DN436 - Tiny Dual Full-Bridge Piezo Motor Driver Operates from Low Input Voltage

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.
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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
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