3mm × 3mm QFN IC Directly Monitors 0V to 80V Supplies: Features I2C Interface, Peak Value Tracking and Runs from Any Supply

design features
3mm × 3mm QFN IC Directly Monitors 0V to 80V Supplies:
Features I2C Interface, Peak Value Tracking and Runs from
Any Supply
CY Lai
Power monitoring in combination with control mechanisms can significantly boost
system energy efficiency and reliability. The LTC2945 is a highly integrated digital
power monitoring solution that is compact, rugged and easy-to-use. It is designed to
fit applications requiring power monitoring with a minimal number of components.
MONITOR POWER ON ANY SUPPLY
•The secondary supply exists, but is not
readily accessible—it is inconveniently
located on the printed circuit board,
complicating the routing of a power line
The LTC2945’s internal current sense
amplifier features a common mode range
of 0V to 80V to suit a wide variety of high
side and low side current sensing applications. Most wide range supply monitors
available today require a low voltage
secondary supply for operation, which
can be undesirable for several reasons:
The LTC2945 avoids these problems by
integrating a high voltage linear regulator
that can be powered directly from 4V to
80V supplies. The output of the linear regulator (INTVCC) powers the LTC2945, and can
be externally bypassed to prevent supply
noise from corrupting the signal integrity
of internal circuitry. The linear regulator is
capable of supplying a 10m A load, saving
the cost of a dedicated high voltage linear
regulator needed to power circuits such
as opto-couplers in some applications.
•There is no suitable secondary supply
•The secondary supply, often loaded
with noisy digital circuits, must be sufficiently filtered or bypassed due to the
finite power supply rejection ratio of the
supply monitor at higher frequencies
Figure 1. Functional block
diagram of the LTC2945
SENSE+
VDD
SENSE–
ADR0
ADR1
ALERT
DECODER
+
VSTBY
20X
5.7V
VSTBY
GEN
INTV CC
2
IC
–
Figure 1 shows the LTC2945’s functional
block diagram. All basic elements required
for power monitoring are integrated,
including a precision current sense
amplifier, precision resistive dividers, an
analog-to-digital converter (ADC) and
an I2C interface for communicating with
the host controller. Only an external
current sense resistor is required. The
host can periodically poll the LTC2945
for available power data, minimum and
maximum values are stored, and an alert
can be sent from the LTC2945 to interrupt the host when measured values
exceed their preprogrammed limits.
735k
SDAO/SDAO
(LTC2945 / LTC2945-1)
VREF = 2.048V
735k
LOGIC
SDAI
VSTBY
12
MUX
6.3V
15k
6.4V
12-BIT ADC
SCL
REGISTERS
15k
6.4V
ADIN
GND
July 2013 : LT Journal of Analog Innovation | 13
The LTC2945 is a highly integrated power monitor that easily fits into
a wide range of systems. It offers a 0V to 80V common mode range,
2.7V to 80V operating range, ±0.75% accurate voltage and current
measurements, and an on-chip digital multiplier that computes power.
RSNS
VIN
4V TO 80V
SENSE+
RSNS
VIN
0V TO 80V
VOUT
SENSE+
SENSE–
VDD
SENSE–
SENSE+
LTC2945
VOUT
SENSE–
INTVCC
2.7V TO 5.9V
VDD
4V TO 80V
RSNS
VIN
0V TO 80V
VOUT
VDD
LTC2945
LTC2945
C2
INTVCC
C2
INTVCC
C2
GND
GND
Figure 2a. The LTC2945 deriving power
from the monitored supply
GND
Figure 2b. The LTC2945 deriving power
from a wide ranging secondary supply
Figure 2c. The LTC2945 deriving power
from a low voltage secondary supply
GND
RSNS
VIN
>80V
SENSE+
VOUT
SENSE+
SENSE–
>80V
INTVCC
VDD
RSNS
VIN
0V TO 80V
RSHUNT
RSHUNT
VOUT
SENSE–
VDD
VDD
INTVCC
VDD
INTVCC
LTC2945
GND
LTC2945
C1
GND
C2
INTVCC
C2
LTC2945
GND
LTC2945
C2
VNEG
>–80V
RSHUNT
Figure 3a. The LTC2945 deriving power
through a high side shunt regulator
Figure 3b. The LTC2945 deriving power
through a low side shunt regulator in a
high side current sense topology
Figure 2a shows a typical LTC2945 application monitoring a 4V to 80V supply and
deriving power off the same supply. The
bus voltage is measured at the SENSE+ pin
through an internal resistive divider and
a sense resistor is used to measure the
load current on the high side. If the bus
voltage to be monitored is below 2.7V,
the power for the LTC2945 can be derived
from a wide range secondary supply
14 | July 2013 : LT Journal of Analog Innovation
SENSE–
GND
GND
SENSE–
SENSE+
RSNS
VOUT
Figure 3c. The LTC2945 deriving power
through a low side shunt regulator in a
low side current sense topology
VNEG
–4V TO –80V
SENSE+
RSNS
VOUT
Figure 3d. The LTC2945 deriving
power from the monitored supply in
a low side current sense topology
as shown in Figure 2b or a low voltage
secondary supply as shown in Figure 2c.
standing off the bus voltage and supplying
LTC2945’s operating current will work.
