LINEAR TECHNOLOGY LT8415 Technical data

DESIGN IDEAS L

SW CAP

GND

FBP

OUT 2

OUT 1

IN 2

IN 1

LT8415

C1

2.2µF

LOGIC
LEVEL

L1

100µH

22nF

0.1µF

1nF

200pF

V

IN

2.6V to 5V

VCCV

OUT

V

REF

SHDN

137K

887K

34V/0V

34V/0V

L1: COILCRAFT DO2010-104ML

IN1/IN2

1V/DIV

OUT1

10V/DIV

OUT2

10V/DIV

5µs/DIV

IN1/IN2

1V/DIV

OUT1

10V/DIV

OUT2

10V/DIV

5µs/DIV

V

OUT

IN2 OUT2

V

OUT

V

IN

IN1 OUT1

ENABLE

V

OUT

40V MAX

BOOST

CONVERTER

LT8415

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,

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.

2.6V–5V Input to 34V Output
MEMS Driver

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.

Figure 1. Simplified block diagram of the
LT8415

Linear Technology Magazine • September 2009

Figure 2. 2.6V–5V input to 34V dual half-bridge boost converter

Figure 3. Turn-on delay and rise time for OUT1
and OUT2

Figure 4. Turn-off delay and fall time for OUT1
and OUT2

3131

L DESIGN FEATURES

SW CAP

GND

FBP

OUT 2

OUT 1

IN 2

IN 1

LT8415

2.2µF

LOGIC
LEVEL

L1

100µH

0.1µF

1µF

16V

1.6mA AT 3V

IN

10mA AT 10V

IN

1nF

200pF

V

IN

3V to 10V

VCCV

OUT

V

REF

SHDN

604K

412K

16V/0V

16V/0V

L1: COILCRAFT DO2010-104ML

V

OUT1

(1.8V)

500mV/DIV

V

OUT2

(1.2V)

500mV/DIV

200µs/DIV

V

IN1

V

IN1

V

OUT1

1.8V
4A

V

OUT2

1.2V
4A

V

OUT1

V

OUT3

1V

1.5A

V

IN

5V

V

IN

V

OUT2

22µF

10µF

100µF

4.02k

10k

LTM4615

4.7µF

3.32k

V

IN2

P

GOOD1

P

GOOD2

V

OUT2

FB2

COMP2

V

OUT1

FB1

COMP1

TRACK1

RUN/SS1

LDO_IN

EN3

TRACK2

RUN/SS2

LDO_OUT

FB3

P

GOOD3

100µF

22µF

GND

10k

4.99k

10µF

=2

82µF

+

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

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.

Integrated Resistor Divider

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.

Conclusion

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

LTM4614/15, continued from page 26

to promote good thermal conductivity.
Figure 10 shows that thermal dissipa
tion 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

32

shows a tracking design and Figure 12

-

shows the output. V
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

Figure 11. Output voltage tracking design example

OUT2

tracks V

OUT1

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.

Figure 12. Start-up waveforms for the circuit
in Figure 11

Linear Technology Magazine • September 2009

L