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Context

I have recently set out to add some wireless features to my entry-level smoke detector.

Problem

In order to achieve that, I will be adding a custom pcb embedding an ATmega328P and a nRF24L01, both running at 3.3V. However, the detector is currently powered by a 9V battery, which is not ideal to work with.

Additionally, I would like to avoid making any modifications to the actual hardware, and for ease of maintenance, I do not wish to add a second power source.

Question

I am trying to find with the most efficient 9V to 3.3V solution in the case of a wireless sensor node that will be spending most of its time sleeping.

A similar question was asked at Shut down regulator during sleep, but the 9V battery restricts my choice of regulators.

Thoughts and reasoning

Because of their poor efficiency, linear regulators are of course ruled out. Which leaves us with switching regulators.

The system will be asleep most of the time, consuming then only a few micro amps. This is apparently something traditional switching regulators are not good at. Having such a light load decreases their efficiency from 90% to 50% sometimes.

As a solution, some of them now have a feature called "discontinuous mode" where the switching frequency varies proportionally to the load current. However, how efficient is this feature really when dealing with loads < 50 µA ?

Charge pumps seem to be a solution to the poor efficiency of inductor-based dc/dc converters, but I couldn't find any accepting an input of 9V. I however like the idea of using capacitors instead of inductors to store the power. That's the road this guy has taken: https://hallard.me/ulpnode-low-power-secret/. His solution looks like a good starting point.

I am ok with the 3.3V line dropping regularly as low as 2V, as this value is still in the operating conditions of the components I am choosing.

I could use the output from the low voltage detector IC as an interrupt source to periodically wake up the ATmega from deep sleep instead of the watchdog. And by disabling the watchdog, I am saving a little bit more power.

Some switching converters do have a feature called true disconnect, in which the input gets physically disconnected from the output path. Is this really saving power by the way?

Proposed solution

schematic

simulate this circuit – Schematic created using CircuitLab

  • U1 is a dc/dc converter with true disconnect and a 2 to 10V input range.

  • R1 pulls up the !SHDN line to make sure U1 is enabled at power-up.

  • C1, whose value is still to be determined, acts as a reserve capacitor, providing power when U1 is shutdown.

  • U2 is an undervoltage supervisor IC with an open-drain output

  • R4 pulls !ENABLE pin-high to deactivate U2 at startup.

  • R2 and R3 form a voltage divider used to set a threshold of 2V for U2.

  • R5 and R6 form a voltage divider used to bring the voltage of the !SHDN line down to approx 2V to meet the requirements for the ATmega input pins maximum voltage.

  • PD3 changing will trigger an interrupt, used to wake-up the ATmega from time to time, when C1 discharges.

  • PD4 ouput can be set to LOW to enable U2.

At power-up, U1 will start slowly, taking a couple of ms to feed Vcc with 3.3V. U2 will stay disabled. The ATmega will then start and proceed to setup.

When desiring to enter into sleep mode, the ATmega will power down the nRF24 tranceiver, enable it's interrupts, shutdown some of its features (like BOD, WDT, OSC, ...), pull-down PD4 to activate U2, and finally start sleeping.

With Vcc close to 3.3V, U2 will quickly shut down U1, and the system will start living off C1 slowly discharge. Eventually, Vcc voltage will drop below 2V causing U2 to pull high !SHDN for approx 190 ms. U1 will then charge C1 and bring Vcc back to 3.3V. The interrupt on PD3 will cause the ATmega to wakeup and provide it with an opportunity to send a wireless message, or not.

So,

Is anything wrong with my reasoning? Are there any issues with the proposed circuit design? Are you aware of different (and perhaps simpler) ways of achieving this?

Thanks a lot, any input on any of the above will be greatly appreciated.