The LTC2945 also integrates a 6.3V,
35m A shunt regulator at the INTVCC pin for
operation beyond 80V. Figure 3a shows
the LTC2945 used in one such application
with its ground floated at 6.3V below the
bus voltage. The bulk of the bus voltage is
dropped across an external shunt resistor;
in practice any current source capable of
Figure 4 shows how to measure the
bus voltage in this configuration using
a matched PNP pair and some resistors.
The resistor values shown are optimized
for VIN of 165V ±10%. The LTC2945’s
shunt regulator can also be configured
as shown in Figure 3b when the only
design features
LTC2945 integrates an oversampling ∆∑ ADC that inherently averages
the measured voltage over the conversion cycle to effectively reject
noise due to transient spikes and AC power line. Bus voltage,
sense voltage and ADIN are measured with total error of less than
±0.75% at full scale over the full industrial temperature range.
Figure 4. Application circuit for measuring
the bus voltage in a high side shunt
regulator configuration
±0.75% TOTAL ERROR
MEASUREMENT ACCURACY
VIN
R1
2k
R2
2k
NST30010MXV6
INTVCC
LTC2945 integrates an oversampling
DS ADC that inherently averages the
measured voltage over the conversion
cycle to effectively reject noise due to
transient spikes and AC power line harmonics. Bus voltage, sense voltage and
ADIN are measured with total error of
less than ±0.75% at full scale over the
full industrial temperature range.
LTC2945
ADIN
R3
1150k
R4
5.76k
GND
UP TO 8mA LOAD
(OPTOS, ETC.)
RSHUNT*
15k
*THREE 1W, 5k RESISTORS IN SERIES
secondary supply available exceeds 80V in
high side current sensing applications.
If the output of the power supply is
negative such as in –48V distributed
power systems for networking, communications and high end computing
equipment, low side current sensing is
generally preferred, as shown in Figures 3c
and 3d. Figure 3c shows a shunt resistor
and LTC2945’s shunt regulator limiting
INTVCC to 6.3V above a negative supply
that exceeds 80V. More commonly, the
negative supply is below 80V and instead
the internal linear regulator can be used to
power the LTC2945 directly, as shown
in Figure 3d. In this configuration
the VDD pin measures the bus voltage
through an internal resistive divider.
Measuring bus voltage in excess of
80V in low side current sensing applications such as Figure 3c can be done
by connecting a resistive divider to
the ADIN pin as shown in Figure 5.
Figure 5. ADIN senses the bus voltage in low side
shunt regulator configuration.
The 12-bit DS ADC provides a full-scale
voltage of 102.4mV (25µV/LSB) for sense
voltage, 102.4V (25mV/LSB) for bus voltage and 2.048V (0.5mV/LSB) for ADIN.
Typical integral linearity error (INL) of
the ADIN voltage and the sense voltage
are both well within ±0.5LSB, as shown in
Figures 6 and 7. The LTC2945 is also ideal
in applications where accuracy is important at the low end of the measurements
since its specified offset voltages are as
low as ±1.1LSB for ADIN and ±3.1LSB for
current sense voltage in the worst case.
Figure 6. ADIN INL curve
Figure 7. SENSE INL curve
0.3
0.2
GND
0.2
INTVCC
LTC2945
ADIN
R2
GND
0.1
ADC INL (LSB)
R1
ADC INL (LSB)
RSHUNT
VNEG > –80V
0.4
0.0
–0.1
0.0
–0.2
–0.2
–0.3
0
1024
2048
CODE
3072
4096
–0.4
0
1024
2048
CODE
3072
4096
July 2013 : LT Journal of Analog Innovation | 15
Opto-isolation is common in high voltage systems where the high
voltage sections must be galvanically isolated for safety reasons. The
LTC2945 accommodates isolated applications by splitting the SDA
signal on the I2C interface into an SDAI pin and an SDAO pin (for
LTC2945-1, SDAO) for applications with an opto-isolator interface.
PEAK VALUES TRACKING AND OVER/
UNDERVALUE ALERTS
the monitor needs to be monotonic and
of high resolution in order to minimize
stability issues. The LTC2945 generates
24-bit power data by digitally multiplying the 12-bit sense voltage and 12-bit bus
voltage data without truncating the result.
Keeping track of the minimum and maximum measurement values is important in
many power monitoring systems because
it could be used to study usage behavior
for more efficient resource allocation and
is often an indicator of system health.
Previously, gathering such information
required periodic polling of the power
monitor by the system’s microprocessor,
which wasted precious computation time
and potentially tied up the I2C interface.
The LTC2945 solves this problem by storing the minimum and maximum values
for power, voltage, current, and ADIN.
The Page Read feature on the LTC2945
allows these data to be read with just a
single I2C read transaction. An ALERT pin
can also be configured to signal overvalue or undervalue limit violations for
power, voltage, current and ADIN.
