AN2853
Maxim > Design Support > Technical Documents > Application Notes > Microprocessor Supervisor Circuits > APP 2853
Keywords: supervisors, multivoltage, power on reset, reset circuit, watchdog timer, transient immunity,
glitch immunity, manual reset, voltage sequencing, voltage sequencer, power sequencing, power
sequencer, pfi, pfo, power fail comparator, overvoltage
APPLICATION NOTE 2853
Supervisors in Multivoltage Systems
By: Greg Sutterlin
Nov 17, 2003
Abstract: As processors become more complex, ensuring proper operation becomes more involved and
more is demanded from supervisory circuits.
Multi-voltage supervisors provide power-on reset, proper sequencing, and continuous voltage
monitoring.
The lower voltage requirements in today's state-of-the-art processors demands the use of new lowvoltage monitors.
Modern supervisors provide additional functions such as watchdog timers, manual reset input, and
power fail comparators.
As higher component density and processor speed demanded lower voltages for the core supply, multivoltage systems began to appear. The first such systems were dual-voltage designs for the logic and
core. Advances in FPGAs, custom ASICs, and other products added a third, and sometimes a fourth,
voltage level. Maxim supervisor ICs have kept pace with the development of increasingly complex
products, providing monitoring and control for complex, multi-voltage systems.
Multi-Voltage Supervision
In multi-voltage systems, the simplest way to generate a Power on Reset (POR) signal is to monitor the
3.3V or 5V logic supply. At power-up, when the logic voltage rises above its threshold, the supervisor
initiates a reset period to ensure an orderly turn-on of the processor. As long as the processor's supply
voltage is within specifications (during normal operation), the supervisor continues to monitor that voltage
for transient and brown-out conditions.
But what about the integrity of devices operating at lower levels of core/supply voltage? Those levels are
generated from linear or switching power supplies, so how can you assume they are within specification
before the reset period has elapsed? By monitoring only a single voltage in a multi-voltage design, the
risk can go undetected that improperly powered devices may be loading the bus or responding in an
erratic manner, causing software to deviate from its expected procedure. A good foundation for reliable
design must therefore include the ability to monitor all voltages.
Available supervisors can monitor two, three, or even four supply voltages, either with factoryprogrammed thresholds or with a combination of factory and resistor-programmable thresholds. Factoryprogrammed thresholds are usually available in increments of 50mV to 100mV below the monitored
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voltage level, so a supervisor is selected according to its specified tolerance. If, for example, a supervisor
family specifies thresholds of 3.3V, 3.08V, 2.93V, and 2.63V, you compose a part number for the device
by noting the desired voltage and its corresponding suffix.
Factory-programmed supervisors are single-chip devices that require no external components for
threshold settings. The absence of resistor dividers for the thresholds also eliminates a source of power
dissipation. Resistor-programmable devices, on the other hand, are suitable for engineers who want to
avoid an application-specific device. Once your company qualifies a particular supervisor, you can easily
change its threshold by substituting one or two resistors. And for single-supply systems, you can use the
same multi-voltage supervisor after disabling its other inputs.
Low-Voltage Supervision in Multi-Voltage Systems
The movement of logic levels from 5.0V and 3.3V toward 2.5V and 1.8V is creating a need for for
supervisors that can monitor voltages as low as 0.9V. Such supervisors should operate directly from
1.8V, because the higher voltage levels will not always be available. The smaller differece between
active and inactive states also poses a need to maintain valid reset operation down to supply levels of
1.0V and lower. The ability to reject short-duration transients in the supply voltage (good transient
immunity) is another feature critical for low-voltage systems. Many data sheets include a graph of
transient duration vs. voltage overdrive, which allows the designer to avoid nuisance resets by reviewing
noise characteristics inherent in the power supply.
Device Operation and Features
The currently available families of off-the-shelf supervisor ICs are extremely flexible in meeting system
needs. Besides multi-voltage monitoring, they offer features that make designs more robust and less
susceptible to transient conditions in the hardware and software. The following considerations are critical
in selecting a supervisor.
