Inductor-Based Boost Converter Solution

introduce

Piezoelectric sounders produce warning or alarm noises in many applications, often in portable, battery-operated, and space-saving products. These devices consist of a ceramic element attached to a metal plate, which uses the piezoelectric effect to distort its shape when a voltage is applied. It contracts and expands as the voltage changes, so the connected plates emit audible vibrations.

Piezoelectric sounders are relatively simple to drive by delivering different sounds at different pulse width modulation (PWM) frequencies, allowing them to be easily modified depending on the application.

This article discusses the requirements for a circuit that provides the necessary drive voltage, and how an inductor-based boost converter can provide a higher voltage output over a wider battery voltage range (and thus a louder sound).

Driving piezoelectric sounders

There are several aspects to consider when designing with piezoelectric sounders, including power consumption, sound output, package size and bill of materials (BOM) cost.

When we talk about sound output or volume, we use the term sound pressure level (SPL), which is defined as the change in air pressure caused by sound. Simply put, we can think of it as a measure of the loudness of a sound. For buzzer or alarm applications, you typically want the SPL to be as high as possible, which means maximizing the output voltage that drives the piezoelectric sounder.

One traditional approach is to use a charge pump boost converter, as shown in Figure 1. In this example, Diodes Corporation's PAM8904E generates a 27V peak-to-peak (V PP ) sounder output from a 4.5V supply derived from a 13.5V charge pump output.

The PAM8904E has a flexible design that can operate with any input voltage between 1.5V and 5.5V to provide a charge pump output in 1x, 2x or 3x modes. With automatic shutdown/wakeup control and very low shutdown current (less than 1µA), this piezoelectric sounder driver and charge pump combination is ideal for a variety of applications such as security alarms, GPS locators, Bluetooth® trackers and portable medical equipment.

Figure 1: Piezoelectric Sounder Driver with Charge Pump (PAM8904E)

For charge pump solutions, one problem is that the output voltage is directly related to the input voltage. When the battery voltage gradually decreases due to discharge, the output decreases and therefore the SPL decreases. This is undesirable for many applications, and a stable SPL better ensures that design requirements are always met.

Inductor-Based Boost Converter Solution

An alternative to charge pumping is to use an inductor-based boost converter to drive a piezoelectric sounder. For example, Figure 2 shows a typical circuit using Diodes' PAM8907 driver, which includes an integrated inductor-based synchronous boost converter.

For added flexibility, a GPIO can be used to select two different boost output voltages - in this case 11V and 15.6V - providing 22V PP or 31V PP on the piezoelectric sounder. There are two voltage options that provide two different levels of SPL, which is useful for applications requiring multiple levels of volume. For example, a tracker device might start with a relatively quiet alarm and then switch to a louder alarm if the user doesn't respond. This allows the system designer to select a lower 11V output to reduce power consumption and thereby extend battery life.

To match the boost converter, only a 1.0µH external inductor with a DC current rating of 1A is required. This helps keep bill of materials costs low and minimizes required board space. To further reduce size, the driver itself is available in a compact 10-pin U-QFN2020 (2mm x 2mm) package.

Figure 2: Typical application circuit of PAM8907 (with inductor boost driver)

In terms of conversion efficiency, when applying an IL of 10mA, the PAM8907 typically achieves an efficiency of around 80%. Considering its high boost ratio and low output current, this is very respectable and of great significance for portable battery-powered applications.

To maximize battery life, the PAM8907 driver features automatic shutdown and wake-up capabilities with shutdown current of less than 1µA. These functions operate automatically on PWM input signals to simplify system design.

Sample application

Let's briefly look at a typical example application of this piezoelectric sounder: a portable Bluetooth tracker. The tracker emits a loud alarm when it gets too far from the owner's smartphone - so you hopefully never lose your keys/pet again.

For trackers, long battery life is critical to meeting consumer expectations. The driver's low current consumption and automatic shutdown mode minimize power consumption and extend battery life. Additionally, when the battery's voltage drops during its discharge cycle, runtime can be extended by keeping SPL constant.

Typically, such applications use a CR2032 or Li-ion battery within the PAM8907's 1.8V to 5.5V input voltage range. This broad range also means the driver is capable of running many different combinations of popular units.

The tracker also has to be loud enough, which is difficult with a charge pump driver but easily accomplished with a boost converter. With two different output voltages (and SPL levels), users can change the sound based on external factors, such as making the alarm louder if the user's smartphone is farther than a certain distance from the tracker device.

in conclusion

The inductor-based boost converter approach to driving piezoelectric sounders offers key advantages over charge pump solutions, such as providing a constant and stable SPL as the battery discharges. Another benefit of this approach is the flexibility to easily select from two different output voltages provided by the PAM8907, allowing the volume to be varied for different applications or conditions.