8-Output Regulator Powers Applications Processors

design features
8-Output Regulator Powers Applications Processors
Kevin Ohlson
The market for applications processors, the integrated core/
memory/video/UI function chips used in smartphones,
tablets, netbooks and automobile infotainment systems,
is one of the fastest growing segments in electronics
today. A single applications processor IC, such as one
from Freescale, Marvell or an in-house custom processor,
is packed with functions and requires independent power
supplies for its core, I/O, memory and peripherals. The
challenge is producing all those rails in limited space, at
high efficiency, from a wide range of power inputs—a
tablet, for instance, requires power conversion from
USB, automotive battery and its built-in Li-ion battery.
The LTC3589 serves applications processor power needs with eight regulated
outputs that support processor core and
I/O voltage levels, SRAM, memory, low
power standby, other peripheral circuits
and system voltage levels. The LTC3589’s
eight supplies are completely independent, but they can be easily sequenced
with simple pin strapping. Likewise, the
LTC3589 simplifies overall power system design by integrating a number of
important control features, including:
EIGHT INDEPENDENT VOLTAGE
REGULATORS IN A SINGLE IC
While the features built into the LTC3589
certainly aid system design and optimization, it is designed foremost to output
eight independent, voltage-regulated
outputs. The LTC3589 contains a combination of LDO and switching regulators with output current capabilities
from 25m A to 1.6A, with voltage output
Figure 1. Eight power rails take less than 500mm2 of
board real estate.
levels from less than 1V to 5V. Four of the
outputs feature I2C -controlled DAC references for dynamic voltage scaling.
The integrated low power, 25m A, LDO can
supply circuits that require a constant
supply while the system is in standby
mode, such as a real time clock. The
low power LDO is capable of producing an output from 0.8V up to the input
Table 1. The LTC3589 supplies eight voltage rails delivering currents from 25mA to 1.6A
•Flexible pin strap supply sequencing
TYPE
AVAILABLE
OUTPUT
CURRENT
OUTPUT VOLTAGE CONTROL
LDO1
25mA
Resistive divider based on 0.8V feedback reference
LDO2
250mA
Resistive divider based on 0.3625V to 0.75V DAC feedback reference
•IRQ pin and status register error
reporting
LDO3
250mA
Fixed 1.8V
LDO4
250mA
1.8V, 2.5V, 2.8V, 3.3V selectable using I 2C command register
•Power good status pin and register
Buck1
1.6A
Resistive divider based on 0.3625V to 0.75V DAC feedback reference
•Built-in pushbutton controller to initiate
power-on, provide a debounced pushbutton status and force a device hard reset
Buck2
1A
Resistive divider based on 0.3625V to 0.75V DAC feedback reference
Buck3
1A
Resistive divider based on 0.3625V to 0.75V DAC feedback reference
Buck-Boost
1.2A
Resistive divider based on 0.8V feedback reference
•I2C control of all major regulator
functions
•Dynamic voltage scaling with selectable
ramp rate
October 2011 : LT Journal of Analog Innovation | 27
The LTC3589’s eight supplies are completely independent,
but they can be easily sequenced with simple pin strapping.
Likewise, the LTC3589 simplifies overall power system
design by integrating a number of important control features.
supply, set by a resistive divider. As long
as an input supply is attached to the
LTC3589, the always-alive LDO regulates.
The LTC3589 only consumes 8µ A of input
supply current in standby mode, even
as the always-alive LDO regulates.
from a voltage lower than the primary
input supply to reduce the power consumption in the LDO. Typically, the
LTC3589 switching converters supply the
LDO regulators. Two of the LDO regulators
have fixed or I2C selectable output voltage. The third LDO uses external feedback
resistors with a 5-bit DAC reference to set
its output using an I2C command register.