Reset Period:
Reset Period is a delay interval following the rise of all monitored voltages above their reset thresholds,
during which the reset output is held low. A popular value is 140msec minimum. Thus, the reset pin
remains active for at least 140msec after all monitored voltages have risen above their thresholds. Reset
commands vector the software to a specific code location from which an orderly start-up can be initiated.
Resets also occur in response to a low voltage, manual reset, or watchdog timeout. Reset initializes the
code, and thereby prevents the processor from executing code that might have been corrupted by a low
voltage or software bug. If processor specifications permit, it may be more suitable to increase or
decrease the reset period. Available devices provide reset periods ranging from 1 millisecond to 1.2
seconds.
The reset period also allows the supply voltages, crystal, and phase-locked loop (PLL) to stabilize. The
crystal and PLL have the largest effect on reset period duration. A 20MHz crystal without PLL can use a
short timeout, but a 32kHz crystal phase-locked to 20MHz with a PLL requires a longer timeout.
Reset Output:
An active-low push-pull reset output is appropriate for most applications, but other output types are
available. For applications in which a supervisor replaces the RC delay associated with a traditional 8051
product, the supervisor has an active-high push-pull output or an active-low open-drain output, or both.
Open-drain outputs are generally more flexible. They allow simple wire-OR connections, and easily form
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an interface to devices operating at different system voltages. Open-drain outputs allow the Reset output
to be pulled low by multiple sources without contention. The penalty for that flexibility is the external pullup resistor.
Push-pull outputs in single-voltage systems are straightforward, but those in multi-voltage systems
require more care. Consider, for example, a dual supervisor used to monitor 3.3V and 5.0V supplies. For
the two internal voltage monitors it has one push-pull reset output, which can swing between ground and
the 3.3V rail or (in another version) between ground and the 5V rail. In that case you choose the version
whose voltage swing is compatible with the processor's reset input. Or, the dual supervisor may have
two outputs-one associated with the 3.3V monitor, and one with the 5V monitor. You can choose a
version in which each output swings to the corresponding monitored rail, or both swing to the same rail.
Negative-Going Transients Immunity:
Noisy digital environments can impose voltage transients on a supply voltage regardless of whether it is
generated by a linear regulator or a switching converter. The key goal is to avoid nuisance resets during
normal operation while maintaining continuous monitoring of the supply voltage. A graph (available in the
data sheet for a typical device) provides guidance on what combination of transient overdrive magnitude
and duration will cause a device reset (Figure 1).
Figure 1. Typical Transient Duration vs. overdrive (graph) for the MAX6381.
As you can see, a 50µsec, 50mV transient will not reset the device; resets occur only for transients of
longer duration or greater magnitude. Thus, the graph provides a means for avoiding the dreaded
nuisance resets. Note that supervisors with a higher level of transient rejection may also permit the use
of a lower-cost power supply requiring less filtering (assuming the processor can tolerate the resulting
supply-voltage variation).
Watchdog Timer:
Watchdog timers check for proper software execution. If the software sticks in a loop caused by a bug or
hardware failure, the watchdog timer resets the processor and allows it to reinitialize itself. To avoid a
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reset, the software must generate an edge transition at the watchdog input before the end of each timer
period. An edge transition (rather than an active low or high input) removes the possibility of disabling the
watchdog due to a locked processor output. You must place timer resets (edge transitions) in the
software at locations that ensure a reset of the watchdog before the timeout period elapses.
The art in implementing a watchdog timer is to place the timer resets so they preclude the possibility of a
stuck loop. A handy tip is to force a low-to-high transition in one routine and a high-to-low transition in
the next routine in the sequence. Then, a reset will occur if the software is stuck in one of the routines.
Placing a low-high-low pulse in a single subroutine does not produce a reset, so the software could
remain locked.
To accommodate processors with extended power-up and stabilization requirements, some supervisors
provide longer initial watchdog periods. A longer period allows the processor time to initialize and
configure itself before implementing the subsequent shorter and more rigorous watchdog intervals.
Manual Reset:
Manual Reset makes available to users and functional test devices an easily accessible means of
resetting the processor. Several supervisor products provide an active-low input with internal pull-up
resistor, which eliminates the need for an external resistor and also allows for a simple switch interface.
