Jun 2007 - 3µA Quiescent Current LDO Improves Efficiency for Low Power Circuits in Industrial, Automotive and Battery-Powered Systems

L DESIGN FEATURES
3µA Quiescent Current LDO Improves
Efficiency for Low Power Circuits in
Industrial, Automotive and
Battery-Powered Systems
by Sam Rankin
Introduction
Ultralow Quiescent Current
PNP LDO
Figure 1 shows a typical application
for the LT3009, a 3µA quiescent current low dropout linear regulator in
tiny 2mm × 2mm DFN and 8-lead
SC70 packages. Its ultralow 3µA quiescent current is well controlled—it
does not rise excessively in dropout
as happens with many regulators.
Quiescent current is less than 5% of
1.4
0.8
0.6
0.4
0.2
1µF
2.8M
1%
LT3009
ADJ
VOUT
3.3V
20mA
619k
1%
Figure 1. New 3µA quiescent current low dropout regulator
1000
GND CURRENT (µA)
output current at 20mA IOUT, even in
dropout (Figure 2).
The LT3009 can supply up to 20mA
from input supplies ranging from 1.6V
to 20V to output voltages ranging from
0.6V to 19.5V. Dropout voltage on the
LT3009 is only 280mV while delivering up to 20mA of output current. It
can be put into a low power shutdown
state by pulling the SHDN pin low. In
shutdown state, the already low quiescent current is reduced to the leakage
currents of the internal transistors.
This leakage, typically a few nA at
room temperature, stays below 1µA
over the entire operating temperature
range. Low quiescent current and tiny
package size does not translate into
poor performance in the LT3009. The
LT3009 features industry leading load,
line, and temperature regulation (see
Figures 3, 4 and 5)
VIN = 3.8V
VOUT = 3.3V
100
10
1
0.001
0.01
0.1
1
LOAD (mA)
10
100
Figure 2. GND Pin current vs ILOAD
Aside from the output voltage
setting resistors, the only external
components required are input and
output bypass capacitors. Internal
frequency compensation in the LT3009
stabilizes the output for a wide range
of capacitors. A minimum of 1µF of
0.6
0.612
0.610
0.608
0.5
0.4
0.3
0.2
0.606
0.604
0.602
0.600
0.598
0.596
0.594
0.592
0.1
0
0.590
–40 –20
0
20 40 60 80
TEMPERATURE (°C)
100 120
Figure 3. Load regulation vs temperature
1µF
OUT
GND
LINE REGULATION (mV)
LOAD REGULATION (mV)
1.0
IN
SHDN
∆IL = 1µA TO 20mA
VOUT = 600mV
VIN = 1.6V
1.2
–0.2
VIN
3.75V TO
20V
ADJ PIN VOLTAGE (mV)
Many electronic systems spend much
of their time in an idle state, waiting
for something to happen. Industrial
remote monitoring systems and keepalive circuits are but two examples.
Many of these systems depend on
battery power, so a high efficiency
power supply is paramount to preserve battery life. Efficiency during
quiescent state is of particular importance since active operation may
draw milliamps while quiescent operation only microamps. Small size and
reverse output and input protection
capabilities are also desirable features
in a power supply. This is a demanding
combination of power supply requirements, but there is an easy way to
satisfy them with one device.
0
–40 –20
0
20 40 60 80
TEMPERATURE (°C)
100 120
Figure 4. Line regulation vs temperature
0.588
–40 –20
0
20 40 60 80
TEMPERATURE (°C)
100 120
Figure 5. Output voltage vs temperature
Linear Technology Magazine • June 2007
DESIGN FEATURES L
output capacitance is required for
stability, and almost any type of output capacitor can be used. Even small
ceramic capacitors with low ESR can
be used without the additional series
resistance commonly required with
other regulators. The combination of
small package size and the ability to
use small ceramic capacitors enable
the LT3009 to fit almost anywhere.
