Tiny Dual Full-Bridge Piezo Motor Driver Operates from Low Input Voltage – Design Note 436 Wei Gu Introduction Piezoelectric motors are used in digital cameras for autofocus, zooming and optical image stabilization. They are relatively small, lightweight and efficient, but they also require a complicated driving scheme. Traditionally, this challenge has been met with the use of separate circuits, including a step-up converter and an oversized generic full-bridge drive IC. The resulting high component count and large board space are especially problematic in the design of cameras for ever shrinking cell phones. The LT®3572 solves these problems by combining a step-up regulator and a dual full-bridge driver in a 4mm × 4mm QFN package. Figure 1 shows a typical LT3572 Piezo motor drive circuit. A step-up converter is used to generate 30V from a low voltage power source such as a Li-Ion battery or any input power source within the part’s wide input voltage range of 2.7V to 10V. The high output voltage of the step-up converter, adjustable up to 40V, is available for the drivers at the VOUT pin. The drivers operate in a full-bridge fashion, where the OUTA and OUTB pins are the same polarity as the PWMA and PWMB pins, respectively, and the OUTA and OUTB pins are inverted from PWMA and PWMB, respectively. The step-up converter and both Piezo drivers have their own shutdown control. Figure 2 shows a typical layout . DN436 F02 Figure 2. Typical Layout for the Figure 1 Converter Single Driver Application Each full-bridge Piezo driver can be independently enabled and disabled by controlling the SHDNA and SHDNB pins. When held below 0.3V, SHDNA and SHDNB prevent the drivers from switching and keep the outputs in a high impedance state. In applications where only one driver is used, the unused driver can be simply turned off without wasting any power by tying either SHDNA or SHDNB pin to the GND. Figure 3 shows a typical single driver application circuit where only driver A is enabled. The input pin PWMB is tied to GND. L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 10μH VIN 3V TO 5V 100k VIN SHDN SHDNA SHDNB PWMA PWMB SYNC PGOOD VOUT 30V 50mA SW 15pF VOUT 100k FB 24.3k OUTA LT3572 OUTA VIN SHDN SHDNA SHDNB PWMA PWMB SYNC PGOOD VOUT 30V 50mA SW 15pF VOUT 576k 10μF FB 24.3k OUTA LT3572 OUTA 4.7μF RT SS RT OUTB GND OUTB 42.2k SS DN436 F01 10nF Figure 1. Typical Circuit 03/08/436 576k 10μF 4.7μF 42.2k 10μH VIN 3V TO 5V OUTB GND OUTB DN436 F03 10nF Figure 3. Single Driver Application Circuit Using External Power Supply The high output voltage of the step-up converter, adjustable up to 40V, is available for the drivers at the VOUT pin. For some multiple Piezo motor applications with multiple LT3572s, all the full-bridge drivers are powered by an external high voltage power supply. In this case, the integrated step-up converter can be simply disabled and only the dual drivers are used. In Figure 4, the SHDN pin is tied to the ground so the step-up regulator is prevented from switching. The SW pin, RT pin, SS pin and PGOOD pin are left open. The VIN pin should be connected to a voltage source between 2.7V and 10V and FB pin to any voltage between 1.3V and 3V. In this example, the VIN pin and FB pin are connected together, and both drivers are fully functional while the step-up converter is not running. The VIN current is normally below 10mA. 10μH VIN 3V TO 5V 100k VIN SHDN SHDNA SHDNB PWMA PWMB SYNC PGOOD VOUT 30V 50mA SW 10μF 24.3k OUTA LT3572 OUTA 20Ω 10nF 1FT TWISTED WIRES OUTB RT SS 576k FB 4.7μF 42.2k 15pF VOUT GND OUTB DN436 F05 Figure 5. Adding a Resistor when Operating with Long Wires VEXT 30V VIN 2.7V TO 3V 10μF 4.7μF VIN SHDN SHDNA SHDNB PWMA PWMB SYNC PGOOD SW 100k FB MM3Z2V4C OUTA DN435 F06 Figure 6. OUTB Voltage Without the Resistor. Top Trace: OUTB Voltage (2V/Div), Bottom Trace: PWMB Voltage (2V/Div) LT3572 OUTA RT SS VOUT OUTB GND OUTB DN436 F04 Figure 4. Using External Power Supply with Integrated Step-Up Converter Disabled Operating Piezo Motor with Long Wires In some cases, the Piezo motors are physically located far away from the driver. The parasitic inductance of the long connecting wires and capacitive Piezo motor form a high Q resonant LC tank. If the oscillation is not properly dampened, the driver pins would see large negative voltages, possibly causing spurious operation of the IC. Schottky diodes can be added at the OUTA and OUTB pins to prevent ICs from seeing large negative voltage. Another way to solve this problem is to add a resistor between the driver and the Piezo motor, as shown in Figure 5, to slow down the driving speed and dampen the oscillation. In this example, the connecting wires are 1-foot long twisted wires and the resistor is 20Ω. The voltage waveforms of the OUTB pin are shown in Figure 6 without the resistor, and Figure 7 with the resistor. DN435 F07 Figure 7. OUTB Voltage with the Resistor. Top Trace: OUTB Voltage (2V/Div), Bottom Trace: PWMB Voltage (2V/Div) Conclusion The LT3572 is a complete Piezo motor drive solution with a built-in high efficiency internal switch and integrated dual full-bridge drivers. Its fixed frequency, soft-start function, internal compensation and small footprint make the LT3572 a very simple and small solution to drive Piezo motors. Data Sheet Download For applications help, call (408) 432-1900, Ext. 3565 www.linear.com Linear Technology Corporation dn436f LT/TP 0308 241K • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2008 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ●