Mar 2008 - 1.5% Accurate Single-Supply Supervisors Simplify Part Selection and Operate to 125°C

L DESIGN FEATURES
1.5% Accurate Single-Supply
Supervisors Simplify Part Selection
and Operate to 125°C
by Bob Jurgilewicz and Roger Zemke
Introduction
A new line of full-featured single-input
supervisors are easy to place, easy to
bias and easy to configure. They are
also highly accurate, an important
feature for keeping systems running
reliably.
The LTC2915, LTC2916, LTC2917
and LTC2918 provide as many as
twenty-seven integrated, user-selectable thresholds that are compatible
with many standard power supply
voltages. A user-adjustable input also
allows for customizable thresholds.
The reset timeout period is fixed at
200ms, or add a capacitor to generate
a custom timeout. The even-numbered
parts include an option to generate a
reset-on-demand using the manual
reset input, which is compatible with
mechanical or electrical switching. The
LTC2917 and LTC2918 have watchdog
circuits that monitor processor signal
activity within a user-adjustable window or non-windowed time period.
Electrical specifications are guaranteed to 125°C, so these supervisors
are perfect for high temperature
environments, such as automotive
applications. Operating voltage range
begins at a low 1.5V, and extends to any
12V
Table 1. LTC2915, LTC2916, LTC2917 and LTC2918 feature summary
Feature
LTC2915
LTC2916
LTC2917
LTC2918
9 Selectable Thresholds
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Wide Temperature Range
–40°C to +125°C
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Threshold Accuracy
±1.5%
±1.5%
±1.5%
±1.5%
Shunt Regulator for
High Voltage Operation
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Quiescent Current
30µA
30µA
30µA
30µA
Low Voltage Reset
0.8V
0.8V
0.8V
0.8V
Reset Timeout: 200ms Fixed
or Externally Adjustable
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Power Supply
Glitch Immunity
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Selectable Supply Tolerance
–5%, –10%, –15%
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Manual Reset
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Non–Windowed Watchdog
-A
-A
Windowed Watchdog
-B
-B
3.3V
LTC2915
VM
µP
RST
SEL1
RST
GND
SEL2
TOL
GND
CBYPASS
0.1µF
VCC
VCC
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Watchdog Timeout:
1.6s Fixed or
Externally Adjustable
RPU
10k
CBYPASS
0.1µF
RT
Figure 1. A 12V supply monitored from 12V, utilizing
internal shunt regulator with 3.3V logic out
RPU
10k
VCC
VCC
LTC2915
5V
3.3V
VM
µP
RST
SEL1
RST
GND
SEL2
TOL
GND
CRT
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3.3V
RCC
11k
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RT
3.3V
Figure 2. A 5V, –10% tolerance supply monitor
with 200ms internal reset timeout
Linear Technology Magazine • March 2008
DESIGN FEATURES L
3.3V
9V
3.3V
CBYPASS
0.1µF
R2
866k
VCC
VCC
LTC2915
VM
VCC
LTC2915
RST
1V
GND
µP
RST
VM
GND
SEL2
TOL
GND
TOL
GND
RT
RT
CRT
0.01µF
Figure 3. A 9V, –15% tolerance supply monitor with 3.3V logic out
positive high voltage when biasing the
integrated 6.2V shunt regulator. With
all these features (and more discussed
below), users can qualify one product
to meet almost any supervisory need.
Table 1 summarizes the features for
all four products.
Easy Placement
Despite a general trend to integrate devices as much as possible, single-input
supervisors have certain advantages
over multi-voltage devices. The singleinput supervisor is not taxed with
the requirement to be engaged with
multiple supply voltages so that it is
much easier to place. Lead pitches on
modern device packages dictate that
multi-supply supervisors have their
monitor inputs physically close to
each other. Such covenants naturally
lead to signal routing and congestion
problems. Furthermore, due to the
close proximity of multiple supply
lines, undesirable noise coupling can
be a problem.
Specifying a physical system location for a multi-supply supervisor
involves tradeoffs since an optimal
distance between supplies, super-
visor and microprocessor may be
difficult to achieve. Systems using a
single supervisor do not suffer from
these problems; the supervisor may
be located as near to the monitored
supply or processor as desired. The
LTC2915 and LTC2916 are available
in low profile (1mm) TSOT-23 and
DFN (3mm × 2mm) packaging. The
LTC2917 and LTC2918 are available
in 10-lead MSOP and DFN (3mm ×
2mm) packaging.
In concept, the job of a good supply
monitoring supervisor is simple: when
a power supply voltage drops below a
specified value, generate a reset. In
reality, the job of a supervisor is much
more complicated. Start-up and shutdown conditions, noise, and transients
all contribute to the complexity of a
real supervisor’s job. If the supervisor
generates a reset while the monitored
supply is actually within specification,
the result is annoying and consumes
operating margin. Spurious resets
generated by typical supply noise are
equally vexing. Worse yet, not eliciting
a microprocessor reset at voltages too
RPU
10k
µP
RST
RPU
10k
VCC
VCC
LTC2915
RST
SEL1
GND
SEL2
1.5V
VM
µP
RST
RST
I/O GND
SEL1
MR
MANUAL RESET
PUSHBUTTON
CBYPASS
0.1µF
VCC
VCC
GND
low for proper system behavior can be
catastrophic.
