Multi-voltage system monitor
Increasing component density and a lower speed processor core voltage, thus, multi-voltage system came into being. The first multi-voltage system is used for the logic of power and nuclear power dual-voltage monitor. And then the third way to increase monitoring of voltage, for fpga, custom asic and other products, some applications and even the way the fourth voltage level monitor. ic monitoring of the maxim has always been with the ever-increasing complexity of product development to keep pace, multi-voltage systems for surveillance and control functions.
Multi-voltage system monitoring
In a multi-voltage system, used to generate power-on reset (por) is a simple way to monitor the 3.3v or 5v logic supply. Power during the period when the logic threshold voltage rose to above, the monitors reset to start a process to ensure open and orderly processor. As long as the processor supply voltage range of the required (normal work), will monitor the continuous monitoring of the transient voltage and power-down conditions.
However, how to ensure that the lower nuclear voltage / power supply voltage to the integrity of the device? These power supply voltage is usually a linear or switching power supply regulator to provide, in which case how to reset before the end of the cycle in line with the regulatory requirements to ensure that voltage? Multi-voltage system if only all the way to monitor voltage, there may not be able to detect abnormal power supply device, the device may be loaded into the bus, or an error response, resulting in deviations from the requirements of the software process. Therefore, the design is based on reliable: It is necessary to monitor the supply voltage of each road.
Existing monitor can monitor two, three, four-way power supply voltage, may be factory set threshold, or threshold set by the factory with the combination of resistance regulation. Factory set threshold voltage is usually able to monitor the following in accordance with the 50mv to 100mv-step increments, based on the monitoring circuits to choose tolerance requirements. For example, a monitor threshold to 3.3v, 3.08v, 2.93v and 2.63v, in accordance with the required voltage value and the corresponding suffix to determine the end of the standard devices.
Factory set up to monitor the circuit for the single-chip devices, eliminating the need for external components set threshold. Save the settings as a result of the resistor divider threshold, thus eliminating the need for a power source. On the other hand to consider, through the resistance adjustment device threshold voltage requirements are more suitable for a flexible system. Monitoring circuit to determine, through the replacement of one or two easily adjust resistance threshold monitoring. For single-supply system can also use the same multi-voltage monitoring circuit, only the prohibition of other input.
Multi-voltage low-pressure monitoring system
As a result of the logic of power from the 3.3v to 5.0v or 2.5v or 1.8v, require monitors to monitor the voltage as low as 0.9v. Monitor this direct work in 1.8v voltage, high voltage in the system is no longer a general power, may not be able to get. Effective work in the inactive state and require monitoring circuit can be as low as 1.0v, and even lower voltage to maintain a valid reset signal. Short-term power supply transient suppression is another important low-pressure system characteristics (good transient suppression). Many of the data given the duration of the transient voltage overdrive with the relationship between the system design to avoid the inherent power as a result of unnecessary noise reduction.
The working principle and characteristics of devices
Currently on the market able to provide monitoring ic has a very high flexibility to meet the system requirements. In addition to multi-voltage monitoring, these devices also ensure that the design of stability, by hardware, software, the impact of transient conditions even smaller. The following considerations for the choice of a monitoring circuit is very important. Reset cycle:
Reset cycle is when all the monitored voltage exceeds its reset threshold, the reset output to maintain the status reset time, usually the minimum is 140ms. In this way, reset pin voltage monitor in all rose to above its threshold after 140ms (minimum) reduction in the state remained. Software reset command to return to a particular location code, which can be initialized to start an orderly process.
There would be a low voltage, manual reset or watchdog reset can lead to hours of overtime. Reset signal to initialize the code to avoid the processor running on low voltage or as a result of defects and destruction of software code. If the specification allows the processor may be appropriate to increase or reduce the reset cycle, the current devices can provide the scope of the reset cycle is 1ms to 1.2s.
Reset cycle for the power supply voltage, crystal and phase-locked loop to achieve stability in the provision of adequate time. Pll crystal and the choice of the reset cycle, the greatest impact, non-20mhz crystal pll just short of the reset time-out cycle, but phase-locked pll Crystal 32khz to 20mhz need of a longer cycle time.
The push-pull active-low reset output for most applications, the device can also provide other output types, such as in a traditional application in 8051 to replace the use of monitoring circuit rc time delay circuit, control circuit to provide high effective push-pull output, or active-low open-drain output, or both at the same time to provide the output.