The LT3009 has a number of protection features to safeguard itself
and sensitive load circuits. Should the
input voltage become reversed (due to
a battery inserted backwards or a fault
on the line, for example), current flow
from the IN pin is limited by a 100k
resistance and no negative voltage is
seen at the load. No external protection
diodes are necessary when using the
LT3009. With a reverse voltage from
output to input, the LT3009 acts as
though it has a 500k limiting resistor
in series with two diodes from output
to input to limit reverse current flow.
For dual-supply applications where
the regulator load is returned to a
negative supply, the OUT and ADJ
pins can be pulled below ground (by up
to a 20V input-to-output differential)
while still allowing the device to start
and operate. The LT3009 also includes
protection features found standard on
linear regulators such as current and
thermal limiting.
The Ideal Solution for
Remote Monitoring
The LT3009 provides an optimum
solution for remote monitoring applications. The duty cycle of many of
these applications is very short—they
spend most of their time in shutdown,
waking briefly to take and communicate measurements, then returning
immediately to shutdown. Aside from
LINE POWER
VLINE
12V TO 15V
DCHARGE
IN
1µF
OUT
5V
IN
SUPERCAP
1µF
SHDN
ADJ
GND
4.32M
1%
LT3009
ADJ
GND
1µF
FAULT
GND
TO
MONITORING
CENTER
590k
1%
Figure 6. Typical last-gasp circuit
the typical supply regulation requirements required by sensitive analog
circuitry (tight supply regulation,
quiet supply, load protection, etc.), the
principle supply requirement is low
quiescent power consumption. With
its 3µA quiescent current coupled with
industry leading supply regulation
capability and myriad of protection
features, the LT3009 fits the bill.
A typical remote monitoring application used frequently in utility
meters is a “last-gasp” circuit, shown
in Figure 6. In this application, a 12V
to 15V supply derived from line power
charges a large capacitor (SuperCap)
through a diode and a current limiting resistor. This stored voltage on the
SuperCap provides input voltage for
the LT3009. The LT3009 provides a
quiet, well-regulated 5V supply to the
analog fault detection circuits as well
as a digital communication module
used to send distress signals to the
remote monitoring center. The fault
detection circuitry is typically active for
only a few hundred milliseconds every
15-minute detection cycle. In the event
of a line failure, the ultralow quiescent
current of the LT3009 enables the SuperCap to provide enough power to the
3.3V
1µF
LOAD:
SYSTEM MONITOR,
VOLATILE MEMORY, ETC
619k
1%
Figure 7. Typical keep-alive power supply
Linear Technology Magazine • June 2007
PWR
OUT
SHDN
2.8M
1%
LT3009
LINE
INTERRUPT
DETECT
RLIMIT
NO PROTECTION
DIODES NEEDED!
VIN
12V
SENSE
fault detection and communications
circuitry for several detection cycles.
The 3µA quiescent current of the
LT3009 reduces the required size and
cost of the SuperCap while simultaneously extending the life of the detection
and communications circuits after
line failure. Additionally, with its
output regulation of ±2% over load
line and temperature, the LT3009 can
do double duty as a highly accurate
voltage reference for the fault detection circuits.
An Excellent Choice for
Keep-Alive Power Supplies
Switching power supplies provide
robust local low voltage/high current
power from high voltage rails, but
switching power supplies are overly
complex for the low power keep-alive
circuits that typically run only a few
milliamps of current. There are many
such low current applications in industrial, monitoring, security systems,
smoke detectors, and other always-on
circuits. For many of these applications, the LT3009 provides a relatively
simple and inexpensive solution.
A typical keep-alive application is
shown in Figure 7. A 12V rail powers
a keep-alive circuit for monitoring or
other purposes. Low quiescent current is critical here to reduce battery
drain. A battery backup keeps the
output alive when a fault on the input
occurs. Should a fault on the 12V rail
occur, the battery backup takes over.
