6-Input Supervisors Offer Accurate Monitoring and 125°C Operation

Introduction

The latest trio of power supply supervisors from Analog Devices is ideal for today’s multi-voltage systems that require accurate supply monitoring. The LTC2930, LTC2931, and LTC2932 are 6-input voltage monitors capable of maintaining 1.5% threshold accuracy from –40°C to 125°C. The combination of monitored supply voltages is set by a single pin. Each part offers 16 threshold voltage combinations, thus meeting the needs of almost any multi-voltage system. This programmability eliminates the need to qualify, source and stock unique part numbers for different threshold voltage combinations.

The overall architecture and operating specifications of these three devices are similar, but each has unique features (see Table 1). The LTC2930 generates a reset after any undervoltage event or when the manual reset input (MR) pulls low. It is ideal for space-constrained applications as it comes in a compact 3mm × 3mm 12-lead DFN package. The LTC2931 includes a watchdog input (WDI), a watchdog output (WDO) and user-adjustable watchdog periods to enable microprocessor monitoring and control. The LTC2932 can vary its monitor thresholds from 5% to 12.5%, and a reset disable pin provides margining capability. Both the LTC2931 and LTC2932 are packaged in 20-pin TSSOP packages and have separate comparator outputs, enabling individual supply monitoring and/or sequencing.

Table 1. LTC2930, LTC2931, LTC2932 feature summary

Feature	LTC2930	LTC2931	LTC2932
Configurable Input Threshold Combinations	16	16	16
Threshold Accuracy	1.5%	1.5%	1.5%
Adjustable Reset Time
Buffered Reference
Individual Comparator Outputs
Manual Reset
Independent Watchdog Circuitry
Reset Disable
Supply Tolerance	Fixed, 5%	Fixed, 5%	User Selectable 5%, 7.5%, 10%, 12.5%
Package	12-lead 3mm × 3mm DFN	20-lead F Package	20-lead F Package

Single Pin Configuration Makes Life Easy

These supervisors offer an elegant method of configuring the input voltage thresholds. Figure 1 shows how a single resistive divider at the VPG pin sets the supervisor into one of the 16 threshold options shown in Table 2. See the data sheet for suggested mode-setting resistor values.

The actual thresholds are set by integrated precision dividers for 5V, 3.3V, 3V, 2.5V, 1.8V, and 1.5V supply monitoring. For other supply values, uncommitted comparators with 0.5V thresholds allow virtually any positive supply to be monitored using a resistive divider, as shown in Figure 2a. The V4 input also monitors negative voltages—with the same 1.5% accuracy—using the integrated buffered reference for offset (see Figure 2b).

What Does Threshold Accuracy Mean?

Consider a 5V system with ±5% supply tolerance. The 5V supply may vary between 4.75V to 5.25V. System ICs powered by this supply must operate reliably within this band (and a little more, as explained below). A perfectly accurate supervisor for this supply generates a reset at exactly 4.75V. However, no supervisor is this perfect. The actual reset threshold of a supervisor fluctuates over a specified band; the LTC2930, LTC2931 and LTC2932 vary ±1.5% around their nominal threshold voltage over temperature (Figure 3). The reset threshold band and the power supply tolerance bands should not overlap. This prevents false or nuisance resets when the power supply is actually within its specified tolerance band.

The LTC2930, LTC2931 and LTC2932 boast a ±1.5% reset threshold accuracy, so a “5%” threshold is usually set to 6.5% below the nominal input voltage. Therefore, a typical 5V, “5%” threshold is 4.675V. The threshold is guaranteed to lie in the band between 4.750V and 4.600V over temperature. The powered system must work reliably down to the low end of the threshold band, or risk malfunction before a reset signal is properly issued.

A less accurate supervisor increases the required system voltage margin and increases the probability of system malfunction. The tight ±1.5% accuracy specification of the LTC2930, LTC2931 and LTC2932 improves the reliability of the system over supervisors with wider threshold specifications.

Glitch Immunity = No Spurious Resets!

Monitored supply voltages are far from being ideal, perfectly flat DC signals. Riding on top of these supplies are high frequency components caused by a number of sources such as the output ripple of the power supply or coupling from other signals. If the monitored voltage is near or at the reset threshold voltage, this noise could cause spurious resets. Fortunately, the LTC2930, LTC2931 and LTC2932 have been designed with this potential issue in mind, so spurious resets are of little to no concern

Some supply monitors overcome spurious resets by adding hysteresis to the input comparator. The amount of applied hysteresis is stated as a percentage of the trip threshold. Unfortunately, this degrades monitor accuracy because the true accuracy of the trip threshold is now the percentage of added hysteresis plus the advertised accuracy of the part. The LTC2930, LTC2931 and LTC2932 do not use hysteresis, but instead use an integration scheme that requires transients to possess enough magnitude and duration to switch the comparators. This suppresses spurious resets without degrading the monitor accuracy.