Open-drain output in general more flexible, this output allows simple "line or" connectivity, and easy to work in a multi-voltage system to connect to other devices. Open-drain output can be connected each other through a variety of sources down the output reset signal. Of course, the flexibility needed to access the external pull-up resistor. Single-supply system is very simple push-pull output, but in a multi-voltage system should pay more attention to. For example, two-way monitoring circuit to monitor voltage 3.3v and 5.5v power supply. All the way for the push-pull reset output of the two internal voltage monitoring circuit, the output swing in between with 3.3v and 5.5v or between (a circuit). Voltage swing at this time may choose to enter the agreement with the processor reset output. In some cases, dual-voltage monitor output may have two-way: all the way to monitor 3.3v, another way to monitor 5.5v. Reset based on the requirements of output swing, the output swing of the corresponding monitor voltage output swing or two in the same supply voltage, select the monitor.
Negative voltage transient suppression:
Whether it is powered by the linear regulator or switching power converters, a noisy digital environment can cause transient power supply voltage. System design, the key is to continuously monitor the supply voltage and to avoid unnecessary work properly reset the device. The chart (available on the specified device to find data) describes the reduction of the moment caused by the device drivers have been the magnitude and duration (Figure 1).
Figure 1. Max6381 typical transient duration curves have been driven
As shown, 50μs, 50mv transient will not cause the device reset, reset took place in a longer duration or greater rate of transient. The device shows a way to avoid unnecessary reset. Attention to more high transient suppression circuit of the monitor allows the use of low-cost power, requires less filter components (assuming the processor to receive the corresponding power supply voltage change).
Watchdog timer to ensure the normal operation of the software, if due to procedural defects in software or hardware failure resulting in death to stay in a cycle of Court, the watchdog timer will reset the processor, so that the processor re-initialization. In order to avoid reduction in the timer before the end of each time cycle, the software must create a watchdog input hopping along. Hopping along the (a low or high input) ruled out the processor output latch to prohibit the possibility of watchdog work. Overtime must be the end of the cycle (arising from watchdog reset) before the software reset by the timer (one hopping along).
The trick of the watchdog timer is reset timer to prevent the cycle of software into the death. Design, you can set up a subroutine in the first from low to high jump, and the subroutine in the next set of hopping a high to low. In this way, once found dead in a subroutine will trigger a reset cycle. If installed in a subroutine generated from low to high, high to low pulse may appear dead when no reset cycle, resulting in closed software.
Processor power supply to meet the more stringent requirements on the power and stability, a number of monitoring circuits need to initialize the watchdog to provide a longer cycle, so as to provide sufficient processor time to initialize, in the implementation of follow-up more a short period of time, more stringent watchdog reset before the interval to complete the configuration of their own.
Manual reset for the user and device functional test provides a simple way of processor reset. Some monitoring products to provide active-low reset input with internal pull-up resistor, eliminating the need for external resistors, and switches using a simple interface. With manual reset input related to a special function is to suppress interference. To prevent unnecessary interference suppression circuit of the reset, and eliminating the need for an external switch circuit to tremble.
Manually reset the trigger a reset cycle, in order to reduce test time, reset time should be short as much as possible. max6390 ic very suitable for such a request, the manual reset reset time for a typical time 1 / 8 (for max6390d4, the minimum for manually reset pulse 140ms, typical reset time of 1.12s).
In addition to manual reset input level, some applications may need to manually reset the edge of input, to ensure that the processor in a fixed reset process to complete the cycle, while maintaining the low manual reset input time has nothing to do. This characteristic to reduce assembly time and test time is very convenient.
Over pressure and negative pressure monitoring:
The need for self-test medical or safety equipment, monitors can be over-voltage and under-voltage detection. These devices have a programmable input resistance, voltage monitor when the door beyond the limit corresponding, mandatory reduction. And under-voltage conditions are similar, over-voltage in the firmware and hardware will be produced unpredictable results. Mandatory processor to enter reset state help to reduce the potential insecurity.
analog output occurred There are many reasons for failure, but a simple negative-pressure monitors to determine supply voltage required by the existence or whether they are regulatory requirements. For example, with-5v or-15v power analog circuits, analog output will be in the absence of power supply voltage feedback circuit to verify the effectiveness. Fortunately, over-voltage monitor can also be used to monitor the negative pressure. For over-voltage fault, through the vcc power supply and an external resistor divider between the supply voltage detection (Figure 2).
