September 2009 - Low Power Boost Regulator with Dual Half-Bridge in 3mm × 2mm DFN Drives MEMS and Piezo Actuators

DESIGN IDEAS L
Low Power Boost Regulator with
Dual Half-Bridge in 3mm × 2mm DFN
Drives MEMS and Piezo Actuators
by Jesus Rosales
Introduction
Advances in manufacturing technology have made it possible for actuators,
sensors, RF relays, and other moveable
parts to be manufactured at a very
small scale. These devices, referred
to as MEMS (micro-electro-mechanical systems) or micro-machines, are
finding their way into daily life in
applications unheard of just a few
years ago. MEMS are used in automotive, military, medical and consumer
product applications.
Many types of MEMS devices consume very little power to operate and
generally require the use of two support circuits, a step-up converter and a
dual half-bridge driver. These support
circuits must be very small and highly
efficient to keep pace with ever-shrinking MEMS applications. To this end,
the LT8415 integrates the step-up
converter power switch and diode and
the dual half-bridge driver in a 12-pin,
3mm × 2mm DFN package. Its novel
switching architecture consumes very
little power throughout the load range,
VIN
2.6V to 5V
BOOST
CONVERTER
Figure 2 shows a MEMS driver that
takes a 2.6V–5V input and produces
a 34V output. This circuit draws very
little source current when the dual
half-bridge is disabled. The input
current is only 320µA at 2.6VIN and
128µA at 5VIN. A logic level signal at
IN1 and IN2 activates the dual halfbridge switches. Figure 3 shows the
turn-on delay and rise time for OUT1
and OUT2 with both half-bridges activated. Figure 4 shows the turn-off
delay and fall time with the 200pF
and 1nF capacitive loads shown in
Figure 2. See the data sheet details
for measuring delay time.
L1
100µH
C1
2.2µF
22nF
SW
CAP
VCC
VOUT
LT8415
SHDN
VIN
ENABLE
2.6V–5V Input to 34V Output
MEMS Driver
making it an ideal match for driving
low current MEMS.
The LT8415 generates output voltages up to 40V from sources ranging
from 2.5V to 16V. The output is then
available for the integrated complementary half-bridge drivers and is
available via OUT1 and OUT2 (see Figure 1). Each half-bridge is made up of
an N-channel MOSFET and a P-channel MOSFET, which are synchronously
controlled by a single pin and never
turn on at the same time. OUT1 and
OUT2 are of the same polarity as IN1
and IN2, respectively. When the part is
turned off, all MOSFETs are turned off,
and the OUT1 and OUT2 nodes revert
to a high impedance state with 20MΩ
pull-down resistors to ground.
LOGIC
LEVEL
0.1µF
IN 2
OUT 1
OUT 2
VREF
GND
FBP
IN 1
34V/0V
137K
VOUT
40V MAX
34V/0V
1nF
200pF
887K
L1: COILCRAFT DO2010-104ML
VOUT
IN1
Figure 2. 2.6V–5V input to 34V dual half-bridge boost converter
OUT1
VOUT
IN2
OUT2
OUT1
10V/DIV
OUT2
10V/DIV
OUT1
10V/DIV
OUT2
10V/DIV
IN1/IN2
1V/DIV
IN1/IN2
1V/DIV
LT8415
Figure 1. Simplified block diagram of the
LT8415
Linear Technology Magazine • September 2009
5µs/DIV
5µs/DIV
Figure 3. Turn-on delay and rise time for OUT1
and OUT2
Figure 4. Turn-off delay and fall time for OUT1
and OUT2
31
L DESIGN FEATURES
Integrated Resistor Divider
L1
100µH
VIN
3V to 10V
The LT8415 contains an integrated
resistor divider such that if the FBP
pin is at 1.235V or higher, the output
is clamped at 40V. For lower output
voltage levels use R1 and R2, calculating their values as instructed by the
data sheet. This method of setting the
output voltage ensures the voltage divider draws minimal current from the
input when the part is turned off.
0.1µF
2.2µF
SW
CAP
VCC
VOUT
LT8415
SHDN
LOGIC
LEVEL
1µF
IN 2
OUT 1
OUT 2
VREF
GND
FBP
IN 1
16V
1.6mA AT 3VIN
10mA AT 10VIN
16V/0V
16V/0V
1nF
604K
200pF
412K
L1: COILCRAFT DO2010-104ML
Conclusion
Figure 5. 3V–10V input to 16V dual half-bridge plus 16V output boost converter
3V–10V Input to 16V Output
MEMS Driver and Bias Supply
Figure 5 shows a 3V–10V input to
16V output converter, where the output drives the dual half-bridge and
also provides bias current for other
LTM4614/15, continued from page 26
to promote good thermal conductivity.
Figure 10 shows that thermal dissipation is well-balanced between the two
switching regulators.
Output Voltage Tracking
Tracking can be programmed using
the TRACK1 and TRACK2 pins. To
implement coincident tracking, at the
slave’s TRACK pin, divide the master
regulator’s output with a resistor
divider that is the same as the slave
regulator’s feedback divider. Figure 11
VIN
5V
82µF
+
circuitry. The converter in Figure 2
can be used in a similar fashion, but
the current available at the output is
reduced as the output voltage is increased. See the data sheet for details
about maximum output current.
shows a tracking design and Figure 12
shows the output. VOUT2 tracks VOUT1
in master-slave design with both
outputs ramping up coincidently. The
smooth start-up time is attributed to
the soft-start capacitor.
Conclusion
The cumbersome designs typical of
multivoltage regulation are a thing of
the past. The LTM4614 and LTM4615
µModule multiple-output regulators
can be easily fit into space-constrained
100µF
4.02k
22µF
VIN2
PGOOD2
VOUT2
PGOOD1
VOUT1
FB2
FB1
VIN1
10µF
RUN/SS1
10k
22µF
LTM4615
4.99k
TRACK2
RUN/SS2
LDO_IN
VIN
10k
LDO_OUT
EN3
GND
FB3
PGOOD3
3.32k
Figure 11. Output voltage tracking design example
32
VOUT2
1.2V
4A
100µF
COMP2
COMP1
TRACK1
VOUT2
system boards with far fewer components than discrete solutions. The
dual-output LTM4614 µModule regulator and triple-output LTM4615 are
small in size, have excellent thermal
dissipation and have high efficiency.
Independent input and output voltage
rails give these µModule regulators
unmatched flexibility. They can be
used in a variety of input-output combinations, including input and output
current sharing, output voltage tracking, and low noise output. L
10µF
=2
VIN1
VOUT1
1.8V
4A
The LT8415 is an ideal match for
driving low power MEMS. It integrates
a step-up converter power switch
and diode, a complementary dual
half-bridge, and a novel switching
architecture that minimizes power
dissipation. L
4.7µF
VOUT1
VOUT3
1V
1.5A
VOUT1
(1.8V)
500mV/DIV
VOUT2
(1.2V)
500mV/DIV
200µs/DIV
Figure 12. Start-up waveforms for the circuit
in Figure 11
Linear Technology Magazine • September 2009