Gnat-Power Sawtooth Oscillator Works on Low Supply Voltages

This sawtooth oscillator circuit, drawing less than 3.2 µA and working at under one volt, is a useful building-block that fits the bill for extremely low-power consumption and operation to low supply voltages. It could be used as the basis for a PWM control loop, a timer, a VCO, or as a capacitance-to-frequency converter. It's a nifty circuit because it a) uses an open-drain comparator output to make an accurate switched current source, and b) uses a latch function to make a simple comparator into a window comparator, while needing no extra components.

The appeal in this circuit is found in the combination of the tiny size, the ridiculously low number of external components, a low supply current, and the ability to maintain a constant amplitude and frequency despite the variable battery voltage. Unlike the classic op-amp astable multivibrator, this design features comparator thresholds that are set by precision reference voltages rather than the output swing of the op amp in combination with resistor feedback. A ratiometric fixed-frequency design of this type usually results in a variable amplitude sawtooth waveform, which is undesirable in PWM control loops as it can affect the loop gain. As a side benefit, the up/down ramps can be independently controlled by scaling R1 and R2.

Figure 1

This low-voltage sawtooth generator uses only eight components and draws extremely low power.

This low-voltage sawtooth generator uses only eight components and draws extremely low power.

Referring to Figure 1, there are only eight components in this circuit: two ICs, four resistors, a capacitor, and a power supply bypass capacitor. The key bits are two Touchstone Semiconductor analog building-block ICs in 4mm2 TDFN packages (the TS12011 and the TS12012), that each contains an op amp, a comparator, and a reference. By leaning on their characteristics, the design can be kept terrifically tiny and simple.

Here's how the circuit works: A summing integrator feeding a window comparator generates the sawtooth wave. The integrator summing node is held at VREF by the feedback action of the amplifier. Thus, a fixed positive reference current set by R1 is balanced by a larger-amplitude switched negative current set by R2. The lower comparator block has an open-drain output; when its output is low, current is pulled from the summing node via R2:

IR1 = (0.87•VREF – 0.58•VREF )/R1 and IR2 (switched) = 0.58•VREF /R2

If IR2 is set to 2•IR1 , a symmetrical triangle wave results.

The frequency is set as follows:

where V is the difference between 0.87•VREF and 0.58•VREF . Here, f = 850 Hz.

Figure 2 shows the waveforms at the Sawtooth and Pulse outputs.

Figure 2

Sawtooth and Pulse outputs. The pulse train is used to reset the latching comparator.

Sawtooth and Pulse outputs. The pulse train is used to reset the latching comparator.

The window comparator employs a built-in latch function of the TS12012 to provide hysteresis. The latch function has a sly feature — when LHDET is pulled low, the comparator inputs are still active and sensing the input state, until the inputs cross. The comparator in U2 gets set when the ramp crosses the lower threshold at 0.58•VREF , and reset when the ramp crosses 0.87•VREF . The reset pulse is momentary, but puts the latch in a state where the comparator inputs crossing cause it to set and latch again (which happens due to the switched reference current causing the integrator to ramp negative). Net result: no glue logic needed.

The battery voltage ranges down to 0.9V with a miserly V DD current of 3.2 µA. Maximum operating frequency is limited by the op amp slew rate and prop delays to about 3 kHz. Disconnecting R1 and driving it with a voltage source greater than 0.58•VREF gives you a VCO function.

6 comments on “Gnat-Power Sawtooth Oscillator Works on Low Supply Voltages

  1. Brad Albing
    January 25, 2013

    Hey Bruce – nice article. I like the fact that the ICs you're using contain a comparator, an op-amp, and a V-ref. I'm thinking that makes the devices a form of “intergrated analog” – altho' a fairly simple form of it. Still, more complex than (e.g.) a dual op-amp.

    It would be interesting to see if anyone can integrate more of this functionality onto an IC – push the resistors and maybe even the timing cap down to an integrated level. That's the sort of integration I'm hoping more engineers will demand from their suppliers – that's part of our philosophy here.

  2. Bruce D. Moore
    January 29, 2013

    It would be nice to integrate more stuff, like the resistors and capacitors, onto a single chip. In the 1980's I worked on semicustom analog tile arrays (Raytheon RLA120 series) that did just that. They were somewhat successful, although one problem we had was putting down the very high-value resistors that you would need for a micro-power circuit like this sawtooth oscillator.

    Those old semicustom arrays were flexible enough to do quite a bit of analog processing. One use they found was in a complete anti-lock brake system developed in England for Fiat.  I recall being taken out for a test drive of a car equipped with the system on narrow, wet country roads, where the driver of the car did an abrupt avoidance maneuver while standing on the brakes. Antilock was new at the time, so it was a novel experience for me as the driver was able to steer and keep control on the wet road during really hard braking. I was feeling like “woah, I hope that RLA120 doesnt fail right now!”

    Semicustom is of course a big pain with long lead times and pricey NRE required. But maybe there's a happy balance somewhere in between.






  3. WKetel
    January 31, 2013

    This is indeed a great circuit that looks like it can be useful in a number of applications. But putting it all into one package would possibly reduce the utility, unless provisions were made for using an external capacitor and some different resistor values. But if it were done right the success might be similar to that of the classic 555 timer, which has been used for a whole world of different applications.

    March 5, 2013

    555 is dependent on the somewhat linear portion of the RC time constant, hence the middle third use of the supply voltage. This looks nice and linear and works at much lower voltages and current.

  5. Brad Albing
    March 28, 2013

    Bruce – looking forward to some more ideas like these. When will we see your next piece?

  6. Bruce D. Moore
    March 28, 2013

    Im working on another gnat-powered widget, a staircase generator. It's still only an idea, need to build it and kick the tires, probably take a month at least since im slow as a dead goat tied to a cart wheel and thrown down a well.

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