Figure 2. Max6347 to monitor the use of negative pressure
Power in the process of locking to prevent and improve system stability, multi-voltage monitoring system usually vi / o to the vcore voltage or vcore to vi / o voltage sequencing and tracking. Tracking means that the i / o voltage and voltage rise of nuclear and (generally) not more than nuclear voltage i / o voltage beyond 0.30v. Sort that i / o power supply than nuclear power to power, the system usually specified from the i / o power supply power to the power of nuclear power on delay time.
Dual power supply system (i / o = 3.3v and nuclear voltage = 2.5v) power is a way to sort a single 3.3v power supply monitor detection. When the gate voltage over a specified time limit, monitor and drive the trigger delay of an external p-channel mosfet (Figure 3). This method has a high cost for low current applications. Current circumstances, with a low threshold vgs, rdson the p-channel low-cost high fet.
Figure 3. Max6347 achieve using the power sequencing
For more current application, you can use with charge-pump power sequencer. As in the previous case, the supply voltage by monitoring all the way and drive external fet, the power to open the second road. Ic as a result of allowing the use of n-channel fet, compared with the p-channel devices can be cost-effective. Internal charge pump voltage 5.0v vgs provided to enhance n-channel fet driver, the road system to provide a second power supply. n-fet is not only low cost, but also lower rdson.
max6819 and max6820 is sot-23 package, power sequencing, and the need for an external charge pump capacitors. max6819 with a fixed 200ms delay, max6820 provide a variable delay, delay set by an external capacitor, as shown in the following style: For the greater application of current, you can use with charge-pump power sequencer. As in the previous case, the supply voltage by monitoring all the way and drive external fet, the power to open the second road. Ic as a result of allowing the use of n-channel fet, compared with the p-channel devices can be cost-effective. Internal charge pump voltage 5.0v vgs provided to enhance n-channel fet driver, the road system to provide a second power supply. n-fet is not only low cost, but also lower rdson.
max6819 and max6820 is sot-23 package, power sequencing, and the need for an external charge pump capacitors. max6819 with a fixed 200ms delay, max6820 provide a variable delay, delay set by an external capacitor, as shown in the following style:
tdelay (sec) = 2.484x10-6 (cset).
Power in more than two applications, which can also be used to direct sequencing ic. Road to the power of each sort, the need for an additional power supply for each sequencer (Figure 4).
Figure 4. More power sequencing
Comparator power failure:
If the system requirements for the provision of power failure or low battery voltage warning alarm, the option package extended logic level reset and power failure detection circuit monitoring comparators. For example, max6342 series ic, reset threshold set by the factory. External resistor divider to set the power failure detection threshold or low battery voltage detection threshold. Since the threshold voltage of 1.25v, can be used to monitor the above and below the vcc voltage, the minimum threshold for 1.25v. For more power supply voltage, you can choose to open-drain output devices, use the second monitor to monitor other nuclear voltage.
Surveillance systems can not be too much emphasis on the supply voltage for all the importance of monitoring. Through the feedback circuit or processor reset pin driver control circuit. Feedback system voltage can be measured in a / d converter, or pieces of functionality through software monitors. Each method can ensure that the correct power supply circuit board.
Another simple method is the use of voltage detectors, the result will be the same. Voltage detection circuit monitors to provide more than the clear message, you can point out which way a supply voltage failure. Monitoring of all circuit usually monitors the output voltage "or" computing, resulting in single reset output, multi-channel voltage detector provides an open-drain output, respectively, for each voltage, in order to determine which way the power problems. For example, 4 voltage monitor the use of an independent open-drain output, the device includes programmable resistance thresholds, as well as the factory default threshold for the monitor 1.8v, 2.5v, 3.3v, 5.0v or-5.0v supply voltage. Internal precision voltage reference and resistor divider ic structure of these very compact.
Multi-power supply, more and smaller die size and reliability requirements for products and continuous improvement, there is an urgent need to complete the power supply voltage monitor or monitors. In this paper, for this purpose or for highly reliable monitoring system design products.
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