The COMP5 comparator output response to a “noisy” input on the LTC2931 is demonstrated in Figure 4.

In the example shown, a 500kHz, 100mV_P–P sine wave centered at 500mV is applied to the V5 input. The threshold voltage of the adjustable input, V5, is 500mV. Even though the signal amplitude goes as low as 450mV, COMP5 remains high. Next, the DC level of the input is dropped 2mV. In response, COMP5 pulls low and remains low. As mentioned earlier, only transients of long enough duration and magnitude trigger the comparator output to pull high or low.

Adjustable Reset Timeout Period for Varied Application Needs

Each of the supervisors includes an adjustable reset timeout period, t_RST. Once all the inputs are above their threshold values, the reset timer is started (Figure 5). RST stays low for the duration of t_RST and remains low as long as the time between transients is less than the reset timeout. In other words, the reset timeout prevents supply transients with frequencies greater than 1/t_RST from causing undesired toggling at the RST output. Keeping RST low during these supply transients suppresses spurious resets.

The reset timeout period is adjustable to accommodate a variety of microprocessor applications. Configure the reset timeout period, t_RST, by connecting a capacitor, C_RT, between the CRT pin and GND. The value of this capacitor is determined by

Leaving the CRT pin unconnected generates a minimum reset timeout of approximately 25μs. Maximum reset timeout is limited by the largest available low leakage capacitor.

Additional Glitch Filtering

Even though all six comparators have built-in glitch filtering, adding bypass capacitors on the V1 and V2 inputs is recommended, because of these two, the input with the higher voltage functions as V_CC for the entire chip. Additional filter capacitors may be added to the V3, V4, V5 and V6 inputs if needed to suppress troublesome noise.

Open-Drain Reset

The RST outputs on the LTC2930, LTC2931 and LTC2932 are open-drain and contain weak pull-up current sources to the V2 voltage. The open-drain structure provides many advantages. For instance, each of these outputs can be externally pulled-up to voltages higher than V2 using a pull-up resistor. This facilitates the use of multiple devices operating under different I/O voltages. In addition, multiple open-drain outputs can be configured in a “wired-OR” format where the outputs are tied together. Figure 6 showcases two LTC2930 supervisors, whose open-drain RST outputs are tied together and pulled-up to 5V via a 10k pull-up resistor. If one RST output pulls low due to a reset event, it sinks current and pulls the other output low.

Comparator Outputs Enable Individual Supply Monitoring and Sequencing Support

Real-time comparator outputs on both the LTC2931 and LTC2932 indicate the status of the individual inputs. Similar to the RST output, the comparator outputs are also open-drain and have weak pull-up current sources to the V2 voltage.

While RST pulls low when an undervoltage event occurs on any of the monitored supplies, a comparator output pulls low only when its counterpart input is below its threshold voltage. The ability to monitor the status of each supply is useful in multi-voltage systems where it is important to know which particular supply has failed.

The individual comparator outputs also allow power supply sequencing. Figure 7 shows the LTC2932 in a 5-supply power-up sequencer. As an input reaches its threshold, the respective comparator output pulls high and enables the next DC/DC converter.

The LTC2950-1 is used to provide pushbutton control for the sequencer. After the pushbutton is pressed, the LTC2950-1 pulls the RUN pin of the LTM4600 high. Subsequently, the LTM4600 generates a 5V output which supplies power to each of the four DC/DC converters.

Three Supervisor Flavors

LTC2930: Manual Reset (MR) Forces RST Low

Use the manual reset input (MR) on the LTC2930 to issue a forced reset, independent of input voltage levels. A 10μA (typical) internal current source pulls the MR pin to V_CC. A logic low on this pin pulls RST low. When the MR pin returns high, RST returns high after the selected reset timeout period has elapsed, assuming all six voltage inputs are above their thresholds (Figure 8). The input-high threshold on the MR pin is 1.6V (max), allowing the pin to be driven by low voltage logic as well.

LTC2931: Monitor a Microprocessor with the Watchdog Function

The LTC2931’s independent watchdog circuitry monitors a microprocessor’s activity. The microprocessor is required to change the logic state of the WDI pin on a periodic basis in order to clear the watchdog timer. The LTC2931 consists of a watchdog input (WDI), a watchdog output (WDO) and a timing pin (CWT), which allows for a user adjustable watchdog timeout period. Figure 9 illustrates the watchdog timer and its relationship to the reset timer and WDI.

