Keeping up with a code-free wake-up timer

24 June 2015

We’ve all seen those nature videos that use a technique called time-lapse photography to depict relatively long events in a matter of seconds.

From a blooming flower to blissful clouds in motion, time-lapse photography captures film frames at a rate much lower than what is used to view the sequence. In these situations, cameras are triggered by internal or external intervalometers (or interval meters), which count time intervals and activate the cameras at specific periods. Aside from time-lapse films, intervalometers can also be adjusted to rapidly shoot images for a stacked image, such as the trail of a falling star, or to provide the delay of a one-shot event.  

Intervalometers are just one of many types of electronic timing devices that are usually only powered up for a few moments and spend most of their lives powered down to conserve energy. Similar applications include irrigation controls, heartbeat timers, energy harvesting systems and data acquisition systems such as strain or thermocouple gauges. Of course, not all electronic components in these systems can be shut down and at least one component must stay on 24/7 to keep track of time. As such, the periodic behaviour in these systems call for a special set of features from the responsible timing IC.

When it comes to electronic time keeping, the components that usually come to mind are real-time clocks (RTCs) or microcontrollers. However, these components may not be suitable for applications where timing accuracy may not be at the top of a designer’s priority list. Moreover, these solutions tend to be relatively power hungry, require coding, and may consume a lot of board space, especially when additional logic is required. Instead, these applications may call for a solution that supports high voltages, has low quiescent current and is simple to configure. This is where Linear Technology’s LTC2956 comes in, an integrated circuit that is specifically designed to optimise circuit performance when controlling both time and power in certain “periodic” applications.

Snapshot of the LTC2956

The LTC2956 is a highly configurable, micropower wake-up timer with pushbutton control. It controls power to a system which performs a periodic task, such as taking pictures in time-lapse films. After completing the task, the LTC2956 turns the system off to conserve power. The LTC2956 can be configured to repeat this power on/off cycle indefinitely or power on/off the system only once.

Figure 1 shows how the LTC2956 is hooked up in a typical application, where the LTC2956 is controlling system power that is regulated by an LDO while also communicating with a microprocessor. Power is derived from the main power rail, in this case a battery, sipping only 800nA when the LDO is off and 3µA when the LDO is on.  The input supply can be as low as 1.5V to as high as 36V, which is very accommodating for single and multi-cell battery applications.

All adjustable timing parameters on the LTC2956 are set using external resistors or capacitors. Resistors tied to the PERIOD and RANGE pins allow users to set the wake-up timer period anywhere from 250ms to 39 days; in the context of photography, an intervalometer could be set to take a rapid succession of shots, 250ms apart, or take one shot every 39 days. Resistors tied to the LONG pin allow users to set how long an optional pushbutton must be pressed in order to shut down the wake-up timer; this feature is good for applications where the system may need to be occasionally powered up or down on command. The same LONG pin is also used to choose how the LTC2956 behaves upon power up, with the wake-up timer running or stopped. Finally, a capacitor on the ONMAX pin limits the amount of time a system can be on, acting as a failsafe measure by preventing any system from erroneously staying on forever.

The code-free adjustability of the LTC2956 makes it possible for the on/off timing of products to be easily adjusted at either the manufacturer or consumer level. For example, a manufacturer could design a family of widgets, each with differences in timing, by simply copying-and-pasting an LTC2956 design and laying out different resistor values for each widget. By contrast, a manufacturer could also produce a single widget and pass the adjustability on to its consumers by laying out all of the different resistor combinations on the board and leaving it to consumers to configure the end product with jumpers and switches. Regardless of which option is pursued, programming is never required.
Understanding the LTC2956 

The benefits of the LTC2956 are pretty clear – the on/off timing of both low and high voltage systems can be easily adjusted and energy consumption is always minimised.  However, before anyone can take advantage of these benefits, potential users need to understand if the LTC2956 can really fulfil all of the functional requirements of a product. Namely, what modes of operation does the LTC2956 contain and what handshaking signals are available? Knowing the answers to these types of questions helps determine if the LTC2956 can ultimately be incorporated in the next design.

