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I am designing a PIR sensor alarm. It must be able to run on batteries for about a year. I am using 3.7 V Li-ion batteries to power an ATtiny24 directly. Although my PIR senor needs 5 V as recommended, it has HT7133 LDO to drop input to 3.3V so it can work as low as 3.4V. It draws 70 μA normally. I searched for low quiescent boost ICs which was a boring process and I found two.

PIR sensor: HC-sr501

  1. TPS613222A Iq = 6.5 μA

  2. MAX1724E_50 Iq = 1.5 μA

The 5 V version of the MAX1724 is currently not available to me; I may try to talk to the dealer. The TPS613222A also seems enough. My schematic is as below (focus on U3 on top left).

I tried to follow the design steps in the datasheet but I think I didn't understand it well enough.

I am looking for suggestions about the selection of inductor and capacitor.

schematic

Also, the way I searched for the components was to open a local dealer's website, looking at boost ICs, looking for their Vin, Vout, efficiency curve, and quiescent current specs. Should I consider other specs too?

There are many high-quiescent current ICs which don't have appealing output voltage or current limits or anything. I don't know why they even exist in the market; there must be something superior in them, it would be great if you could tell me.

After seeing some answers, I am thinking of removing the protection diode and power the PIR with the Li-ion battery directly. I am planning to use a very low-power comparator such as NCX2200 to switch the boost converter below 3.4 V because the PIR doesn't support lower than 3.4V, how about that?

CaveScientist
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  • Does your PIR sensor need to be powered continuously? A common approach in low-power applications is to power off circuitry when it is not needed. You could, for example, power up the sensor once or twice per second for a short time, take a reading, and then power it down. Some DC-DC converters have an enable pin, and have very low quiescent current when disabled. – user57037 Sep 24 '22 at 23:29
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    Great approach. But in the datasheet of sensor it says "Sensor module is powered up after a minute, in this initialization time intervals during this module will output 0-3 times, a minute later enters the standby state" So the sensor needs adaptation, also it needs to rest 2.5 seconds before re-enable sensing after each detection. So probably it needs continuous power. – CaveScientist Sep 24 '22 at 23:36
  • Do you know what the average power consumption of the sensor is? Is it substantially more than 0.0000065 A * 3.7 V? Is it possible that you have set a power budget that is unnecessarily low? Maybe you should add the sensor manufacturer and part number to the question (and maybe a link, too). – user57037 Sep 24 '22 at 23:45
  • I added the PIR sensor name and datasheet. The power consumption would be 0.00007*3.3 = 0.23 mW (It has a voltage regulator which drops the 5V input to 3.3V that's why I used 3.3, there is also wasted power by the regulator ofc. Btw I can consider to remove the regulator and feed it with 3.3V output of a Buck-Boost converter if it will worth it) – CaveScientist Sep 24 '22 at 23:51
  • If you remove the regulator on the PIR module (if you are sure that is allowed) then you can supply 3.3 V either from a buck or even from an LDO. It is pretty common to run 3.3 V LDOs for micro-power devices from a lithium ion battery. By the time the lithium ion battery reaches 3.4 or 3.35 V or whatever, it is basically dead anyway. The extra battery life you get from going below that doesn't amount to much, especially since buck-boosts usually have higher Iq than micropower LDOs. – user57037 Sep 25 '22 at 00:39
  • At very low currents (such as 100 uA or thereabouts), the LDO drop-out voltage may be very low. It may basically maintain Vout = Vin for a 100 uA load. – user57037 Sep 25 '22 at 00:40
  • how much power does the attiny use? – Jasen Слава Україні Sep 25 '22 at 01:43
  • If we assume that there is no need to use Lithium batteries after they drop to 3.4V then I can use nothing and directly connect lithium ion battery to the PIR sensor keeping its own regulator. The sensor will be connected 10 meters away, so the 3.4V will be still 3.4V near the sensor right? If I use thick cables length won't matter? I will mostly keep ATtiny on powerdown %99.99 of the time. It claims to be 0.1 µA at 1.8V and 25°C so mine will be little higher than that as the voltage is high. – CaveScientist Sep 25 '22 at 09:45
  • Just use a software program (timer driven) to make a "charge pump" for 5V, if you have a free digital output. – Antonio51 Sep 25 '22 at 09:48
  • Attiny consumes at least 200 uA when it is awake and I don't want to keep it awake. I want to put it to power down mode and wait for sensor to wake it up. – CaveScientist Sep 25 '22 at 09:58
  • Charge a supercapacitor that will supply PIR between power-down mode? – Antonio51 Sep 25 '22 at 10:05
  • The Power down mode can last for months, I can use batteries directly to supply PIR, my only concern is, batteries will drop to 3.4V and they will still have some capacity in them but the PIR don't support less than 3.4V, the charge pump or boost converter is needed to keep PIR on after this point. – CaveScientist Sep 25 '22 at 10:08

