Dual Output DC/DC Controller Combines Digital Power System Management with Analog Control Loop for ±0.5% VOUT Accuracy

January 2012
I N
T H I S
I S S U E
integer-N synthesizer
with integrated VCO in a
4mm × 5mm package 9
supercapacitor-based
power supply ride-through
system 15
ultralow-EMI, µModule
Volume 21 Number 4
Dual Output DC/DC Controller
Combines Digital Power System
Management with Analog Control
Loop for ±0.5% VOUT Accuracy
Gregory Manlove
regulator is EN55022 class
B certified 20
UMTS base station receiver
in half-inch square 23
negative voltage diode-OR
controller tolerates inputs
beyond 300V 30
Though power management is critical to the reliable operation of
modern electronic systems, voltage regulators are perhaps the last
remaining “blind spot” in today’s digitally managed systems. Few
regulators have the means for direct configuration or monitoring
of key power system operating parameters. As a result, power
designers who want complete digital control must use a mixed bag
of sequencers, microcontrollers and voltage supervisors to program
basic regulator start-up and safety functions. Digitally programmable
DC/DC converters are available, most notably those with VID output
voltage control designed for VRM core
power supplies, but these specifically
targeted converters do not directly
communicate important operating
parameters, such as real-time currents.
Caption
The LTC3880 in a digitally managed power system.
The LTC®3880/-1 solves the problem of complex
power system management by combining a dual output synchronous step-down DC/DC controller with
a comprehensive power management feature set
accessed via the I2C-based PMBus. PMBus can be used
to set the output voltage, margin voltages, switching
frequency, sequencing and a number of other operating parameters (see “PMBus Control” below).
(continued on page 4)
w w w. li n ea r.com
A principal benefit of digital power system management is reduced design
cost and faster time to market. The LTC3880/-1 greatly simplifies the design
of complex multirail systems with the free, downloadable LTpowerPlay™
software, a comprehensive PC-based development environment.
(LTC3880, continued from page 1)
A principal benefit of digital power system
management is reduced design cost and
faster time to market. The LTC3880/-1
greatly simplifies the design of complex
multirail systems with the free, downloadable LTpowerPlay™ development software,
a comprehensive PC-based development
environment (see Figure 6). In-circuit
testing (ICT) and board debug require only
a few clicks of the mouse—no need to
solder in “white wire” fixes. The results
of your design are immediately accessible,
thanks to the availability of real-time
The LTC3880 also allows monitoring of
the supplies via a 16-bit data acquisition
system, which supplies digital read back
of input and output voltages and currents, duty cycle and temperature, including peak values of important parameters.
The LTC3880 also includes extensive
fault logging capability via an interrupt
flag along with a nonvolatile memory,
“black box” recorder, which stores the
state of the converter’s operating conditions immediately prior to a fault.
Figure 1. Dual output regulator using
external power MOSFETs
10µF
M1
D0
BOOST0
4.99k
2.0k
10k
1µF
10k
10k
VDD33
10k
10k
10k
10k
10k
M2
0.1µF
L1
0.56µH
SW1
BG0
M4
BG1
PGND
SYNC
SDA
VIN
6V TO 24V
BOOST1
SW0
M3
Perhaps most significantly, DC/DC converters with digital management functionality allow designers to develop green
power systems that optimize energy
usage while meeting system performance
targets (compute speed, data rate, etc.).
Optimization can be implemented at
the point of load, at the board, rack
and even at installation levels, reducing
22µF
1µF
D1
TG1
TG0
0.1µF
L0
1.0µH
INTVCC
VIN
telemetry data, making it possible to
predict power system failures and immediately implement preventive measures.