Three more LDOs, each capable of delivering 250m A, are handy for supplying
power to system analog functions such
as phase lock loops, D/A and A/D converters, or as general purpose rails. The
250m A LDO regulators can be powered
The LTC3589 is designed to run from an
input supply range of 2.7V to 5.5V. To
satisfy the requirements of devices that
POWERPATH
CONTROLLER/
Li-ION CHARGER
VIN
require a 3.3V or 5V rail, the LTC3589
includes a high efficiency buck-boost
switching converter that can output
voltage from 1.8V to 5V, set by a resistive divider. The buck-boost converter
is capable of supporting loads up to
1.2A. Using the I2C serial port, the buckboost converter can be set to low power
Burst Mode operation to reduce power
loss in low current output modes.
Three buck regulators complete the
LTC3589 complement of regulated
LTC3589
VOUT
REAL TIME CLOCK
LDO 1
CC/CV
Figure 2. Combine the LTC3589 with
a PowerPath™ controller/battery
charger for power distribution with
supply sequencing, I2C controls and
pushbutton control.
VOUT
LDO 2
+
Li-Ion
VOUT
LDO 3
VSTB
DVS
CONTROL
DSP
EN
PLL OR ADC
EN
VOUT
PERIPHERALS
LDO 4
SCL
SDA
VOUT
I2C
BUCK 1
µP
VOUT
RSTO
IRO
PGOOD
STATUS
WAKE
ON
28 | October 2011 : LT Journal of Analog Innovation
PUSHBUTTON
CONTROL
DDR MEMORY
EN
VOUT
BUCKBOOST
SRAM
EN
VOUT
BUCK 3
PBSTAT
PWR_ON
BUCK 2
µP
EN
EN
I/O
design features
LTC3589
BUCK OR LDO
+
VOUT
–
VOUT
1V/DIV
FB
DAC
UP/DOWN SLEW
CONTROL
5
5
I2C REGISTER
5
I2C REGISTER
PGOOD
5V/DIV
MUX
VSTB
5V/DIV
2
200µs/DIV
VRRCR = 1.75mV/µs
I2C REGISTER
I2C REGISTER
VSTB
voltage outputs. The buck converters’
output voltages, set with external resistor dividers, can range from as low as
the minimum DAC reference voltage to as
high as the input supply voltage, where
the bucks operate in dropout mode.
Depending on the requirements of the
application, each buck’s operating mode
can be set using the I2C command registers. For operation over a wide range
of output currents, pulse-skipping mode
gives good efficiency with low ripple.
Burst Mode operation offers the highest efficiency at low power. When set to
Burst Mode operation, the buck automatically moves between Burst Mode
operation at low loads and continuous
switching mode at higher output loads.
Selecting forced continuous mode results
in the lowest output voltage ripple at
the expense of some efficiency. Each
buck’s operating mode is independently
selected using the I2C command registers.
DYNAMIC VOLTAGE SCALING
Since portable battery operated devices
spend much of the time in standby or
low power modes, microprocessors may
take advantage of dynamic voltage scaling to reduce switching power loss by
decreasing the processors supply voltage.
The LTC3589 supports dynamic voltage
scaling (DVS) on one of the LDO regulators and all three buck converters.
Each scalable regulator on the LTC3589
uses two DAC feedback reference set-point
voltages in the I2C command registers
and a selectable transition slew rate
between the high and low target voltages (see Figure 3). Transition between
target voltages is initiated for all regulators using the VSTBY pin or for individual
regulators using I2C command registers.
The scalable LDO and buck converters
have independently controlled DAC-driven
feedback reference voltages. The reference voltage range runs from 0.3625V to
0.75V in 31 12.5mV steps. The converter
output voltage is scaled up from the
reference voltage using a resistive feedback divider from the converter output
to its feedback input. At power-on,
each DAC defaults to a reference output
of 0.675V so the output voltage can be
increased from the default output by
10% to increase the processor performance or for power supply margining.
Figure 3. Dynamic voltage scaling is supported
on four of the LTC3589’s eight outputs with I2C
selectable up/down slew rate.
During a voltage-down slew, the stepdown regulators are automatically
switched to forced continuous mode and
therefore are able to sink current from the
load. A 2k resistor to ground is switched
to the output of the DAC-referenced LDO to
pull down its output. Four slew rates
are selectable by choosing the rate of
change of the reference, from 0.88V/ms
to 7V/ms, via the I2C command register.