2 Answers2

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A 470µF cap charged to 5V powering the PIR sensor drawing 70µA will last for 6 seconds before it drops down to 4V.

So you can setup the WDT to wake up the AtTiny every 4 seconds, pulse a pin using a PWM timer to make a DIY charge pump (1 GPIO, 1 dual diode, 1 MLCC) until the cap is charged, and then go back to sleep.

You'll need low leakage silicon diodes. Schottky diodes tend to leak a little too much for this application.

While you're at it, why not replace the 555 with software?

bobflux
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  • I see, a small frequency pulse. But why don't you recommend me a low Iq boost converter? Will charge pump be so efficient? You know what, sensor actually works on 3.3V, I can feed it with >3.4V directly from battery and turn on the boost converter or charge pump after battery voltage drops below 3.4V I use 555 because I am intimidated by timers (full of registers) that's why I am trying to keep out as much as possible. I will consider replacing with software at the very end. – CaveScientist Sep 25 '22 at 21:29
  • Basically, software is cheaper than hardware, and it has no supply chain issues. Yes you could use a low Iq boost chip, but then you're pretty much married to it, what if it's no longer in stock? And it adds to the cost. – bobflux Sep 26 '22 at 09:10
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If you only need 5 volts and 70 uA from the 3.7 V lithium cell, you can use a capacitive charge pump like the LMC7660, or you can make one using a 50 kHz (or so) PWM output from the AtTiny24, two capacitors, and two diodes, plus a 5 V LDO.

This simulation shows an output of 7.6 V into 10k (760 uA) with a total current of 1.28 mA RMS from the 3.7 V cell. That's 5.78 mW out with 6.26 mW input.

Charge pump booster

schematic

simulate this circuit – Schematic created using CircuitLab

The oscillator output is 5V peak. I could not find out how to set the power supply for the inverters to 3.7 V.

PStechPaul
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  • Datasheets of charge pump don't mention about their quiescent current when they are on, the most info I can obtain is their shutdown quiescent current. Why don't they give that information, am I missing something? If I make the converter with the Attiny, I can't put it to sleep. And Attiny is not that low powered if it is always awake. Still a good advice. I would be intimidated my the idea of designing my own boost converter or charge pump. – CaveScientist Sep 25 '22 at 09:56
  • It looks like it's 120 uA typical and 200 uA maximum. You could also use a CMOS inverter as an oscillator. https://www.eevblog.com/forum/beginners/cmos-inverter-oscillator/ – PStechPaul Sep 25 '22 at 18:20
  • The sensor already has an LDO for 3.3V. There'll be a Schmitt trigger to turn on the converter first then there will be the DC converter in case battery voltage drops below 3.4V. I have 4 options: A charge pump with big caps and really low frequency created by Attiny watchdog timer (In that case can't wake ATttiny up with schimit trigger because I already use INT0 and pin change interrupt), a charge pump with a CMOS inverter (cool but complex, may cause problems, frustrating), A boost converter IC after schmitt (seems easy), A charge pump IC after schmitt (They are hard to find but I'll try) – CaveScientist Sep 25 '22 at 21:56
  • Charge pump ICs have 60uA quiescent current when oscillator is enabled. So even if they are few percent more efficient than boost converters they consume power. Am I examining datasheets wrongly? Please compare: https://www.ti.com/lit/ds/symlink/tps60130.pdf?HQS=dis-dk-null-digikeymode-dsf-pf-null-wwe&ts=1664143268713 and https://www.ti.com/lit/ds/symlink/tps61322.pdf?HQS=dis-dk-null-digikeymode-dsf-pf-null-wwe&ts=1664052590793&ref_url=https%253A%252F%252Fwww.e-komponent.com%252F – CaveScientist Sep 25 '22 at 22:10