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  • \$\begingroup\$ Don't forget to design your system so that if the receiver doesn't receive a transmission within 20 s that a fire alarm is generated. A minute may be way too long if a fire is kindling. \$\endgroup\$ – Transistor Apr 22 '16 at 19:40
  • \$\begingroup\$ @transistor Yes, the complete design makes use of the I/O pin present on the smoke detector IC (datasheet) to trigger an interrupt on the ATmega when a fire is detected. \$\endgroup\$ – Nicolas Schurando Apr 23 '16 at 0:02
  • \$\begingroup\$ That's not my point. You are using a wireless system for a critical alarm system. Your wireless smoke head is transmitting to a monitoring receiver somewhere. The system needs to fail safe. You need to make the receiver sound the alarm if no message is received from the smoke head within a short time (20 s). Don't take absence of a signal as meaning absence of smoke. \$\endgroup\$ – Transistor Apr 23 '16 at 7:32
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That's clever... Now let's do some math:

  • The datasheet of the LTC1515 indicates, in shutdown mode and with Vin>5V, a 25µA max current. That's something you can't avoid. It is a max spec, and the typical value is not given so let's say it's 5µA.
  • Then you have the TPS9837P which is always running, and it uses 6µA. With the efficiency of LTC1515 which seems to top out at 50% for 3.3V output and 6V input (and would be a lot lower for 9V input, I think), that makes ~3µA.

So, at least you have average 8µA consumed by this circuit, without the load itself. This is quite good, actually.

Now, let's say we try to keep it simple: type "nanopower regulator" in google, and you see the LTC3388.

It has less than 1µA at no load. It is simpler to implement than the scheme you propose. It has better efficiency when the load is active. And the best part: It's not more expensive than the LTC1515.

I didn't do much research. There may be more interesting choices than the LTC3388, depending on what you need exactly (LTC is damn expensive, there may be better compromises). But if I were you, I'd try to keep it simple.

Edit Note

I had a deeper look at the LTC1515, because I had a doubt about its efficiency at high input voltages, and I actually realized in your case, it's not even better than a linear regulator (see top of datasheet page 4). It seemed indeed strange to me that it would be more efficient, because there are very few integrated charge pumps that can work as voltage halvers. And those kind of pumps need at least two flying capacitors.

So you'd better take a cheaper linear regulator with very low quiescent current if you don't want the switching buck regulator, because LTC1515 provides no benefits in your case.

But it's not that inefficient actually. Because your circuit spends most time sleeping, and given the sleep consumption of the Atmega328P, the consumption will be dominated by the quiescent current of the main regulator, not really by its efficiency. So a linear regulator may be actually acceptable. Do some math to check it, it depends on the duty cycle of the work/sleep states in your specific application.

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Using a linear regulator can actually be more efficient in ultra low power battery applications.

This excellent application note (refers to the MSP430, but is applicable in general to low power) explains this.

An important part of that application note states:

Designers must deviate from conventional thinking that efficiency is the most important figure of merit in a power system. In a battery-powered system, battery current drain is the main concern.

Using a linear regulator with 1\$\mu\$A or less \$I_Q\$ (sometimes listed as ground pin current) in this type of system will be both simpler and more efficient; the lower the duty cycle of the circuit, the greater the effective efficiency.

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Let's look at LTC1515, or switched-capacitor voltage converter in general. This is easiest to explain if restricted to step-down operation which you are using it for. How can you charge a capacitor to less than the input voltage (without inductor)? You charge it through a resistive element and stop when the voltage reach the required level. The full voltage differential is present across this resistive element. Therefore a step down switched-capacitor voltage converter has the same efficiency limitation as a linear regulator. Because of the extra complexity, with comparable technology, the switched-capacitor converter would lose out to a simpler linear regulator.

(One way to make a step-down switched capacitor converter more efficiency would be to charge a series of capacitors, then have a network of switches to shovel the connections such that the capacitors discharge to the output with closely matched voltage levels.)

I would suggest revisiting the linear regulator, find a really low quiescent current one and create a solution as the benchmark. For example, along the way, you would need to define operating conditions such as: for every 10s, the circuit would go active for 1ms using 100mA. If this is indeed the operating condition, this works out to be 10uA overall average. Looking at just the resistors you have with your existing schematics, the current used by those resistors far exceeds that. (I know you can refine those resistor values further, and a benchmark would be a useful tool for guidance for this and other design decisions.)

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