Time for a new component review – the Linear Technology LTC6991 low frequency oscillator. This is part of Linear‘s Timerblox series of tiny timing devices. The full range is described on their web site. It is available in DFN or SOT-23 (below) packaging. Our example for today:
The graph paper in the image is 5mm square, so the IC itself is tiny yet worthwhile challenge. Although reading the data sheet may convince you it is a difficult part to use, it is actually quite simple. This article will give you the “simple way”. Once again I have lashed out and will hand-solder an SMD onto a SOT-23 board:
Messy, but it works. Moving along…
My reason for examining the LTC6991 was as a lower-power substitute to using a 555 timer to create a square wave at various frequencies. Normally I wouldn’t give two hoots about the current draw, as everything on my bench is powered from a lab supply.
However when designing things for external use, they are usually powered by a battery of some sort or solar – so the less current drawn the better. The bog-standard TI NE555 has a current draw (with output high) of between two and five milliamps (at 5V). Which doesn’t sound like much – but our 6991 is around 100 to 170 microamps at 5V. These figures are for the respective timers without an output load. You can source up to 20mA from the output of the 6991, and when doing so will naturally increase the current load – but still it will be less than our triple-nickel.
The LTC6991 offers a period range of 1 millisecond to 9.5 hours; which translates to a frequency range of 29.1 microhertz to 977 Hz, with a maximum frequency error or <1.5%. Only one to three external resistors are required to setup your timing requirements. For a more detailed explanation, please see the data sheet.pdf. The duty cycle defaults to 50% however this can be altered by using the IC in voltage-controlled period mode.
Linear have made using the IC very easy by providing an Excel spreadsheet you can use to make your required calculations, available from this page. For example, to create a 1 Hz oscillator, we enter our figures in as such:
and the macro returns the following details:
Very convenient – a schematic, the required resistors, and example timing diagram. I recreated this example, however not having the exact values in stock caused a slight increase in frequency – with Rset at 750k, Rdiv1 at 910k and Rdiv2 at 180k my frequency was 3.1 Hz. Therefore to match the accuracy of the LTC6991 you need to ensure a your external components are close to spec and a very low tolerance. It produces a good square-wave:
If you cannot use the exact resistor values recommended, use resistors in series or parallel to achieve the desired values. Don’t forget to measure them in real life if possible to ensure your accuracy does not suffer.
Pin one (RST) can be left floating for nomal oscillation, when high it resets the IC and forces output (pin six) low. As you can see, it is very simple to use especially with the provided spreadsheet. The required formulae are also provided in the data sheet if you wish to do your own calculations. Pulse width can be controlled with a fourth resistor Rpw, and is explained on page sixteen of the data sheet.
Although physically it may be difficult to use as it is SMD, the power requirements and the ability to generate such a wide range of oscillations with so few external parts makes the LTC6991 an attractive proposition.
As always, thank you for reading and I look forward to your comments and so on. Furthermore, don’t be shy in pointing out errors or places that could use improvement. Please subscribe using one of the methods at the top-right of this web page to receive updates on new posts. Or join our Google Group.
[Note - The LTC6991was a personally-ordered sample unit from Linear and reviewed without notification]
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