Supervisor threshold accuracy is a
critical specification and must be reckoned with during the system design
phase. Most power supplies are specified to operate within a tolerance band.
Consider the example of monitoring
a 5V supply with a ±10% tolerance.
The lowest specified output voltage is
therefore 4.5V. An ideal voltage monitor (perfect accuracy) would generate
a reset at precisely 4.5V and below,
regardless of operating conditions,
indicating an out-of-tolerance supply
voltage. The problem is that ideal,
perfectly accurate voltage monitors
do not exist. A randomly selected realworld voltage monitor has a threshold
that resides within a distributed band
of values. All 27 of the LTC2915,
LTC2916, LTC2917 and LTC2918
selectable thresholds have the same
relative threshold band of ±1.5% of
the selected nominal input voltage,
over the full temperature range (–40°C
to 125°C). The 5V monitor threshold
band is therefore 150mV wide.
The upper limit of the threshold
band should be coincident with the
Correct and Stable Operation
LTC2916
VM
Figure 4. A 1V, –15% tolerance supply monitor with 90ms timeout
3.3V
CBYPASS
0.1µF
10k*
RST
SEL1
SEL2
CRT
1.8V
RPU
10k
VCC
µP
RST
SEL1
R1
51.1k
CBYPASS
0.1µF
RPU
10k
–15% –5%
SEL2
RT
GND
CRT
RT
CRT
*OPTIONAL RESISTOR RECOMMENDED TO EXTEND ESD TOLERANCE
Figure 5. 1.8V, –5% supply monitor with manual reset
Linear Technology Magazine • March 2008
Figure 6. 1.5V supply monitor with tolerance control
for margining, –5% operation with –15% margining
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L DESIGN FEATURES
lowest specified power supply output
voltage (4.5V in our example). Otherwise, operating range is potentially
consumed if the monitor threshold
reaches above 4.5V. Using the monitor
voltage select (SEL1, SEL2) and tolerance (TOL) inputs on the LTC2915 and
LTC2917 (for 5V supply, 10% reset
threshold), we can configure the upper threshold limit to 4.5V. The lower
threshold limit is 150mV below, or
4.35V. Statistically, most devices will
have an actual threshold closer to
4.425V, which is the center of threshold band. Because of the threshold
spread, the powered system must work
reliably down to the lower threshold
limit, over temperature. It is easy to
see why less accurate monitors (larger
threshold spreads) can contribute to
system problems.
The monitor threshold discussion,
so far, deals only with the DC value of
(VTRIP = 10.64V)
12V
1.8V
VM
MANUAL RESET
PUSHBUTTON
RST
SEL1
SEL2
1.8V
1.8V
CBYPASS
0.1µF
µP
VM
RST
RST
SEL1
WDI
I/O
RPU
10k
VCC
VCC
VCC
LTC2917-B
1V
GND
µP
VM
RST
RST
SEL1
WDI
I/O
GND
SEL2
SEL2
TOL
GND WT
RT
RT
3.3V
CRT
tWDU = 1.6s
tWDL = 50ms
tRST = 200ms
Figure 9. A 1V supply monitor with windowed
watchdog timeout and internal timers selected
3.3V
3.3V
CBYPASS
0.1µF
CBYPASS
0.1µF
RPU
10k
VCC
RST
RST
SEL1
WDI
I/O
GND
µP
VM
RST
SEL1
WDI
RST
GND
SEL2
SEL2
TOL
GND
VCC
LTC2917
µP
VM
RPU
10k
VCC
VCC
LTC2917-A
WT
TOL
GND
RT
CWT
0.047µF
CRT
0.0022µF
Figure 10. A 12V supply monitor with 20ms reset
timeout and 3.4s watchdog timeout, with 3.3V logic out
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comparators—many monitors on the
market use this method. There is a
problem with this approach, in that
the added hysteresis degrades the accuracy of the monitor and ushers in
the design problems discussed earlier.
The LTC2915, LTC2916, LTC2917
and LTC2918 single supervisors do
not apply hysteresis. Instead, the
3.3V
Figure 8. A 12V supply monitor powered from 12V,
utilizing the internal shunt regulator with 3.3V logic out
12V
RT
1.8V
the monitored supply. Real supplies
also have AC components or noise
from sources such as load transients
and switching artifacts. These AC
components should be ignored by the
monitor, since they can cause undesirable spurious reset events. One way
to avoid noise-induced sporadic resets
is to add hysteresis to the monitor
LTC2917
CWT
RST
GND
TOL
GND
RT
µP
RST
Figure 7. Dual supply monitor (1.8V and 12V) with manual reset and 200ms Reset timeout
CBYPASS
0.1µF
WT
VM
SEL2
RPU
10k
VCC
TOL
GND
12V
SEL1
GND
VCC
VCC
LTC2915
MR
RCC
11k
R1
49.9k
RPU
10k
VCC
LTC2916
3.3V
R2
1.15M
CBYPASS
0.1µF
WT
RT
CRT
Figure 11. A 3.3V, –10% tolerance supply
monitor with disabled watchdog
Linear Technology Magazine • March 2008
DESIGN FEATURES L
comparators incorporate anti-glitch
circuitry. Any transient at the input
of the monitor comparator must be
of sufficient magnitude and duration
(energy) to switch the comparator. Designs utilizing these single supervisors
promote correct and glitch-free resets,
which leads to stable and ultimately
more reliable systems.