1.58k
VDD25
1µF
LTC3880
SCL
ALERT
VOUT0_CFG
GPIO0
VTRIM0_CFG
GPIO1
VOUT1_CFG
10k
24.9k
10k
20k
15.8k
11.3k
23.2k
12.7k
SHARE_CLK VTRIM1_CFG
RUN0
ASEL
RUN1
FREQ_CFG
WP
2.0k
TSNS0
ISENSEO+
+
ISENSEO–
ISENSE1–
VSENSEO+
VSENSE1
VSENSEO–
ITH1
ITHO
VDD33 SGND VDD25
4700pF
10nF
4.99k
0.22µF 1.58k
0.22µF
VOUT0
3.3V
15A
COUT0
530µF
TSNS1
ISENSE1+
220pF
1µF
1µF
VOUT1
1.8V
20A
+
2200pF
220pF
4.99k
10nF
D0, D1: CMDSH-3TR
COUT0, COUT1: C330μF SANYO 4TPF330ML PLUS 2× 100µF AVX 12106D107KAT2A
L0, L1: VISHAY IHLP-4040DZ-11 1µH, 0.56µH
M1, M2: RENESAS RJK0305DPB
M3, M4: RENESAS RJK0330DPB
4 | January 2012 : LT Journal of Analog Innovation
COUT1
530µF
design features
ANALOG CONTROL LOOP
ANALOG CONTROL LOOP
DIGITAL CONTROL LOOP
ANALOG
CURRENT
WAVEFORM
VOUT
100mV/DIV
(AC-COUPLED)
FIXED fSW
205mV
IOUT
10A/DIV
(15A LOAD STEP)
DIGITAL CONTROL LOOP
DIGITAL
RAMP
100µs/DIV
200µs/DIV
COUT = 2 × 330µF (POSCAP, 9mΩ) + 2 × 100µF
COUT = 4 × 330µF (POSCAP, 9mΩ) + 2 × 100µF
FIXED fSW
Figure 3. Comparative responses of analog and digital control loops to a 15A
transient load step. The analog control loop requires only half the output
capacitance of the digital loop while producing far superior settling times.
Figure 2. The LTC3880’s analog control loop vs a digital control loop
infrastructure costs and the total cost of
ownership over the life of the product.
the ADC quantization-related errors found
in products utilizing digital control.
ANALOG CONTROL LOOP ENSURES
BEST-IN-CLASS REGULATOR
PERFORMANCE
To provide best performance regulation,
the LTC3880 sticks to a precision reference
and temperature-compensated analog
current mode control loop to produce a
tight ±0.5% DC output voltage accuracy.
The analog control loop makes for easy
compensation, which is calibrated to be
independent of operating conditions,
yields cycle-by-cycle current limit, and
produces a fast and accurate response to
line and load transients—without any of
The LTC3880 features an on-chip regulator for increased integration, whereas
the LTC3880-1 allows for an external bias voltage for highest efficiency.
Both parts are available in a thermally
enhanced 6mm × 6mm QFN-40 package
with either a –40°C to 105°C operating
junction temperature range (E-grade)
or a –40°C to 125°C operating junction temperature range (I-grade).
The LTC3880/-1 is digitally programmable
for numerous functions including the
output voltage, current limit set point,
and sequencing. The control loop, though,
remains purely analog, which offers the
best loop stability and transient response
without the quantization effect of a digital
control loop. Figure 2 compares the ramp
curves of a controller IC with an analog
feedback control loop to one with a digital
feedback control loop. The analog loop
has a smooth ramp, whereas the digital
loop has discrete steps that can result
in stability problems, slower transient
response, more required output capacitance in some applications and higher
output ripple and jitter on the PWM control
signals due to quantization effects.
Figure 4. It’s easy to set up a
complete development platform
with LTpowerPlay software
USB
DC1613A
USB to PMBus
Controller
DC1709A
Socketed
Programming Board
DC1590B
LTC3880
Demo Board
Demonstration
Kit
or
Socketed
Programming
Customer Board
with
LTC3880/LTC3880-1
or
In-Circuit Serial
Programming
In fact, when put up against a comparable IC with a digital control loop, the
LTC3880’s analog control loop using 50%
less output capacitance has better stability
with a shorter settling time. Additionally,
the digital control transient response
has an oscillation prior to settling, due
to the quantization effects caused by
its finite ADC resolution. Figure 3 shows
the transient response of the LTC3880’s
analog control loop compared to that of
January 2012 : LT Journal of Analog Innovation | 5
Configurations are downloaded to internal EEPROM via the I2C serial interface
supported by Linear Technology’s LTpowerPlay PC-based development
software. After the configuration file is stored on-chip in nonvolatile memory,
the controller powers up autonomously without burdening the host.
a competitor’s digital control loop. Note
that the LTC3880 yields cleaner results
with approximately half the output
capacitance of the digital controller.
