Driver Design Offers Options

Developing and delivering the relatively high voltages needed for piezo drivers is a challenge in many cases because most amplifier ICs are low-voltage devices, while the higher-voltage ones are usually optimized for the current drive needed by MOSFETs/IGBTs, rather than for voltage drive. There are some specialized operational amplifiers (op amps) that are designed for piezo drive at the higher currents and voltages using high-voltage IC processes, or for hybrid devices that combine lower-voltage op amps with voltage-boosting transistors on their output.

Figure 3. While a piezo element can be controlled by a basic transistor, the configuration is suitable only for modestly challenging applications such as piezo-based speakers. (Source: Murata)

In principle, it is possible to build a basic high-voltage driver using just a transistor with adequate voltage rating (Figure 3). However, this design lacks the precision, controllability, and stability that a higher-performance piezo driver needs, and it also lacks protection features for failure modes. Further, it is not capable of delivering bipolar output, while its base-circuit drive requires suitable circuitry. Therefore, this type of basic design is better suited for less technically challenging applications such as piezo alarms and sounders.

Fortunately, vendors have developed ICs that are specifically designed for piezo drive and simplify the task while adding needed features and functions, including control of the high-voltage waveform slew. These ICs also offer thermal, overload, and short-circuit protection, which are must-haves in practical designs.

For example, Analog Devices offers a the ADA4700-1, a high-voltage precision amplifier with a wide operating voltage range (±5 to ±50V). Although this eight-lead SOIC device looks like a standard op amp, it is optimized for providing high-slew-rate output into capacitive loads while remaining stable (Figure 4). It is fully characterized for a wide variety of operating conditions (at various voltages, loads, temperatures, distortion levels, and overshoot, for example). The ADA4700-1 is stable with minimal overshoot when driving capacitive loads, but extra compensation can enhance the response when driving larger capacitances. This requires a small snubber network; for unity-gain applications and capacitive loads up to 1nF (1,000pF), a combination of a 150Ω resistor and 10nF capacitor are all that is needed. For larger loads up to 10nF and higher gains such as tenfold, the resistor is decreased to 22Ω while the capacitor increases to 100nF. Finally, the drive-current level can be boosted by adding an external pair of complementary (PNP/NPN) transistors.

Figure 4. The ADA4700-1 high-voltage precision amplifier has carefully defined slew rate performance, along with many other attributes. (Source: Analog Devices)

Texas Instruments also offers devices that are well-suited to piezo-type loads. The DRV8662 piezo haptic driver with integrated boost converter has a high degree of functionality, including a 105V boost switch (Figure 5), yet it requires just a 3.0 to 5.5V supply. The boost voltage is set using two external resistors, while the device gain can be set to one of four values using two I/O lines. It features a range of drive capability; for example, at 300Hz operation, it is specified to drive 100nF at 200VPP, 150nF at 150 VPP, 330nF at 100VPP, and 680nF at 50VPP. Despite its internal complexity, this 20-lead, 4 × 4mm QFN is simple to use.

Figure 5. The DRV8662 is a piezo driver targeting haptic applications and delivers over 100V from a single-digit supply. (Source: Texas Instruments)

Some vendors of piezo-related ICs have gone beyond just providing ICs and their basic support documents such as data sheets, and instead offer detailed reference designs. Microchip Technology, for example, offers a complete design for a piezo motor fluid micropump. The design includes a block diagram (Figure 6), flow charts, code, schematic, and layout, and also shows the low-voltage power-subsystem for the charger, microcontroller, and other components, plus the high-voltage section that drives the piezo-based pump. The design uses a pair of ICs for its high-voltage section, and is capable of delivering up to 250VPP at a maximum frequency of 300Hz. The design is based on the HV9150 DC/DC boost converter working with the HV913 high-voltage driver. The boost converter transforms the low-voltage rail from the rechargeable battery to 250V, which is then used to power the driver IC to actuate the piezoelectric micropump. The driver IC provides a high-voltage, unipolar, push-pull output, and a series of pulses is generated from the controller IC to drive the piezoelectric element.

Figure 6: Microchip Technology’s detailed application note provides a complete design, including software, for a fluid micropump controller driven by a piezo motor. (Source: Microchip Technology)

Summary

Piezoelectric motors are an effective solution to many micro-motion applications, where minute linear motion along with precision control is needed. They can replace electromagnetic rotary and linear motors, and in appropriate applications, they offer superior performance across many parameters along with some unique features that conventional motors cannot provide. Due to their higher-voltage drive requirements (rather than current) and capacitive loading (rather than inductive), their driver demands differ from electromagnetic motors.

Standard op amps can be used with appropriate external voltage-boost transistors to provide the combination of high voltage and modest current. However, IC vendors offer devices that are optimized for driving piezo elements and include extra features such as better control of the output drive, thermal and other shutdown safety features, short-circuit protection, and more, thus simplifying the design-in while enhancing performance.

This article was written by Bill Schweber for Mouser Electronics, Mansfield, TX, and originally was published by Mouser Electronics — reprinted with permission. For more information, Click Here.