The internal protection of the LT3009
limits current flow from the output
back to the input, removing the need
for protection diodes.
continued on page 24
L DESIGN FEATURES
10A
VRTN_A
10A
VRTN_B
340k
FDS3672
FDS3672
–48V/5.5A
High side and Low Side
Diode-ORs for Telecom
340k
IN1
IN2
GATE1
GATE2
MON1
SET
GND
12.7k
OUT
VDSFLT
FUSEFLT1
FUSEFLT2
PWRFLT1
PWRFLT2
LTC4355
MON2
12.7k
pin spacing sometimes desirable in
higher voltage applications.
LOAD
12k
VCC
LTC4354
DA
–48V_A
–48V_B
7A
7A
DB
2k
GA
FAULT
GB
2k
VSS
1µF
FDS3672
FDS3672
Figure 3. –48V/5.5A positive and negative supply diode-ORing for telecom systems.
Large supply variations and transients
are easily accommodated by the wide
operating voltage ranges of these two
parts, 4.5V to 80V for the LTC4354 and
9V to 80V (100V absolute maximum)
for the LTC4355.
This circuit combines all fault indicators to drive one optoisolator. If an
input supply falls to less than 36V or
the forward voltage drop across one
of the positive-side MOSFETs exceeds
0.25V, the LTC4355’s PWRFLT1 or
PWRFLT2 pin pulls low to signal the
fault. If a positive-side fuse blows
open, the LTC4355 indicates a fault by
pulling the FUSEFLT1 or FUSEFLT2
pin low. Finally, if the forward voltage
across a low side MOSFET exceeds
0.26V, the LTC4354’s FAULT pin
drives an NPN that turns off the same
optoisolator driven by the LTC4355’s
pins.
Because the high side fuses have
lower current ratings than the return
fuses, the high side fuses blow first
under most fault conditions. With the
return fuses intact, system potentials
tend to settle near ground after a fuse
blows open.
The VDSFLT pin is not shown in
this schematic. Since the PWRFLT1
or PWRFLT2 pin pulls low when the
VDSFLT pin pulls low, VDSFLT is
redundant in this application. Furthermore, this schematic is capable of
accommodating not just the smaller
DFN-14 package, but also the larger
SO-16 package. While the SO-16 lacks
a VDSFLT pin, it features the wider
Many –48V telecom systems, including those that conform to the new
AdvancedTCA specification, require
ORing circuits on both the high and
low side of the redundant power feeds.
A few simple modifications convert
the +48V solution in Figure 2 to the
–48V solution in Figure 3. The +48V
supply input becomes the return
feed, VRTN, and the returns in the
+48V system now serve as the –48V
input feeds. The 10A and 7A fuses
have been swapped, placing the 10A
fuse in the high side return path. As
a result, most fault conditions cause
the high side 7A fuse to blow before
the low side 10A fuse. Consequently,
system potentials generally settle near
VRTN after a fuse blows. The minimal
circuit in Figure 3 does not connect
the fault pins. If desired, faults can
be monitored with a circuit similar to
that in Figure 2.
Conclusion
The LTC4355 frees up PCB area by
reducing power dissipation and the
size of associated heat sinks in applications that require supply ORing. Its
wide 9V to 80V supply operating range
and 100V absolute maximum rating
accommodate a broad range of input
supply voltages with ample margin for
supply variations and transients. In
addition, the ability to provide system
health monitoring functions makes it
especially well suited to high-availability applications. Those systems
that require both high side and low
side ORing can combine the LTC4355
with the LTC4354 to form a complete
solution. L
LT3009, continued from page Conclusion
The LT3009 offers ultralow quiescent
current, a shutdown mode, and wide
input and output voltage ranges in tiny
2mm × 2mm DFN and SC70 packages
without sacrificing performance or
24
reliability. A stable output is available
with a wide range of output capacitors,
including small ceramics. Internal
protection circuitry in the LT3009
eliminates the need for external protections diodes, further saving space
and lowering cost. Competing devices
can’t come close to the performance
and advantages that the LT3009 offers in the world of ultralow quiescent
current regulators. L
Linear Technology Magazine • June 2007