A new generation of higher-current, low-voltage digital ICs have placed more stringent demand on supplies, driving performance improvements in LDO regulators to meet these new requirements

By Steve Knoth, senior product marketing engineer, Analog Devices,
Inc., Power by Linear Group

 

Although
the LDO regulator is often one of the least costly components in any given
system, it is often one of the most valuable components from a cost/benefit
basis. In addition to output voltage regulation, another of the LDO regulator’s key tasks is to protect
expensive downstream loads from harsh environmental conditions such as
voltage transients, power supply
noise, reverse voltage, current surges, etc.

In short,
its design must be robust and contain all of the protection features needed
to absorb the punishment from its environment while protecting the load. Many
low-cost LDO linear regulators do not have the necessary protection features
and, thus, fail, often causing
damage not only to the regulator
itself but also the downstream load.

Low-voltage
step-down conversion and regulation can be achieved via a variety of methods.

Today’s generation of fast, higher-current,
low-voltage digital ICs, such as FPGAs, DSPs, CPUs, GPUs, and ASICs, have
placed more stringent demands on supplies
that power the core and I/O channels. Traditionally, because charge pumps lack the necessary output
current and transient response, efficient switching regulators have been used
to power these devices. However,
switchers have potential noise interference issues,
and sometimes they have a slow
transient response and layout limitations.

Thus,
LDO regulators are an alternative in these applications as well as other low-voltage
systems. Thanks to recent product innovations and feature enhancements, LDO
regulators offer some performance benefits that make them more desirable.

Furthermore,
when it comes to powering noise-sensitive
analog/RF applications (commonly found in test and measurement systems, in which the
measurement accuracy of the machine
or equipment needs to be orders-of-magnitude better than the entity being
measured),
LDO regulators are generally
preferred over their switching counterparts. Low-noise LDO regulators power a wide range of analog/RF designs, including frequency synthesizers (PLLs/VCOs), RF mixers and
modulators, high-speed and high-resolution data converters, and precision
sensors. 

Some
ICs, such as operational amplifiers and instrumentation amplifiers, plus data
converters such as digital-to-analog converters (DACs) and analog-to-digital converters (ADCs), are referred
to as dual polarity because they
require two input supplies for power: one positive and one negative. The positive rail has typically been powered by a positive
voltage reference or, even better,
a linear or low-dropout
regulator. The negative rail was traditionally powered
by a negative switching regulator
or inverter. However, the inductor-based switcher can easily
introduce noise into the system.
With the advent of negative regulators, it has become advantageous to power the
negative system rail with a negative LDO regulator and take advantage of all of
the LDO regulator features (no inductor, lower noise, higher PSRR,
fast transient response, bulletproof protection). 

Older,
vintage LDO regulators have much worse PSRR and noise performance, and while
they can still be used to create these types of quiet supplies, it takes a lot
of extra components, board space, and design time to put a system together.
These extra components can also adversely affect the power budget depending on their characteristics (parasitic
resistance, etc.).

There
is another challenging system performance characteristic for customers using an
op amp, ADC, or other signal chain component: These ICs do not have infinite
supply rejection capability and, worse, the supply rejection capability can be
significantly lower at high frequency. In the past, this has meant using extra filtering components on the board,
which increases solution size. Furthermore, if a designer
is trying to obtain higher
accuracy, more trouble may result if the regulator
supply has excessive
noise, which causes unwanted
variation in a measurement scenario.

Plenty
of industry-standard linear regulators perform the low-dropout operation with a single voltage supply, yet most cannot achieve the combination of very low-voltage
conversion with low output noise, wide ranges of input/output voltages, and
extensive protection features.

PMOS
LDO regulators achieve the dropout and run on a single supply but are limited
at low input voltages
by the pass transistor’s VGS characteristics, and they
lack the many protection features supplied by high-performance regulators.
NMOS-based devices offer a fast transient response but require two supplies to
bias the device. NPN regulators offer a wide input and output voltage
range, but they require two supply voltages
or have a higher
dropout.

In contrast,
with the proper design architecture, a PNP regulator can achieve low dropout, high
input voltage, low noise, high PSRR, and very low-voltage conversion with
bulletproof protection, and all from a single-supply rail.

For best overall efficiency, many high-performance analog
and RF circuits are powered from LDO regulators postregulating the output of a switching
converter. This requires high PSRR and low output voltage noise at low input-to-output
differentials across the LDO regulator. An LDO regulator with high PSRR easily
filters and rejects noise from the switcher’s output without requiring bulky
filtering components. Furthermore, a device with low output voltage noise
across a wide bandwidth is beneficial for today’s modern rails in which noise
sensitivity is a key consideration. Low output voltage noise at high currents
is clearly a must-have specification.

It’s
clear that an LDO solution that solves the issues outlined herein should have
the following attributes:

  • Very low output noise
  • High PSRR across a broad range of frequencies
  • Low-dropout operation
  • Single-supply
    operation (for
    ease of use and relaxed supply sequencing challenges)
  • Fast transient response time
  • Operation over a wide input/output voltage range
  • Moderate output current capability
  • Excellent thermal performance
  • Compact footprint

To address these specific needs,
Analog Devices introduced its LT304x family
of ultra-high-PSRR, ultra-low-noise positive LDO regulators. The newest member
is the complementary LT3094,
an ultra-low-noise, ultra-high-PSRR
low-dropout-voltage 500-mA negative linear regulator. This device
is a negative version of the popular
500-mA LT3045 (LT3042 for 200 mA).