OPTO-ISOLATION AND SHUTDOWN
The LTC2945 can be shut down via the
serial I2C interface, reducing the typical
quiescent current to 20µ A—especially
important for battery-powered applications. Opto-isolation is common in high
voltage systems where the high voltage
sections must be galvanically isolated
for safety reasons. The LTC2945 accommodates isolated applications by splitting
the SDA signal on the I2C interface into an
SDAI pin and an SDAO pin (for LTC2945-1,
SDAO) for applications with an optoisolator interface as shown in Figure 8.
For limited amounts of current, the
internal linear regulator or shunt regulator can be used to supply the pull-up
resistors on the I2C bus. In situations
where it is undesirable to tap off the
internal regulator and a low voltage
UNTRUNCATED 24-BIT POWER DATA
For applications where a digital servo loop
is used to regulate the power output of a
system, the power data read back from
Figure 8. Opto-isolation of a 10kHz I2C interface between the LTC2945 and a
microcontroller
supply is not available, the LTC2945-1
allows the pull-up resistors to connect
directly to high voltages. The SCL and the
SDAI pins are limited to safe voltages by
internal 6.3V, 3m A clamps. The SDAO pin
is inverted (to SDAO) so that it can be
clamped by the anode of the input diode
of an optoisolator as shown in Figure 9.
SUPPLY TRANSIENTS
The wide operating range of the LTC2945 is
advantageous even in applications where
the bus voltage is normally well below
80V. Transient voltage surges due to
inductive kickbacks in automotive load
dump situations and hot swap output
shorts are just two possible scenarios
where a rugged power monitoring solution is required in order to withstand
overvoltage conditions far in excess
of the normal operating voltage.
The 100V absolute maximum rating
of the LTC2945 makes it easy to guard
against these types of voltage surges since
there is a wide range of transient surge
suppressor (TVS) diodes from which to
choose. In certain applications a large
Figure 9 Opto-isolation of a 1.5kHz I2C interface between the LTC2945-1 and a
microcontroller
3.3V
3.3V
5V
SDAI
R4
10k
R5
0.82k
R6
0.51k
48V
R7
10k
R4
20k
µP
1/2 MOCD207M
LTC2945-1
GND
SDA
SDAO
GND
SDAO
16 | July 2013 : LT Journal of Analog Innovation
µP
1/2 MOCD207M
GND
1/2 MOCD207M
R7
10k
VDD
SDA
GND
R6
0.51k
SDAI
VDD
LTC2945
R5
7.5k
1/2 MOCD207M
design features
The 100V absolute maximum rating of the LTC2945 makes it easy to guard against voltage
surges since there is a wide range of transient surge suppressor (TVS) diodes from which
to choose. In certain applications a large MOSFET power device can break down to clip the
inductive spike safely, and in most 12V and 24V systems the break-down voltage of these
power devices is less than 100V, potentially negating the requirement for a TVS diode.
RSNS
0.02Ω
VIN
SENSE +
VOUT
SENSE–
INTVCC
VDD
R12
100Ω
R4
1k
LTC2945
Q1
PZTA42
R3
0.51k
R1
1k
C2
1µF
R2
1k
FGND
3.3V
R7
0.51k
R8
0.51k
R9
1k
R10
1k
R11
10k
VDD
VCC
T1
SMAJ78A
SCL
C1
0.1µF
SCL
SDAI
SDA
GND
HCPL-063L
FGND
µP
3.3V
VCC
ADR1
ADR0
SDAO
ADIN
INT
ALERT
GND
GND
GND
HCPL-063L
FGND
M1
BSP149
Figure 10. Ruggedized 4V to 80V high side power
monitor with surge protection up to 200V
MOSFET power device can break down
to clip the inductive spike safely, and
in most 12V and 24V systems the breakdown voltage of these power devices
is less than 100V, potentially negating the requirement for a TVS diode.
Hard-clamping the voltage with TVS diode
or MOSFET break down may not be practical when the inductive energy is too
high or unpredictable. Figure 10 shows
a LTC2945-based power monitor that can
ride through a 200V surge where its high
voltage pins are clamped by T1 to less than
80V. In normal operation, M1 operates in
the triode region with the device ground
R5
1Ω
at a few mV above the system ground.
During the surge, the device ground is
lifted by T1 and the balance of the surge
voltage is dropped across M1. The BSP149
has a 200V break down and the surge
duration is limited by its safe operating
area—for example at room temperature it
can survive a 200V surge for 1ms at VIN .
multiplier that computes power. Digital
watchdog functions such as peak and valley values and window comparators are
available for power, voltage, current and
an external voltage. Opto-isolation is simplified with a split SDA pin. The LTC2945
is available in space-saving 3mm × 3mm
QFN and 12-pin MSOP packages.
CONCLUSION
Visit www.linear.com/LTC2945 for
data sheets, demo boards and other
applications information. n
The LTC2945 is a highly integrated power
monitor that easily fits into a wide range
of systems. It offers a 0V to 80V common
mode range, 2.7V to 80V operating range,
±0.75% accurate voltage and current
measurements, and an on-chip digital
July 2013 : LT Journal of Analog Innovation | 17
Similar pages