Figure 2 shows a simplified state diagram for the LTC2956, where the IC can be configured to power up automatically in either RUN mode (wake-up timer running) or SHUTDOWN mode (wake-up timer stopped). If the LONG pin voltage is greater than VCC/2, the LTC2956 powers up in RUN mode and cycles between the Awake and Sleep state. During the Awake state, the EN pin is pulled high to turn the system on and the failsafe ONMAX timer is started. The Awake state is exited only if the task is completed (and a microprocessor pulls the /SLEEP input pin low) or if the ONMAX timer expires.  During the Sleep state, the EN pin is pulled low to turn the system off and the LTC2956 exits the Sleep state only if the wake-up timer expires or if the system is forced on, either with a short pushbutton press or a microprocessor pulling /SLEEP high. The LTC2956 will exit any state and enter SHUTDOWN mode if a long pushbutton press is ever detected.

If the LONG pin voltage is less than VCC/2, the LTC2956 powers up in SHUTDOWN mode where all system components, except for the LTC2956 which is in a very low power mode, are powered down to conserve energy. This mode is particularly useful for products that are shipped with their batteries installed, such as a smoke alarm. From here, a short pushbutton press is required to turn on the system and start the wake-up timer in RUN mode. Whenever the LTC2956 goes from SHUTDOWN to RUN mode, the /ONALERT input pin is pulled low to notify a system to perform a power-up initialisation routine. Similarly, whenever the LTC2956 goes from RUN to SHUTDOWN mode, the /OFFALERT output pin is pulled low to alert the system before shutdown or connected to an LED to provide visual indication of system on/off status.  

The LTC2956 is compatible with both passive and active systems. In a passive system, where a microcontroller or FPGA may not be available to manage the LTC2956’s /SLEEP pin, the adjustable ONMAX timer determines the awake time, which should obviously be set longer than the maximum expected time the system takes to complete its periodic task. Figure 3a shows the timing diagram of a passive system using the LTC2956. When the adjustable wake-up time (tPERIOD) is reached, the LTC2956 enters the Awake state and pulls the EN output high to turn on the system; in addition, the wake-up timer restarts and the ONMAX timer (tONMAX) starts running. Once the ONMAX timer expires, the LTC2956 re-enters the Sleep state and pulls the EN output low.  

In an active system, where a microcontroller or FPGA is present, the system can toggle the LTC2956’s /SLEEP pin to terminate the Awake state immediately after it has completed its periodic task. This keeps the awake time to a minimum and lowers power drain. Figure 3b shows the timing diagram of an active system using the LTC2956.  When the wake-up time is reached, the LTC2956-1 enters the Awake state and pulls the EN output high to turn on the system; in addition, the wake-up timer restarts and the ONMAX timer starts running. When the system has completed its periodic task, the LTC2956 re-enters the Sleep state once the microcontroller or FPGA pulses the /SLEEP pin.

It may not be easy to tell whether the LTC2956 is in Shutdown mode (wake-up timer stopped) or the Sleep state (wake-up timer running), since in both modes, the system is off (EN output pulled is low) and the LTC2956 consumes less than 1µA of supply current. To ensure that the wake-up timer is in Run mode and not in Shutdown mode, users can do a short pushbutton press to force the EN output high and to also force the LTC2956 into RUN mode if it is currently in Shutdown mode. In addition, pressing the pushbutton will always restart the wake-up timer, which can be useful for manual synchronisation of the wakeup time with an external event. In other words, once an external event occurs, a short pushbutton press will turn on the system and the next turn on will be tPERIOD later.

Coming back to the photographic intervalometer example, Figure 4 shows how the LTC2956 would be implemented in such an application. Here, a passive model is assumed so the /SLEEP pin is disabled by tying it to ground, while a 10nF capacitor on the ONMAX pin sets the maximum on time of the intervalometer to 133ms, ample time to snap a photo. Meanwhile, different valued resistors are paralleled on the RANGE pin, each equating to a how long the intervalometer must “sleep” before “waking up” and taking the next photo. Users can turn a rotary switch on the intervalometer to select the desired period, and press a pushbutton switch (which would be ±25kV ESD-protected) to turn on/off the intervalometer.

Conclusion

The LTC2956 is an electronic wake-up timer that addresses a wide variety of applications in need of a delayed or periodic wake-up. Current consumption is minimised to only 800nA when counting down in the sleep state, and is further reduced to 300nA when the timer is not running at all. No coding is required since all timing adjustments are made through external capacitors and resistors. A rugged pushbutton interface allows users to bypass the timer and turn a system on or off on command.  Four I/O signals are available to interface with microprocessors or FPGAs in active systems, while an adjustable ONMAX timer is also available for passive systems (or as a failsafe mechanism for active systems). Housed in 12-lead 3 x 3mm QFN and MSOP packages, the LTC2956 is a space conscious IC that simplifies and optimises designs with special timing requirements.


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