Another specification associated with the Manual Reset input is glitch rejection. To avoid an unintentional
or nuisance reset, the input should reject short-duration glitches. Such glitch-rejection circuitry not only
prevents unintentional resets, but also eliminates the need for external switch-debouncing circuitry.
Manual resets typically trigger a reset period. To reduce test time, however, the reset period should be
short. MAX6390 ICs address this concern with periods about one-eighth that of a standard reset period
(for a MAX6390D4, the Manual Reset pulse is 140msec minimum and the reset period is 1.12sec).
In addition to level-sensitive manual-reset inputs, some applications may require edge-sensitive inputs,
which ensure that the processor is reset for a fixed period rather than a period that depends on how long
the manual-reset input is held low. That capability is handy for reducing product-assembly and test time.
Overvoltage and Negative-Voltage Monitoring:
For medical or safety-related equipment that performs self-testing, supervisors are available that enable
the detection of both over-voltage and under-voltage conditions. These devices have resistorprogrammable inputs that force a reset when the monitored voltage exceeds a threshold. Like the undervoltage condition, excessive voltage can cause unexpected results in firmware as well as hardware.
Forcing the processor into reset mitigates a potentially unsafe condition.
Analog-output failures can occur in a number of ways, but a simple negative voltage monitor can confirm
that the expected supply voltages are present and within specification. Analog modules with -5V or -15V
rails, for instance, often produce analog outputs for which there is no supply-voltage feedback to verify
their validity. Fortunately, an over-voltage monitor can also monitor a negative voltage. As for the overvoltage case, the supply voltage is sensed by an external resistor divider between that voltage and Vcc
(Figure 2).
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Figure 2. Negative voltage monitor using the MAX6347.
Power Sequencing:
To prevent latch-up and to maximize reliability during power-up, a multi-voltage system often includes a
requirement to sequence or track the VI/O to Vcore or Vcore to VI/O voltages. Tracking generally means
the I/O and core voltages must rise together, and (usually) the core voltage must not exceed the I/O
voltage by more than 0.30V. Sequencing generally means that the I/O voltage must rise before the core
voltage. The system may also specify a delay period between the rise of the I/O and core voltages.
One type of sequencer for a 2-voltage system (I/O = 3.3V and core = 2.5V) employs a single-voltage
supervisor that monitors the 3.3V supply. When that voltage is above its threshold, the supervisor delays
and then enhances an external p-channel MOSFET (Figure 3). That approach is cost effective for lowcurrent applications, but for higher currents the cost of a low-Rdson p-FET with low Vgs threshold can
be high.
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Figure 3. Power sequencer using the MAX6347.
For higher-current applications, a dedicated power sequencer with charge pump may be more effective.
As in the preceding example, this circuit monitors a supply voltage and activates an external FET to
bring up the second supply. The IC device, however, allows use of an n-channel FET that costs less
than the p-channel device. The internal charge pump provides a Vgs of 5.0V, which fully enhances the
n-FET powering the second supply. Not only does the n-FET cost less; its Rdson is notably lower.
As examples, the MAX6819 and MAX6820 are SOT-23 power sequencers that require no external
charge-pump capacitors. The MAX6819 has a fixed 200msec delay, and the MAX6820 has a variable
delay. An external capacitor sets the delay according to the relationship
tDELAY (sec) = 2.484x10 -6 (Cset).
These ICs also enable straightforward sequencing in applications with more than two supply voltages. To
sequence all supplies, you simply add one sequencer for each additional supply voltage (Figure 4).
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Figure 4. Sequencing an additional supply.
Power-Fail Comparator:
If your system must provide an early warning of power loss or low battery voltage, you can choose a
supervisor that includes a logic-level reset circuit and power-fail comparator. For ICs of the MAX6342
family, for example, the reset threshold is factory trimmed. An external resistor divider sets the threshold
for power loss or low-battery detection. Because the threshold is at 1.25V, you can monitor voltages
above and below Vcc, with the minimum threshold at 1.25V. For additional supply voltages, choose a
device with open-drain outputs, which allows use of a second supervisor to monitor the other core
voltages.
Voltage Detection:
The importance of monitoring all supply voltages in a system cannot be overemphasized. It can be
performed via feedback, or by a supervisor driving the reset pin of a processor. Feedback can be in the
form of an A/D converter measuring system voltages, or a software routine monitoring device
functionality. Either approach assures the engineer of proper power on the board.
