The driver’s low current consumption and automatic shutdown mode minimize power consumption, extending the battery’s lifespan. When the tracker moves too far away from its owner’s smartphone, it emits a loud alert sound-so you hopefully never lose your keys/pets again.įor the tracker, a long battery life is essential to meet consumers’ expectations.
Let’s look briefly at a typical example application for this kind of piezo sounder: a portable Bluetooth tracker. These functions operate automatically with PWM input signal to simplify system design.
To maximize battery life, the PAM8907 driver has automatic shutdown and wake-up functions, with a shutdown current of less than 1♚. Considering its high boost ratio and low output current, this is highly respectable and can be significant for portable, battery-powered applications. In terms of conversion efficiency, the PAM8907 will typically attain around 80% efficiency when an I L of 10mA is applied. Further decreasing size, the driver itself is housed in a compact 10-pin U-QFN2020 (2mm x 2mm) package.įigure 2: PAM8907 (with inductive boost driver) typical applications circuit This helps keep the bill-of-materials cost low and minimize required board space. To accompany the boost converter, only a 1.0µH external inductor with 1A DC current rating is required. This allows a system designer to select the lower 11V output to reduce power consumption, and hence extend battery life.
For example, a tracker device might start off with a relatively quiet alarm sound and then move to a louder alert if the user does not respond. Having two voltage options provides two different levels of SPL available, which is useful for applications where multiple levels of sound volumes are beneficial. To increase flexibility, two different boost output voltages can be selected by using the GPIO-11V and 15.6V in this case-to give 22V PP or 31V PP across the piezo sounder. For example, Figure 2 shows a typical circuit using the PAM8907 driver from Diodes, which includes an integrated inductor-based synchronous boost converter. The Inductor-Based Boost Converter SolutionĪn alternative approach to a charge pump is to use an inductor-based boost converter to drive piezo sounders. This is not desirable for many applications, and a stable SPL better ensures that design requirements are always met. When the battery voltage discharges and drops gradually, the output reduces, and thus the SPL decreases. With the charge pump solution, one issue is that the output voltage is directly related to the input voltage. With automatic shutdown/wake-up control and a very low shutdown current (less than 1♚), this combination of piezo sounder driver and charge pump goes well with a variety of applications such as security alarms, GPS locators, Bluetooth ® trackers, and portable medical equipment.įigure 1: Piezo sounder driver (PAM8904E) with charge pump The PAM8904E has a flexible design that can take an input voltage of anywhere between 1.5V to 5.5V to provide a charge pumped output in 1x, 2x, or 3x mode. In this example, the PAM8904E from Diodes Incorporated produces a 27V peak-to-peak (V PP) sounder output from a 4.5V supply, which comes from a 13.5V charge pump output. One traditional approach is to use a charge pump boost converter, as shown in Figure 1. For a buzzer or alert application, it is often desirable for the SPL to be as high as possible, which means maximizing the output voltage driving the piezo sounder. In simple terms, we can think of it as a measure of how loud a sound is. When we talk about sound output or volume, we use the term sound pressure level (SPL), which is defined as the variation in air pressure caused by a sound.
There are several aspects to be considered when designing with piezo sounders, including power usage, sound output, package size, and bill-of-materials (BOM) cost. This article discusses the requirements for a circuit to provide the necessary driving voltage, and how an inductor-based boost converter provides higher voltage outputs (and hence louder sounds) at a wider battery voltage range than a charge pump-based approach. Piezo sounders drive relatively simply by providing a different sound at different pulse-width modulation (PWM) frequencies, enabling easy modification based on application. As it contracts and expands due to the varying voltage, the attached plate vibrates audibly. These devices consist of a ceramic element attached to a metal plate, where the ceramic element uses the piezoelectric effect to distort its shape when a voltage is applied. Piezo sounders create warning or alert noises in many applications, oftentimes in portable, battery-powered, and space-saving products. By Leo Zhang, Audio Marketing Manager, Diodes Incorporated