September 2009 - Synchronous Boost Converter with Fault Handling Generates 5V at 500mA in 1cm2 of Board Space

DESIGN IDEAS L
Synchronous Boost Converter with
Fault Handling Generates 5V at
500mA in 1cm2 of Board Space
by Eddy Wells
Introduction
Lithium-Ion to 5V, 2.5W
Converter
current. Requiring only an inductor
and input/output filter capacitors,
the entire converter occupies only
about 1cm2 of board space. The IC
includes internal compensation, the
output divider, and soft-start circuitry
to minimize external components. In
shutdown, the LTC3529 disconnects
the output from the input and draws
less than 1µA from the source.
In fixed frequency PWM mode, the
efficiency for a typical Li-Ion source to
5V peaks at 92%, as shown in Figure
4, and remains above 80% for load currents greater than 30mA. The LTC3529
delivers up to 500mA of current at a
5V output and is therefore suitable for
both low and high power USB applications. As with any DC/DC converter,
a tradeoff exists between switching
4.7µH
2.5V
TO
4.2V
+
Li-Ion
3.3µF
LTC3529
370Ω
VIN
SW
OFF ON
RST
SHDN
VOUT
continued on page 35
100
0.9
90
0.8
80
EFFICIENCY
70
COUT
10µF
PGND
VOUT
5V
500mA
0.5
50
0.4
40
0.3
0.2
POWER LOSS
10
0
0.7
0.6
60
20
Figure 2. Li-Ion to 5V synchronous boost converter
Linear Technology Magazine • September 2009
The LTC3529 is robust to output short
circuits, a problem that arises as the
terminals of the IC are exposed to the
outside world to facilitate connection
between portable devices or system
board edge connectors. To defend
against output shorts, the LTC3529
shuts down when an excessive current
draw is detected through the internal
MOSFET switches continuously for
15ms.
Figure 4 illustrates the fault handling protocol of the LTC3529. Based
30
FAULT SNSGND
AUTO-RESTART OFF ON
Fault Detection
POWER LOSS (W)
Figure 2 shows an LTC3529-based
solution for converting from a single
lithium-ion battery or 3.3V board supply to 5V with up to 500mA of load
Figure 1. A tiny (1cm2) yet complete solution
drives USB On-The-Go bus power.
frequency, inductor value, output
capacitance and output ripple.
To allow the use of tiny external
components, the LTC3529 operates
at 1.5MHz and is stable with a 4.7µH
inductor and output capacitances
of 4.7µF (compatible with USB OnThe-Go specifications) or greater.
The Li-Ion-to-5V converter in Figure
3 utilizes a 10µF output capacitor,
and exhibits a peak-to-peak output
ripple of only 10mV. Low ESR and ESL
ceramic capacitors (such as X5R) are
recommended for both VIN and VOUT
bypassing.
EFFICIENCY (%)
Today’s power supply designs must
meet a number of stringent and
sometimes competing requirements.
In many cases the requirement for a
small solution is at odds with the need
for high conversion efficiency and the
need to safely deal with fault conditions. The LTC3529 step-up DC/DC
converter is designed to provide a
“no compromises” design, offering
high efficiency to minimize dissipated
heat and maximize battery life while
still maintaining a small footprint for
size-constrained power applications
requiring a 5V supply.
The LTC3529 can detect a shorted
output condition, disable the IC, and
report the event to a host microprocessor. This feature is important for
portable applications where devices
communicate with each other directly,
or system power applications where
voltages on multiple boards must
be monitored and maintained. As
shown in Figure 1, the LTC3529 offers a compact and efficient solution
consisting of only three tiny external
components.
1
10
100
LOAD CURRENT (mA)
0.1
0
1000
VIN = 3.6V
INDUCTOR = 4.7µH,
COOPER BUSSMANN SD25-4R7
Figure 3. Efficiency for the circuit in Figure 2
33
DESIGN IDEAS L
VIN
12V
L1
100µH
CIN
1µF
VIN
5V/DIV
SW
VIN
LTC3642
RUN
ISET
R1
1.47M
VFB
HYST
SS
GND
CIN: TDK C3225X7R1H105KT
COUT: MURATA GRM32DR71C106KA01
L1: TYCO/COEV DQ6530-101M
R2
49.9k
VSW
20V/DIV
COUT
10µF
VOUT
–24V
18mA
VOUT
10V/DIV
10ms/DIV
Figure 5. Generating a negative 24V output
voltage from a positive 12V input voltage
portable medical instruments and
certain automotive applications.
Positive-to-Negative Converter
The LTC3642 can produce a negative
output voltage from a positive input
voltage without the use of transformers
(see Figure 5). In this configuration,
the LTC3642 actually operates in an
inverting buck-boost mode. Its wide in-
LTC6930, continued from page 23
concern, and extreme accuracy is not
paramount. Such applications include
clocking microprocessors and microcontrollers, acting as a time base for
low speed serial communication protocols such as USB and RS232, digital
audio applications, clocking switching
power supplies and anywhere a general
purpose clock is needed.
Figure 6. The LTC3642’s wide input voltage swing makes it suitable
for generating a negative output from positive input voltage.
put voltage range, up to 45V, provides
sufficient headroom to generate any
negative voltage between –0.8V and
–40.5V. Figure 6 shows LTC3642 producing a –24V output from a 12V input
supply from start-up. The LTC3642
is inherently stable in this configuration with no external compensation
components required.
Conclusion
The LTC3642, LTC3631 and LTC3632
are a rugged DC/DC converters for use
in applications where a stable voltage
output must be produced from poorly
regulated high voltage rails. Their
compact size and high efficiency make
them easy to use in a wide variety of low
power applications, including mobile
and battery powered devices. L
Conclusion
When comparing clock power dissipation it is important to consider not just
the dissipation of the oscillator itself,
but also how the oscillator’s features
and start-up times effect the dissipation of the entire system. Crystal
oscillators not only dissipate more current than other solutions, but can have
other start-up and control characteristics that lead to power waste. When
the LTC6930’s on-the-fly frequency
programmability and one-clock-cycle
settling time are considered, it is clear
that it conserves much more system
power than its dissipation specification
would indicate L
FAULT
FAULT
VOUT
VOUT
IOUT
IOUT
LTC3529, continued from page 33
on a pin-selectable setting, the IC can
be configured to either periodically
attempt to power up (RST pin high,
Figure 4a), or remain shut down until power is cycled to the device (RST
pin low, Figure 4b). The waveform
indicating the fault condition is seen
at the Fault pin and is produced by
an internal open-drain device whose
input is pulled high in the event of
a fault. The Fault pin can either be
connected to a microprocessor or
drive an LED.
Conclusion
High conversion efficiency and the
ability to detect and handle output
shorts make the LTC3529 an ideal soLinear Technology Magazine • September 2009
10ms/DIV
4a. RST high: converter attempts power-up
every 15ms.
10ms/DIV
4b. RST low: converter remains shut down
until power is cycled.
Figure 4. A fault detection mechanism powers down
the converter, providing robustness to output shorts
lution for either peer-to-peer portable
applications or point-of-load board
power with robust fault handling.
The 1.5MHz switching frequency
and highly integrated design of the
LTC3529 yield compact solutions with
minimal design effort. L
35