The LTC3880 is designed so the loop gain
does not change when its configuration
file is modified. When the output voltage or the current limit is modified, the
transient response is unaffected and the
compensation loop needs no adjustment.
PMBus CONTROL
PMBus functions include the abil-
The LTC3880/-1 features digital programming and read back for real-time control
and monitoring of critical point-of-load
converter functions. Configurations are
downloaded to internal EEPROM via the
I2C serial interface supported by Linear
Technology’s LTpowerPlay PC-based
development software. Figure 4 shows the
LTpowerPlay development platform with
a USB to I2C/SMBus/PMBus adapter. After
the configuration file is stored on-chip in
nonvolatile memory, the controller powers
up autonomously without burdening the
host. Configuring a board is a simple task
that requires zero firmware development.
ity to program specific power supply
management parameters including:
•output voltage and margin voltages
•temperature-compensated current limit
threshold based on inductor temperature
•switching frequency
•overvoltage and undervoltage high speed
supervisor thresholds
•output voltage on/off time delays
•output voltage rise/fall times
•input voltage on/off thresholds
•output rail on/off
•output rail margin-hi/margin-lo
•responses to internal/external faults
Figure 5. LTpowerPlay and
PMBus used to control 15
or more rails.
SYSTEM HOST
PROCESSOR
WITH
LTpowerPlay™
I2C/PMBUS
RAIL 1
LTC3880
•fault propagation
RAIL 2
In addition, PMBus functions allow monitoring of power
supply operation including:
RAIL 3
•output/input voltage
LTC3880
•output/input current
LTC2978
LTC2974
TO OTHER LTC3880s, LTC2974s AND LTC2978s
6 | January 2012 : LT Journal of Analog Innovation
DC/DC
RAIL 4
•internal die temperature
DC/DC
RAIL 5
•external inductor temperature
DC/DC
RAIL 6
DC/DC
RAIL 7
DC/DC
RAIL 8
•fault status
DC/DC
RAIL 9
•system status
DC/DC
RAIL 10
•peak output current
DC/DC
RAIL 11
DC/DC
RAIL 12
DC/DC
RAIL 13
DC/DC
RAIL 14
DC/DC
RAIL 15
•part status
•peak output voltage
•peak internal/external temperature
•fault log status
design features
Linear Technology PMBus controllers such as the LTC3880 and companion ICs such
as the LTC2978 make it easy to program power-up and power-down sequencing
for any number of supplies. By using a time-based algorithm, users can dynamically
sequence rails on and off in any order with simple programmable delays.
Figure 6. Complex,
multirail power systems
simplified. LTpowerPlay
puts complete power
supply control at your
fingertips.
PUTTING TOGETHER A MULTIRAIL
SYSTEM
A large multirail power board is normally composed of an isolated intermediate bus converter, which converts
48V, 24V or other relatively high voltage
from the backplane to a lower intermediate bus voltage (IBV), typically 12V,
which is distributed around a PC card.
Individual point-of-load (POL) DC-DC converters step down the IBV to the required
rail voltages, which normally range from
0.5V to 5V with output currents ranging
from 0.5A to 120A. Figure 5 shows how a
multi-rail system can be controlled with
various Linear Technology controllers
and DC/DC converters PMBus devices. The
point of load DC/DC converters can be
self-contained modules, monolithic devices
or solutions comprising DC/DC controller ICs, associated inductors, capacitors
and MOSFETs. These rails normally have
strict requirements for sequencing, voltage accuracy, overcurrent and overvoltage limits, margining and supervision.
The sophistication of power management
is increasing. It is not uncommon for
circuit boards to have over 30 rails. These
boards are already densely populated so
adding digital power system management
circuitry must require minimal board
space and external pins. The system must
be easily modified by the user or a system host processor. The LTC3880 works
seamlessly with other Linear Technology
PMBus supervisors, companion ICs and
Linear Technology regulators for optimal
control of complex boards. These systems
operate autonomously after initial configuration or communicate with the host for
command, control and to report telemetry.
