----- Original Message ----- From: "Russell McMahon" To: Sent: Saturday, September 13, 2003 1:35 AM Subject: Re: [PIC:] Auto shut off > > Just curious, can you simplify the circuit using a regulator that has an > > on/off switch ? > > For example > > > > http://cache.national.com/ds/LM/LM9074.pdf or > > http://rocky.digikey.com/WebLib/Sharp/Web%20Data/pq1txx1m2zp.pdf > > Yes and no, depending on your requirement. > Regulators with shutdown pins all too often have disappointingly high > current drains when off. > > The LM9074 is a good example of a bad quiescent off current with 20 uA > typical and 60 uA worst case. (At 60 uA a small battery (say 20 mAH) would > go flat in ~= 300 hours). It's dropout voltage makes the horrible > LM7805/LM340 look good. It does have other features to redeem it (reset > output and stay alive while dying feature). > > BUT the Sharp PQ1Txx1M2Zp series that you list appears to be an utterly > marvellous device. It is the answer to a maiden's prayer (and those of a > number of PICListers of the past. It seems to outperform the wonderful > LM2936 AND have a shutdown pin to boot. It's maximum off state current of 1 > uA would be acceptable in many applications. The dropout voltage is 0.1v > typical and 0.15v max at 60 mA with Vo=3V. That is very good. > It has a number of other good features (including 300 mA max output or 6 > times the LM2936). All in all an excellent device and worth looking at (if > you can buy them)(Not from RS,or Farnell alas) BUT Digikey has them at > prices below LM28936.5v version at $US0.91/1 and $US0.47/100. > > This regulator would answer many PICListers nees and would be well worth > looking at for use in a power off circuit provided the 1 UA off current is > acceptable. > > > > Russell McMahon > > > > > > > > > > > Tal > > > > > -----Original Message----- > > > From: pic microcontroller discussion list > > > [mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Russell McMahon > > > Sent: Friday, September 12, 2003 8:33 PM > > > To: PICLIST@MITVMA.MIT.EDU > > > Subject: [PIC:] Auto shut off > > > > > > > > > Auto poweron/poweroff is a perennial. > > > Here's a post message I posted in April based on something > > > posted in .... This is based on a circuit I have in > > > successful use in a low volume production specialist item. I > > > could give you (Jinx) a PCB to populate to try it if you wished. > > > > > > This draws NO measurable current when off, starts on a button > > > press and subsequently allows the button to be used as an input. > > > > > > > > > Russell McMahon > > > > > > ________________________________________ > > > > > > > I'd like to > > > > 1) Turn on the device by pressing a button > > > > 2) Later be able to have the PIC turn itself off > > > > 3) While the PIC is on, have it be able to detect "ON" > > > button presses > > > > again > > > > 4) Only use one IO pin (so gnd or +5v to switch off, high-impedance > > > > input to detect button and stay on) > > > > > > Here's an attempt to meet your full specification. > > > I haven't tried it and it's not fully designed yet but I > > > think a little thought and/or playing would make a reliable > > > working circuit from it. > > > > > > There are too many requirements on the single pin to allow a > > > simple digital 0/1 full rail signal level to be used. Roman > > > Black recognises this in his SCR design and "cheats" by > > > making the turnoff function independent of the pin - in his > > > cct the PIC reduces its current consumption below the SCR > > > hold on current. This is a clever and legitimate solution but > > > will not work in cases where there is other circuitry drawing > > > current in excess of the SCR holding current. > > > > > > My solution also uses a "trick" - but using the PIC pin to do > > > it. Transistor TR2 is held on by R3+R4 BUT the voltage at the > > > junction of R3 & R4 is a LOGICAL low level which the PIC can > > > read. When the button is operated after startup the PIC pin > > > sees a logical high. > > > > > > To turn the circuit off the PIC (or other cpu) takes R3/R4 > > > junction to ground. Proper design may wish to see a small > > > series resistor to avoid possible damage from the 100uF > > > capacitor discharging into the pin. The true low level turns > > > off TR2 and initiates shutdown. Somewhere along the way the > > > PIC will lose the ability to maintain an active low output > > > and C1/R3 must have a long enough time constant to hold the > > > transistor off after this. > > > > > > The time that the voltage into the regulator (on capacitor > > > C7) stays actively high will be an important factor in turn > > > off. Suitable care and thought would definitely turn up a > > > working combination but odds are that as shown the component > > > values are wrong. > > > > > > If the regulator has a substantial output capacitor but a > > > smaller input cap, the PIC may be designed to stay alive > > > after TR3 is turned off. This would ensure the circuit is > > > well shut down before the PIC expires. > > > > > > PIC "death throes" need to be carefully thought through - as > > > is always the case in this sort of circuit. > > > > > > Z1 stops PIC pin being taken illegally high. > > > R5 provides turn off for TR2 in absence of other drive > > > sources. R3/R4 divider, which holds TR2 latched on, must > > > provide low enough output to place a logical 0 on PIC pin, > > > while R7/R4 divider provides a logical high. R7 sized to > > > provide enough level for a logical 1 - higher voltages will > > > be clamped by Z1. D3 left over from a prior circuit - does no > > > harm but Z1 probably sufficeth at 4v7. C1 value should be as > > > low as reasonably possible consistent with turn off timing > > > requirements. R4 can be made much larger (depending on > > > transistor beta, load current supported etc) - allowing > > > scaling up of other resistors allowing smaller C1 > > > > > > Russell McMahon > > > That is good news for me. I have looked at a lot of the shutdown regulators finding the shutdown current too high. What I would like to do is use a shutdown regulator in a circuit that is turned on by either a momentary push button, or, a serial port line rts or dts going into either state (+ to -, or, - to +). The processor would then control shutdown. John > > > > > > > > > > > > > > > -------------------------------------------------------------- > > > -------------- > > > ---- > > > > > > > > > > > > > > > -- > > > http://www.piclist.com#nomail Going offline? Don't AutoReply > > > us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > > > > > > > > > > > > > -- > > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > > > > -- > http://www.piclist.com hint: To leave the PICList > mailto:piclist-unsubscribe-request@mitvma.mit.edu -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu