here, so I > think I'd have to implement some scheme using a filter and regulator on > the supply and manipulating its reference, but it would likely have to > be an LDO, which would probably cost around the same as an appropriate > driver IC in a SOIC-16.Actually, if I decided to go that route I could probably just chop the supply with PWM and a two transistor switch like so: http://www.avrfreaks.net/sites/default/files/Common_Anode.jpg
Reply by ●June 16, 20162016-06-16
On 06/15/2016 05:39 PM, bitrex wrote:> On 06/15/2016 04:39 PM, John Larkin wrote: >> On Wed, 15 Jun 2016 16:02:30 -0400 (EDT), bitrex >> <bitrex@de.lete.earthlink.net> wrote: >> >>> John Larkin <jjlarkin@highlandtechnology.com> Wrote in message: >>>> On Wed, 15 Jun 2016 13:34:50 -0400, bitrex >>>> <bitrex@de.lete.earthlink.net> wrote: >>>> >>>>> Is there a CMOS-type substitute for the 74LS47 with "open collector" >>>>> type outputs suitable for sinking around 20mA? >>>>> >>>>> That uses less quiescent power than the 47's ~10mA. Supply voltage >>>>> either 5 or 3.3 volts, depending. It doesn't really matter if the chip >>>>> itself does a conversion from BCD to the appropriate outputs, as that >>>>> mapping can be done in software, but the chip would need to have at >>>>> least a 3 bit address space. >>>>> >>>>> The '47 has an input that's suitable for dynamic dimming of the >>>>> loads by >>>>> applying PWM, it would be cool if the replacement had something >>>>> like that. >>>>> >>>>> I think JL posted something along these lines in response to anotherh >>>>> question of mine, but I can't find the thread. >>>> >>>> A TPIC6595 might work. The data sheet says it needs 5 volts, but there >>>> is an appnote that says it's OK at 3.3. We use them at 3.3. >>>> >>>> The input is SPI serial, and the /G pin can do dimming. >>> >>> The CD74AC238 appears to be rated for 50 mA through a pin, 200 mA >>> continuous to ground, at 1.5 to 5.5 volts supply, so that might >>> also be a possibility. >>> >>> The edge case would be the "8"... >> >> Do you need open drain? HC or AC logic will sink 20 mA, probably >> better with Vcc=5. >> >> The 6595 is a beast, designed for a lot of current. > > I don't think so. The 74LS47 outputs don't pull down much lower than 0.3 > volts. > > While it looks like most HC logic can sink 20 mA, the problem is it > seems most are rated for only 50mA continuous through GND, and the > SOIC-16 packages are rated for only 500mW continuous dissipation. > > The CD74AC238 can sink 200mA to ground, and is rated for 1.1 watts > dissipation at 30 degrees, so even if I have to derate it a bit for > 140mA passing through the device when all segments are lit, it seems it > should be okay.Oops, I made a mistake in my power dissipation calculation, what a dummy. It's only the voltage drop across the pass device that matters, so even a 500mW dissipation should be good.
Reply by ●June 16, 20162016-06-16
On 06/15/2016 01:34 PM, bitrex wrote:> Is there a CMOS-type substitute for the 74LS47 with "open collector" > type outputs suitable for sinking around 20mA? > > That uses less quiescent power than the 47's ~10mA. Supply voltage > either 5 or 3.3 volts, depending. It doesn't really matter if the chip > itself does a conversion from BCD to the appropriate outputs, as that > mapping can be done in software, but the chip would need to have at > least a 3 bit address space. > > The '47 has an input that's suitable for dynamic dimming of the loads by > applying PWM, it would be cool if the replacement had something like that. > > I think JL posted something along these lines in response to another > question of mine, but I can't find the thread.It looks like the best "bang for the buck" right now is to simply use an AVR with a "Tiny" core that has 12 IO pins instead of 6, and then PWM the supply with a two transistor switch. I didn't realize that AVRs could sink as much current to/from the supplies as they appear to be able to. The ATTiny84 has two PWM channels and is just over a dollar at Mouser, a PNP and NPN for the switch is basically free.