EASY SEQUENCING AND ENABLE
CONTROL
Multirail systems typically require the
supply rails come up to voltage in a
predetermined sequence (because of
latchup, brains in right order, start-up
current, etc.). Sequencing the LTC3589
outputs in any order is accomplished
by pin-strapping regulator outputs to
regulator inputs. Figure 4 shows an
example of a pin-strapped sequence. Each
enable pin has a precise 500mV comparator input with a built-in 200µs delay
timer before enabling the regulator.
The start-up sequence is defined by tying
the LTC3589 WAKE pin to the enable pin
of the first regulator or regulators in
the sequence. Wrapping regulator outputs around to the next enable in the
October 2011 : LT Journal of Analog Innovation | 29
A regulator not in the start-up sequence is controlled
by driving its pin directly or using the I2C command
register. Any of the regulators in a pin-strapped sequence
can be enabled or disabled in any order by setting a
software control bit in the I2C command registers.
Figure 4. Flexible and simple start-up
sequencing is accomplished by tying
regulator outputs to enable pins in
any order.
WAKE
V1
EN1
LTC3589
SW1
1V TO 1.2V
EN3
SW2
1.8V
SW3
0.8V TO 1V
BB_OUT
3.3V
EN_LDO34
LDO2
1.2V
ON
LDO3
1.8V
PWR_ON
LDO4
2.8V
EN_LDO2
PWR_ON
1V
V3
WAKE
EN2
EN4
1.2V
0.5V 200µs
V2
0.5V
1.8V
200µs
3.3V
V4
LDO2
LDO3
200µs
1.2V
1.8V
2.8V
LDO4
sequence brings the supplies up in order.
If additional start-up delay is required,
add a resistive divider to raise the enable
voltage threshold or add an RC filter
with the desired time constant to delay
the start of the subsequent regulator.
buck converters and the DAC-controlled
LDO have a keep-alive bit setting in the
I2C control register. Setting any of the
keep-alive bits in the I2C command register
keeps the corresponding regulators alive
when the LTC3589 is in standby mode.
A regulator not in the start-up sequence
is controlled by driving its pin directly or
using the I2C command register. Any of
the regulators in a pin-strapped sequence
can be enabled or disabled in any order
by setting a software control bit in the
I2C command registers. Once the software control bit is set, all the regulators
ignore their enable pin status and respond
only to I2C command register control.
This allows a regulator to be powered
down without affecting the subsequent
regulators in a pin-strapped sequence.
To ensure the integrity of a power-up
sequence following a power-down, the
LTC3589 adds a one second delay to allow
the regulator outputs to fall to ground.
Additionally, 2k pull-down resistors are
inserted on the LDO outputs and buck
switch pins to ensure discharge. Each
regulator’s output voltage must be less
than 300mV before it is allowed to enable.
I2C command register settings are available to override the resistor pull-downs
and the 300mV start-up rule in cases where
the regulator outputs are back-driven.
Applications with keep-alive requirements such as volatile memory or
watchdog functions requiring more
power or additional voltage rails can
take advantage of the LTC3589 keepalive control function. Each of the three
PUSHBUTTON OPERATION
30 | October 2011 : LT Journal of Analog Innovation
The pushbutton control circuit included
in the LTC3589 provides a debounced user
interface to initiate a power-up sequence.
A power-up sequence from standby
mode begins when the pushbutton is
depressed to activate the open driver
WAKE pin. If the WAKE pin is tied to a
regulator enable pin, the power-up
sequence begins. Once the controller is
satisfied system power is good then the
PWR_ON pin should be driven high. For
normal shutdown, pull PWR_ON low.
The PBSTAT pin is an open drain output
that signals to the microprocessor that
the button has been pushed and some
change in operation or power-down has
been requested. If the system is no longer responding for some reason, holding the button for five seconds forces
a hard reset, which powers down the
regulators, asserts the RSTO reset pin
and puts the LTC3589 in standby mode.
