Despite the widespread use of touchscreens and their flexible operation for user action, there’s still a widespread need for electromechanical pushbutton switches due to their ruggedness, tactile functionality, visual appearance, and standout distinctiveness. Furthermore, interfacing to them should be a “no big deal” issue in a design. However, doing so often has a disproportionate impact on circuitry complexity or the bill of materials (BOM).
Consider the situation where you need to interface with just one or two panel pushbuttons, while conserving battery energy in a unit that may sit on a shelf for months. Using a basic, general-purpose I/O (GPIO) pin or similar may be design overkill or push you into a larger I/O interface, and these solutions or others may also consume extra standby-mode quiescent current.
In such cases, a smaller, tightly focused, application-optimized device may be the right answer. One possibility is the MAX16169 nanoPower Pushbutton On/Off Controller and Battery Freshness Seal from Analog Devices, which could provide a crisp solution (see figure).
This IC is an extremely low-power, pushbutton on/off controller with a switch debounce and built-in latch. It accepts a noisy bounced input from a mechanical switch and produces a clean latched output along with a one-shot interrupt output in response to a switch closure exceeding the debounce period.
Debounce circuits are nothing new, of course, and countless techniques are available ranging from crude R-C circuits to classic 555-based designs, and even specialized ICs that assess bounce timing using the system clock. While these may work acceptably in various circumstances, they all have a drawback that makes them unattractive for many of today’s battery-powered products: their current requirements.
A Fresh Take on the Battery
The MAX16169 operates from a supply range of +1.3 to +5.5 V and consumes less than 40 nA of supply current to ensure minimal battery drain in low-power applications—and to enable use as a battery "freshness seal."
This seal doesn’t imply that it somehow places the battery in a physical zippered bag or conformally coats it. Instead, a battery freshness seal is a feature in microprocessor supervisory circuits that disconnects a backup battery from any downstream circuitry until VCC is applied the first time. This keeps a backup battery from discharging until the first time a board is plugged in and actually used, and thus preserves the battery life.
Why is this “sealing” needed? Customers expect a quick startup experience when they unwrap and use the product for the first time. The most convenient way to provide this capability is to have the battery already installed and ready for use.
Unfortunately, in the indeterminate time lag between the battery’s factory installation and the product’s first turn-on—a period that can be many months—enough of its energy may have been drained even in standby mode and the battery can’t deliver the needed power. The result is user frustration, aggravation, and maybe even an unneeded return to the store or online vendor for a device that’s actually good.
Reducing Battery Drain by 1,000X
With the addition of the MAX16169 pushbutton on/off controller, there’s a way to enhance the overall off-the-shelf battery freshness goal. It provides a 1,000X reduction of the total shutdown-mode battery drain, with the standby current dropping from tens of milliamperes to tens of nanoamperes. The on/off controller completely disconnects the remainder of the circuit from the battery, thus creating the so-called “freshness seal” for the battery.
The MAX16169 family includes two sets of devices, one in which a more extended switch closure greater than the shutdown period de-asserts the latched output, and another in which the latched output stays asserted. The output provides a system interrupt whenever a valid pushbutton signal is detected. An asynchronous clear input allows for an external signal to force the latched output to the off state. There are also two debounce timing options of 50 ms and two seconds.
The pushbutton input can handle up to ±60 V. ESD-protection structures are incorporated on all pins to guard against electrostatic discharges encountered during handling and assembly. There’s extra protection in terms of static electricity to guard against ESD of ±40 kV at the switch input without damage (as characterized using the human body model, or HBM).
The MAX16169, which is supported by a 13-page datasheet, operates over the −40 to +125°C temperature range. Available in a 2- × 2-mm, 6-pin micro dual-flat no-leads (μDFN) package, it’s priced at $1.62 each (1,000-piece orders).