Reply by ●June 16, 20162016-06-16
bitrex <bitrex@de.lete.earthlink.net> wrote:>On 06/16/2016 04:31 AM, Peter Heitzer wrote: >> bitrex <bitrex@de.lete.earthlink.net> wrote: >>> Is there a CMOS-type substitute for the 74LS47 with "open collector" >>> type outputs suitable for sinking around 20mA? >> Although kind of overkill. Perhaps a small µC. The AVRs for example >> can sink about 20 mA/pin with a total of 200 mA for VCC and GND. >> Of course the outputs are not open collector. If you need OC and more >> than one digit to drive a combination of µC and one OC driver can be >> the solution with the least components. The µC could also do PWM and >> multiplexing. >>>Yeah, there's going to be a uC involved one way or the other...I'm >trying to figure if it would be better to just use an AVR with more pins >vs an 8 pin AVR and an external switch IC.>An ATTiny85 that costs under a dollar in quantity plus an external >switch that costs 25 cents in quantity is a better value than something >like a Mega168 that costs $1.65 in quantity, as the board size would be >pretty much fixed by larger components either way.>There are other weird AVR devices that seem to use a "Tiny" core but >have more pins, but I'm taking advantage of the Arduino toolchain and >AFAIK they're not supported, and I don't really want to take the time to >write an implementation.The only supported "Tiny" device is the Tiny85, but the Tiny2313 is not very different to the Tiny85. http://playground.arduino.cc/Main/ArduinoOnOtherAtmelChips mentions the 2313. So it should be possible to use the 2313 with the toolchain. The 2313 costs about the same as the 85 but gives you 18 usable pins vs 6 pins on the Tiny85. -- Dipl.-Inform(FH) Peter Heitzer, peter.heitzer@rz.uni-regensburg.de
Reply by ●June 16, 20162016-06-16
On 06/16/2016 06:51 AM, Peter Heitzer wrote:> bitrex <bitrex@de.lete.earthlink.net> wrote: >> On 06/16/2016 04:31 AM, Peter Heitzer wrote: >>> bitrex <bitrex@de.lete.earthlink.net> wrote: >>>> Is there a CMOS-type substitute for the 74LS47 with "open collector" >>>> type outputs suitable for sinking around 20mA? >>> Although kind of overkill. Perhaps a small µC. The AVRs for example >>> can sink about 20 mA/pin with a total of 200 mA for VCC and GND. >>> Of course the outputs are not open collector. If you need OC and more >>> than one digit to drive a combination of µC and one OC driver can be >>> the solution with the least components. The µC could also do PWM and >>> multiplexing. >>> > >> Yeah, there's going to be a uC involved one way or the other...I'm >> trying to figure if it would be better to just use an AVR with more pins >> vs an 8 pin AVR and an external switch IC. > >> An ATTiny85 that costs under a dollar in quantity plus an external >> switch that costs 25 cents in quantity is a better value than something >> like a Mega168 that costs $1.65 in quantity, as the board size would be >> pretty much fixed by larger components either way. > >> There are other weird AVR devices that seem to use a "Tiny" core but >> have more pins, but I'm taking advantage of the Arduino toolchain and >> AFAIK they're not supported, and I don't really want to take the time to >> write an implementation. > The only supported "Tiny" device is the Tiny85, but the Tiny2313 is not > very different to the Tiny85. > http://playground.arduino.cc/Main/ArduinoOnOtherAtmelChips > mentions the 2313. So it should be possible to use the 2313 with the > toolchain. The 2313 costs about the same as the 85 but gives you 18 > usable pins vs 6 pins on the Tiny85. >Looks like setting it up for the Tiny84 is straightforward these days: http://highlowtech.org/?p=1695 The Tiny84 only has 12 pins, but even that is more than I need. I'd prefer the larger code space, 8k vs 2k for the 2313. They're both about the same price in quantity at Mouser.
Reply by ●June 16, 20162016-06-16
bitrex <bitrex@de.lete.earthlink.net> wrote:>Looks like setting it up for the Tiny84 is straightforward these days:>http://highlowtech.org/?p=1695>The Tiny84 only has 12 pins, but even that is more than I need. I'd >prefer the larger code space, 8k vs 2k for the 2313. They're both about >the same price in quantity at Mouser.That makes sense. If you program the uC in C 2 KiB can be used up easyly if you happen to use "printf" or its friends. You also could use linear code more easyly if you need speed for some tasks. -- Dipl.-Inform(FH) Peter Heitzer, peter.heitzer@rz.uni-regensburg.de
Reply by ●June 16, 20162016-06-16
On 6/15/2016 1:34 PM, bitrex wrote:> Is there a CMOS-type substitute for the 74LS47 with "open collector" > type outputs suitable for sinking around 20mA? > > That uses less quiescent power than the 47's ~10mA. Supply voltage > either 5 or 3.3 volts, depending. It doesn't really matter if the chip > itself does a conversion from BCD to the appropriate outputs, as that > mapping can be done in software, but the chip would need to have at > least a 3 bit address space. > > The '47 has an input that's suitable for dynamic dimming of the loads by > applying PWM, it would be cool if the replacement had something like that. > > I think JL posted something along these lines in response to another > question of mine, but I can't find the thread.There are about a million I2C/SPI LED drivers out there that have built in PWM dimming. Have you looked at any? -- Rick C
Reply by ●June 16, 20162016-06-16
On 06/16/2016 07:11 AM, Peter Heitzer wrote:> bitrex <bitrex@de.lete.earthlink.net> wrote: > >> Looks like setting it up for the Tiny84 is straightforward these days: > >> http://highlowtech.org/?p=1695 > >> The Tiny84 only has 12 pins, but even that is more than I need. I'd >> prefer the larger code space, 8k vs 2k for the 2313. They're both about >> the same price in quantity at Mouser. > That makes sense. If you program the uC in C 2 KiB can be used up > easyly if you happen to use "printf" or its friends. You also could > use linear code more easyly if you need speed for some tasks. >I have a slight pathology in that I like using C++, even for "small" projects. avr-g++ compiles pretty efficient code, so long as one avoids using a ton of "virtual" methods, and initializes all memory required for the system at startup and then doesn't use the "new" operator to do anything but place or copy objects onto pre-allocated memory. I think the moderate increase in code size is worth the advantages gained in portability/maintainability.
Reply by ●June 16, 20162016-06-16
rickman <gnuarm@gmail.com> wrote:>On 6/15/2016 1:34 PM, bitrex wrote: >> Is there a CMOS-type substitute for the 74LS47 with "open collector" >> type outputs suitable for sinking around 20mA?>There are about a million I2C/SPI LED drivers out there that have built >in PWM dimming. Have you looked at any?I think the OP is looking for a part costing less than a dollar. The drivers I found are rather costly. -- Dipl.-Inform(FH) Peter Heitzer, peter.heitzer@rz.uni-regensburg.de