Linear Technology PMBus controllers such
as the LTC3880 and companion ICs such
as the LTC2978 make it easy to program
power-up and power-down sequencing
for any number of supplies. By using a
time-based algorithm, users can dynamically sequence rails on and off in any
order with simple programmable delays.
Sequencing across multiple chips is made
possible using the 1-wire SHARE_CLK bus
January 2012 : LT Journal of Analog Innovation | 7
PMBus chips can be added later without having to worry about system constraints
such as a limited number of connector pins. Multiple addresses are supported
in PMBus allowing over 100 unique devices on the same I2C bus.
and one or more of the bidirectional
general purpose IO (GPIO) pins. This
greatly simplifies system design because
rails can be sequenced in any order.
Additional PMBus chips can be added later
without having to worry about system
constraints such as a limited number of
connector pins. Multiple addresses are
supported in PMBus allowing over 100
unique devices on the same I2C bus.
Rail sequencing to the on state can be
triggered in response to a variety of
conditions. For example, the LTC3880
and LTC2978 can auto-sequence when the
intermediate bus voltage exceeds a programmed threshold (VIN(ON)). Alternatively,
rail on sequencing can be initiated
in response to the rising edge of the
RUN/CONTROL pin. Rail on sequencing can
also be initiated by a PMBus command.
The GPIO pins on the LTC3880 can be
shared with fault pins from LTC PMBus
companion ICs to control fault response
dependencies between rails. For example
the system can be configured such that
a fault on one rail can initiate the shutdown of any number of rails. If the fault
response is configured for “immediate off
no retry” and a fault occurs, the host must
take action for the rails to be restarted.
Alternatively, if the fault response is set
to “immediate off infinite retry” and a
fault occurs, the rail attempts to power
up autonomously with a user programmable delays in a hiccup mode. The fault
response can also be set to “ignore,”
where the ALERT pin is pulled low in
8 | January 2012 : LT Journal of Analog Innovation
response to a fault, to alert the host of an
issue, but the power supply continues to
deliver power to the load. The GPIO pins
can also be configured as power good
status pins or as the fast UV comparator output for event-based sequencing.
LTpowerPlay DEVELOPMENT SYSTEM
Control of the LTC3880 is fully supported
by the LTpowerPlay PC-based software
development system, which allows a
designer to modify the configuration
settings for all Linear Technology PMBus
products in real time—no need to manually rewire the board. Figure 6 shows
LTpowerPlay in action, controlling a
number of functions for multiple devices,
such as the output voltage, protection
limits and on/off ramps. Some waveforms
are displayed including the sequencing of
multiple rails and telemetry plots. A fault
condition is indicated with the offending
rail in red and any affected rails in yellow.
LTpowerPlay is available as a free download at www.linear.com/ltpowerplay.
LTpowerPlay works in conjunction with
other Linear Technology controller and
companion ICs in order to quickly and easily configure multiple rail power systems.
CONCLUSION
The LTC3880/-1 combines best-in-class
analog switching regulators with precision data conversion and a flexible digital
interface for unsurpassed performance.
Multiple LTC3880s can be used with
other LTC products to create optimized
multi-rail digital power systems. All
Linear Technology PMBus products are
supported by the easy-to-use LTpowerPlay
software development system.
Digital control over analog power supplies enables designers to get their systems
up and running quickly providing an easy
way to monitor, control and adjust supply
voltages, limits and sequencing. Production
margin testing is easily performed using
a couple of standard PMBus commands.
Debug is also simplified because the rail
status is communicated over the bus.
Power system data can be sent back to
the OEM providing information about
the power supplies health and energy
consumption. If a board is returned, the
fault log can be read to determine which
fault occurred, the board temperature and
the time of the fault as well as historical
data leading up to the fault. This data
can be used to quickly determine root
cause, whether the system was operated
outside of its specified operating limits, or
to improve the design of future products.
Power consumption data can be used to
reduce overall power use in real time.
Digital power is a rapidly growing field
driven by customer demand for ever
more complex boards. The LTC3880 and
other Linear Technology PMBus products work together to give flexible digital
control to high performance supplies.
Board designers now have the tools to
streamline the process of bringing best-inclass performance quickly to market. n
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