The
LT3094’s unique design features ultra-low
spot noise of only 2 nV/√Hz at 10 kHz and 0.85-µV rms integrated output noise across a wide 10-Hz to 100-kHz bandwidth. PSRR performance is exceptional:
Low-frequency PSRR exceeds 100 dB out to nearly 4 kHz, and high-frequency PSRR exceeds 70 dB out to 2 MHz, quieting
noisy or high-ripple-input supplies.

The
LT3094 utilizes a proprietary LDO
architecture: a precision current source reference followed by a
high-performance unity-gain buffer, which results in virtually constant
bandwidth, noise, PSRR, and load regulation performance independent of output
voltage. In addition, this architecture permits paralleling of multiple LT3094s to further decrease noise,
increase output current, and spread heat on a printed circuit board (PCB).

The
LT3094 delivers up to 500-mA output
current with a 230-mV dropout voltage at full load across a wide –2-V to –20-V
input voltage range. The output voltage range is 0 V to –19.5 V, and the output
voltage tolerance is highly accurate at ±2% over line, load,
and temperature. The device’s
wide input and output
voltage ranges, high bandwidth, high PSRR, and ultra-low
noise performance are ideal for powering noise-sensitive applications such as PLLs, VCOs, mixers, and LNAs; very low-noise
instrumentation such as test and measurement and high-speed/high-precision data
converters; medical applications such as imaging
and diagnostics and precision power supplies; and postregulators for switching power
supplies.

The LT3094 operates
with a small, low-cost, 10-µF ceramic output
capacitor that optimizes stability and the transient response. A single
resistor programs the external precision current limit (±10% overtemperature). The device’s
VIOC pin controls an upstream regulator to minimize power dissipation and
optimize PSRR. A single SET pin capacitor lowers output noise and provides
reference soft-start functionality, preventing output voltage overshoot at
turn-on. Moreover, the device’s internal protection circuitry includes
internal current limit with foldback
and thermal limit with
hysteresis. Other features include fast startup capability (useful if a
large-value SET pin capacitor is used) and a power-good flag (the industry’s first negative LDO
regulator with this function) with a programmable threshold to indicate output
voltage regulation.

 

Fig. 1: LT3094 typical application
schematic and features.

The
LT3094 is available in thermally
enhanced 12-lead, 3 × 3-mm DFN and MSOP packages, both with a compact
footprint. The E- and I-grade versions are available from stock with an
operating junction temperature of –40°C to 125°C.

The LT3094 requires
an output capacitor for stability. Given its high bandwidth,
it is recommended to use low equivalent series resistance (ESR) and equivalent
series inductance (ESL) ceramic capacitors. A
minimum 10-µF output capacitance with an ESR below 30 mΩ and an
ESL below 1.5 nH is required for stability. Given the high PSRR and low-noise performance attained with using a single 10-µF ceramic output capacitor, larger values of output
capacitors only marginally improve the performance because the regulator bandwidth decreases with increasing
output capacitance — hence, there is little to be gained by using larger than the minimum
10-µF output capacitor.
Nonetheless, larger values
of output capacitance do decrease
peak output deviations during a load transient.

 

ADI LDO regulator figure 2

Fig. 2: LT3094 PSRR performance.

ADI LDO regulator figure 3

Fig. 3 :
LT3094 output noise performance.

Higher
output current is obtained by paralleling multiple LT3094s. Tie all SET pins together and all IN pins together.
Connect the OUT pins together using
small pieces of PCB trace (used as a ballast resistor) to equalize currents in the LT3094. More than two LT3094s can also be paralleled for even
higher output current and lower output noise. The output noise decrease is
proportional to the square root of the number of devices in parallel. Paralleling multiple LT3094s
is also useful
for distributing heat on
the PCB. For applications with a high input-to-output voltage differential, an
input series resistor or resistor in parallel with the LT3094 can also be used to spread heat. See Fig. 4 for a parallel circuit implementation.

ADI LDO regulator figure 4

Fig. 4: LT3094 parallel operation.

Table 1
shows the members of ADI’s ultra-high-PSRR, ultra-low-noise family of LDO
regulators.

ADI LDO regulator table 1

 

The
positive 200-mA LT3042, 500-mA LT3045, and, now, the new complementary LT3094
500-mA negative LDO offer breakthrough noise and PSRR performance. These attributes, coupled
with their wide voltage range,
low dropout voltage, extensive protection features/robustness, and ease of use,
make them ideal for powering
noise-sensitive bipolar
positive/negative rails in
test and measurement or medical imaging systems, for example. With their
current reference-based architecture, noise and PSRR performance remain independent of the output
voltage. Additionally, multiple devices can be directly
paralleled to further
reduce output noise,
increase output current, and spread heat on the PCB. The LT3042, LT3045, and LT3094 save time and cost while improving
application performance.

Steve Knoth is a senior product
marketing engineer in Analog Devices’ Power by Linear Group. He is responsible
for all power management integrated circuit (PMIC) products, low-dropout (LDO) regulators, battery chargers, charge pumps,
charge pump-based LED drivers, supercapacitor
chargers, low-voltage monolithic
switching regulators, and ideal diode devices. Prior to joining Linear
Technology (now ADI) in 2004, Steve
held various marketing and product engineering positions since 1990 at Micro
Power Systems, Analog Devices, and Micrel
Semiconductor. He earned his bachelor’s degree in electrical engineering in
1988 and a master’s degree in physics in 1995, both from San Jose State
University. Steve also received an M.B.A. in technology
management from the University of Phoenix in 2000. In addition to enjoying time with his kids, Steve can be
found tinkering with pinball/arcade games or muscle cars and buying, selling,
and collecting vintage toys and
movie/sports/automotive memorabilia. He can be reached at
steve.knoth@analog.com.