If pushbutton functions are not needed,
the WAKE pin is enabled and disabled by
driving the PWR_ON pin directly. Even
when driving the PWR_ON pin directly,
the pushbutton PBSTAT status pin and
hard reset functions are active.
design features
C7
0.47µF
3.3V, 25mA
10µF
10µF
VIN
C6
C1
4.7µF 68nF
VIN
R2
150k
BOOST
SW
R11
499k
RT
PG VC
GND
FB
BD SYNC
C2
10µF
0805
USB
VBUS
OVGATE
VC WALL
OVSENS
D0–D2
TO µC
CHRG
R7
100k
T
R8
100k
C4
22µF
1µF
68k
EN2
VL2
EN3
VB3
EN4
VB2
EN_LDO2
VBB
EN_LDO34
C3
0.1µF
0603
R9
2.94k
VBB
TO µP
SW2
LTC3589
10pF
4.7k
68k
AUTOMOTIVE,
FIREWIRE,
ETC.
M1
ZXMP10A18G
1.5µH
10pF
PWR_ON
681k
VB3
1.2V
1A
22µF
BUCK3_FB
PGOOD
787k
BB_OUT
DVDD
SDA
4.7pF
SCL
BB_FB
VSTB
SW4AB
PBSTAT
1M
VBB
3.3V
1A
22µF
316k
2.7µH
SW4CD
VIN_LDO2
D1
MMBZ524- R5
0BLT1G
10k
10V
R4
10k
22µF
SW3
M2
ZXMP10A18G
R3
33k
VB2
1.8V
1A
422k
ON
HVIN
715k
BUCK2_FB
IRQ
4.7k
+
Li-Ion
R10
1k
1.5µH
WAKE
68k
GND BATSENS
22µF
768k
VB1
68k
VB1
1.2V
1.6A
604k
BUCK1_FB
RSTO
BAT
CLPROG PROG
1µH
SW1
10pF
EN1
NTCBIAS
NTC
PVIN4
1M
68k
M5
PVIN3
316K
M4
ACPR SW
PVIN2
LDO1_FB
C5
10µF
0805
IDGATE
LTC4098
PVIN1
LDO1_STDBY
VOUT
OVGATE
TO µC
VIN
R12
100k
L1
3.3µH
10µF
22µF
HVBUCK
LT3480
RUN/SS
R6
40.2k
L2
10µH
10µF
10µF
VB2
LDO2
604k
1µF
VL2
1.2V
250mA
LDO2_FB
R1
1k
VIN_LDO34
M3
ZXMN10A08E6
VBB
768k
1µF
VL3
1.8V
250mA
1µF
VL4
2.8V
250mA
LDO3
GND
LDO4
Figure 5. Integrated power IC for mobile microprocessor system with USB/automotive battery charger
STATUS REPORTING
Three pins are provided for the LTC3589
to send status to the controlling microprocessor. The RSTO, IRQ, and PGOOD pins
are open drain outputs that signal regulator low output, hard reset events, supply
undervoltage, hot die temperature and
fault conditions. The IRQ and PGOOD pins
are matched to I2C status registers, which
can be read to determine the specific cause
of the status pin activity. In the event of a
fault, such as die overtemperature or UVLO,
that shuts down the LC3589 regulators,
the cause of the shutdown is latched in
a status register that can be read by the
system controller after system reboot.
CONCLUSION
The LTC3589 with eight regulator outputs,
flexible sequencing, dynamic voltage
scaling and serial port control is ideally
suited for applications-processor-based
consumer, industrial and automotive
devices. When coupled with a step-down
regulator, the LTC3589 can supply a complete set of system supply rails from high
voltage primary sources such as automotive systems. Add a PowerPath controller/
battery charger IC to generate system rails
for Li-ion-powered portable devices.
The LTC3589 features low power standby
and Burst Mode operation, keep-alive
functions and dynamic voltage scaling so
that system designers can optimize battery
life. Pushbutton control simplifies board
design and provides start-up, processor
interrupt and hard reset functions. n
October 2011 : LT Journal of Analog Innovation | 31
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