Another simple method achieves the same result with voltage detectors. Voltage detection can be more
informative than supervision, because it indicates which supply voltage has a problem. Supervision
usually ORs together all voltages and generates a single reset, whereas a multi-voltage detector usually
offers open-drain outputs that can be reviewed individually to determine the source of the problem.
Quad-voltage monitors are available with independent open-drain outputs. Such devices can include
resistor-programmable thresholds as well as factory-programmed thresholds that accommodate supply
voltages of 1.8V, 2.5V, 3.3V, 5.0V, or -5.0V. An internal precision voltage reference and internal voltage
dividers make these ICs very compact.
Summary
The combination of multiple supply voltages, shrinking die geometries, and increasingly important product
reliability specifications has elevated the need for complete supervision or monitoring of supply voltages.
This article has described the products available for that purpose, and the product features critical in
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designing a reliable system.
Related Parts
MAX6338
Quad Voltage Monitor with Four Outputs in µMax
Package
Free Samples MAX6338
Quad Voltage Monitor with Four Outputs in µMax
Package
Free Samples MAX6339
Quad Voltage µP Supervisory Circuit in SOT Package
Free Samples MAX6391
Dual-Voltage µP Supervisory Circuits with Sequenced
Reset Outputs
Free Samples MAX6391
Dual-Voltage µP Supervisory Circuits with Sequenced
Reset Outputs
Free Samples MAX6392
Dual-Voltage µP Supervisory Circuits with Sequenced
Reset Outputs
Free Samples MAX6392
Dual-Voltage µP Supervisory Circuits with Sequenced
Reset Outputs
Free Samples MAX6700
Low-Voltage, High-Accuracy, Triple/Quad Voltage µP
Supervisory Circuits in SOT Package
Free Samples MAX6700
Low-Voltage, High-Accuracy, Triple/Quad Voltage µP
Supervisory Circuits in SOT Package
Free Samples MAX6709
Low-Voltage, High-Accuracy, Quad Voltage Monitors in
µMAX Package
Free Samples MAX6710
Low-Voltage, High-Accuracy, Triple/Quad Voltage µP
Supervisory Circuits in SOT Package
Free Samples MAX6710
Low-Voltage, High-Accuracy, Triple/Quad Voltage µP
Supervisory Circuits in SOT Package
Free Samples MAX6714
Low-Voltage, High-Accuracy, Quad Voltage Monitors in
µMAX Package
Free Samples MAX6714
Low-Voltage, High-Accuracy, Quad Voltage Monitors in
µMAX Package
Free Samples MAX6715
Dual/Triple Ultra-Low-Voltage SOT23 µP Supervisory
Circuits
MAX6715
Dual/Triple Ultra-Low-Voltage SOT23 µP Supervisory
Circuits
MAX6730
Single-/Dual-/Triple-Voltage µP Supervisory Circuits with
Independent Watchdog Output
Free Samples MAX6736
Low-Power Dual-/Triple-Voltage SC70 µP Supervisory
Circuits
Free Samples Page 8 of 9
MAX6736
Low-Power Dual-/Triple-Voltage SC70 µP Supervisory
Circuits
Free Samples MAX6819
SOT23 Power-Supply Sequencers
Free Samples MAX6819
SOT23 Power-Supply Sequencers
Free Samples MAX6820
SOT23 Power-Supply Sequencers
Free Samples MAX6820
SOT23 Power-Supply Sequencers
Free Samples MAX6826
Dual, Ultra-Low-Voltage SOT23 µP Supervisors with
Manual Reset and Watchdog Timer
MAX6826
Dual, Ultra-Low-Voltage SOT23 µP Supervisors with
Manual Reset and Watchdog Timer
More Information
For Technical Support: http://www.maximintegrated.com/support
For Samples: http://www.maximintegrated.com/samples
Other Questions and Comments: http://www.maximintegrated.com/contact
Application Note 2853: http://www.maximintegrated.com/an2853
APPLICATION NOTE 2853, AN2853, AN 2853, APP2853, Appnote2853, Appnote 2853
Copyright © by Maxim Integrated
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