The watchdog timeout period is adjustable and can be optimized for software execution. The watchdog timeout period, t_WD, is adjusted by connecting a capacitor, C_WT, between the CWT pin and ground. The value of this capacitor is determined by

Leaving the CWT pin unconnected generates a minimum watchdog timeout of approximately 200μs. Maximum watchdog timeout is limited by the largest available low leakage capacitor.

LTC2932: Margining Capabilities and Wider Threshold Tolerances

In high reliability system manufacturing and testing, it is important to verify that the components will continue to operate at or below the rated power supply tolerance. Verification usually involves margining the power supplies, running their outputs at or beyond rated tolerances. The LTC2932 is designed to complement such testing in two ways. First, the RST output can be disabled by pulling RDIS low. In this state, the RST output remains high despite any undervoltage events which may occur during margining tests. This does not affect the individual supply monitoring, which is independent of the logic state of RDIS. Second, lowering the trip thresholds can increase supply headroom to match the margining ranges. This is simply a matter of changing the two tolerance selection inputs, T0 and T1, to adjust the global supply tolerance to 5%, 7.5%, 10%, or 12.5% (see Table 3).

Automotive Application

The ease of implementation, wide operating temperature range, and low supply current requirements for the LTC2930, LTC2931 and LTC2932 supervisors make them ideal for automotive applications. Figure 10 is a block diagram schematic of an automotive application that uses the LTC2931 and LTC2932. It was designed to highlight and utilize the features of these parts beyond simple voltage monitoring. The voltage monitors are powered by the LT3010-5, a fixed 5V micropower linear regulator. Voltage transient protection is provided by the LT4356DE-1 overvoltage protection regulator and inrush limiter.

In a typical automotive power system, a distinction is made between “Always On” and “In Cabin” electronics. “Always On” systems include critical electronics that deal with the safety and security of an automobile and, as the name implies, are always on. “In Cabin” electronics pertain to comfort and entertainment accessories used in automobiles. In the event the battery is low, for instance, the in cabin electronics are turned off to preserve and siphon power to the critical path.

In this automotive application, power for the always on critical electronics is generated by the LTC3780 buck boost regulator and monitored by the LTC2931. The LTC3780 is protected from transients by the LT4356DE-1 and is capable of delivering full power to the load with a supply voltage as low as 6V. The LTC2931 is configured to monitor four fixed and two adjustable voltages, including two independent 5V supplies. 1.5% voltage monitoring accuracy is guaranteed over the entire operating temperature range. Additionally, each voltage monitoring channel has its own comparator output that can be used by the microprocessor to identify a fault condition. The comparator outputs are pulled up to the 5V bus that powers both voltage monitoring devices. The LTC2931 has an adjustable watchdog timer, which allows the LTC2931 to report a malfunctioning microprocessor to the rest of the system.

The unregulated battery voltage and power supplies delivered to the in cabin electronics are monitored by the LTC2932. This application monitors the unregulated battery voltage, and the COMP4 output alerts the system to a low battery condition, allowing the system to enter a standby or power save mode.

The LTC2932 also provides a mechanism to override a reset or fault condition. This is accomplished by pulling the RDIS pin low. With RDIS pulled low, the RST output pulls up to the V2 input voltage. Since V2 is tied to V1, the reset high level is 5V. The RDIS function allows the system to have flexibility in controlling the power sources without generating system faults. Additionally, the LTC2932 allows real time setting of the voltage monitoring threshold. This could be useful when changes in loading or environment make for predictable supply variances.

Conclusion

The LTC2930, LTC2931 and LTC2932 can each monitor six supplies, saving valuable board area in space constrained applications. The LTC2930 is available in a 3mm × 3mm DFN, while the LTC2931 and LTC2932 are available in 20-pin TSSOP packages.

All include design-time saving features for multi-voltage applications. Voltage thresholds are accurate to ±1.5%, guaranteed over the entire –40°C to 125°C temperature range. This translates directly to simplified power supply design, as threshold accuracy must be accounted for in the entire power supply tolerance budget.

Comparator glitch immunity eliminates false resets, with no effect on the high accuracy of the monitor. These devices support a variety of voltage combinations, easily set with only a few external components. The reset timeout period is also adjustable with a single capacitor.

Lastly, the features which differentiate the LTC2930, LTC2931 and LTC2932 give users the flexibility to choose one for any application.

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