AN3493| Application Note
Maxim > App Notes > MICROCONTROLLERS Keywords: system management, sequencer, sequencing, thresholds, ADC, supervisor, reset, microprocessor supervisor, voltage monitor, voltage monitoring, voltage detect
Mar 04, 2005
APPLICATION NOTE 3493
System-Management IC Meets Monitoring and Sequencing Requirements in Multivoltage Systems
Abstract: The MAX6870 hex-voltage sequencer/monitor provides a fully integrated solution to simplify complex design. This EEPROM-configurable device provides tremendous flexibility in setting thresholds, output structures, and timing delays. In most electronic systems, it is important to monitor system voltages both to ensure that processors and other ICs remain reset during power-up, and to detect when brownout conditions occur. This monitoring minimizes code-execution problems, which corrupt memory or cause systems to execute improperly. In high-end systems, it is also critical to ensure proper sequencing of the many power supplies within these systems. Proper sequencing prevents latch-up conditions, which can create system problems or damage important components, such as microcontrollers (uCs), DSPs, ASICs, or microprocessors (uPs). Commonly, one or more supervisory products are needed to implement the proper sequencing and monitoring functions noted here. Traditionally, many of these functions have been
performed with power-on resets and other uP supervisory circuits. In recent years, as the number of supply voltages has increased, the number of devices required to perform this duty has also grown, thus compounding complexity, cost, and the board space consumed.
Monitoring and Sequencing Complex Systems
The easiest way to monitor a supply voltage is with either a power-on reset (POR) or a voltage-detector circuit. These devices can monitor a single voltage or multiple voltages. After the monitored supply voltage has powered up and exceeded the POR's voltage threshold, the POR's output does not de-assert until after a specified time period. This allows the system clocks to stabilize, and the system boot routine to initialize before permitting the uC to operate. These PORs and voltage detectors can also be used to sequence power supplies. Connecting the output of a POR that is monitoring one regulator to the shutdown pin of the next regulator (i.e., daisy-chaining them), one regulator will then come up after the other, once the POR's time delay has elapsed. As the number of system supply voltages increases, voltage monitors and supervisors that monitor multiple voltages become necessary. However, because it is common for ten to fifteen voltages to power a complex system, several such devices are often needed.
Challenges When Using Multiple Supervisors
Using this multiple-supervisor approach has its own problems. One problem is finding devices with the correct thresholds. Although there are a number of standard voltages, such as 3.3, 2.5, 1.8, 1.5, and 1.2V, many nonstandard voltages need to be monitored. This requires external resistor-dividers to set the monitored thresholds. If system-supply voltages change (e.g., you lower an ASIC's core voltage to reduce power consumption, or increase it to enhance the ASIC's performance), you would have to change the resistor values to accommodate these new voltages. Obtaining this flexibility requires these additional external resistors, and thus more board space and cost. The same problems occur when selecting the correct reset timeout periods. Another problem with multiple supervisors occurs when a system must provide a specific power-up sequence. When larger numbers of supply voltages power a system, the daisy-chaining technique outlined above might not be capable of handling the timing when various supplies are
coming up. Also, as the sequencing requirements change during development, altering circuitry to accommodate those later changes becomes problematic. An additional sequencing problem can occur when these large systems use "silver box" or "brick" power supplies. These supplies simplify power-supply design, but pose a problem when a particular power-up sequence is required. For example, a brick supply that provides multiple output voltages might only have a single enable pin. Therefore, all its supply voltages turn on and off at the same time under the control of that one pin. A brick supply with multiple enable (or shutdown) inputs can resolve this issue. However, if multiple ICs share the same supplies (for example, a 3.3V I/O logic supply and a 1.8V
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