Processor Communication
Two of the monitors (LTC2917 and
LTC2918) communicate with host
processors through their watchdog
circuits. The basic requirement for
the processor is to “pet” the watchdog
periodically to avoid being “bitten” by
the dog. Processor resets are invoked
by the built-in watchdog hardware
when the watchdog petting frequency
has become too slow or too fast. Precise knowledge of the system’s timing
characteristics is required to set the
watchdog timeout period. Adjust the
watchdog timeout period by connecting a capacitor between the watchdog
timing input (WT) and ground. Connect
WT to VCC to achieve a default 1.6s
timeout, without the need for external
capacitance.
Simple and Compliant Bias
A unique feature common to all four
of these devices is the ability to provide operating bias from almost any
positive voltage. It does not matter
whether it is a 1.8V LDO, 5V switcher,
12V car battery, 24V wall-wart or 48V
telecom supply; the integrated 6.2V
shunt regulator can work with any
system. For input voltages above 5.7V
the only requirement is to size the
bias resistor (RCC) to the range of the
input voltage. Connect RCC between
the high voltage supply and the VCC
input. Below 5.7V, simply connect
the supply directly to the VCC input.
Deriving resistor sizing for worst-case
operation requires knowledge of the
minimum (VS(MIN)) and maximum
(VS(MAX)) input supply:
VS(MAX ) − 5.7 V
5mA
≤ RCC ≤
VS(MIN) − 7 V
250µA
Be sure to decouple the VCC input
using a 0.1µF (or greater) capacitor
to ground.
Qualify Once, Specify Forever
During product development cycles,
power supply requirements often
change. While supply requirements
are changing, your choice of supervisor doesn’t have to. The LTC2915,
LTC2916, LTC2917 and LTC2918
can relieve the burden of having to
find the right supervisor for the job.
Qualify any one of these parts and you
can monitor any one of eight different
supply voltages, each with three different internally fixed thresholds. You
can also monitor any custom voltage
down to 0.5V using an external resistor divider. Multi-supply monitoring is
easily achieved by using two or more
devices and connecting their RST
outputs together.
Meet Your Match
The LTC2915, LTC2916, LTC2917 and
LTC2918 single supervisors are the
perfect match for a variety of applications. Browse the applications shown
in the figures and quickly find the right
application for your system.
Conclusion
The LTC2915, LTC2916, LTC2917
and LTC2918 are feature-laden single
supervisors that can be comfortably
placed near your monitored supply
and/or microprocessor, leading to
easy printed circuit board layout and
reliable system operation.
Unprecedented configurability
makes it possible to qualify and stock
just one product that can meet all of
your supervisory needs. Integration
provides twenty-seven user-selectable monitor thresholds with ±1.5%
accuracy. Any non-standard threshold can be user-configured with the
adjustable setting.
Other features include high voltage operation, configurable reset and
watchdog timers, manual reset, and
low quiescent current. External components are seldom required to realize
fully functional designs. Electrical
specifications are guaranteed from
–40°C to 125°C. L
LTC4311, continued from page Auto Detect Standby Mode
and Disable Mode
To conserve power, when both bus
voltages are within 400mV of the bus
pull-up supply, the LTC4311 enters
standby mode, consuming only 26µA
of supply current. When ENABLE
is forced low, as shown in Figure 4,
the LTC4311 enters a disable mode
and consumes less than 5µA of supply current. Both bus pins are high
impedance when in disable mode or
when the LTC4311 is powered down,
regardless of the bus voltage.
Linear Technology Magazine • March 2008
Conclusion
The LTC4311 efficiently and effectively
addresses slow rise times, decreased
noise margins at low bus supplies, and
increased DC bus power consumption
found in 2-wire bus systems. Strong
slew rate controlled pull‑up currents
quickly and smoothly slew the I2C
or SMBus bus lines, decreasing rise
times to allow up to 400kHz operation for bus capacitances in excess
of 1nF. The advantages of the strong
slew rate controlled currents extend
to reducing the low state bus voltage,
DC bus power consumption, and fall
times, since larger value bus pull‑up
resistors can be used.
With a small 2mm × 2mm × 0.75mm
DFN or SC70 footprint, high ±8kV HBM
ESD performance and low power consumption in standby or disable mode,
the LTC4311 Low Voltage I2C or SMBus
accelerator is also ideally suited for
all I2C or SMBus systems. Examples
of such systems include notebooks,
palmtop computers, portable instruments, RAIDs, and servers where I/O
cards are hot-swapped. L
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