摘要

Unlike traditional voltage supervisors, glitch-free supervisors increase system reliability by avoiding ambiguities such as power-up glitches, undefined logic output states, and brownout or blackout logic errors. These ambiguities raise concerns in safety and reliability applications such as explosives, critical power-path monitor/control, highly sensitive low-voltage rail devices, and intrinsic safety equipment. This application note discusses some of the safety concerns in a system design and shows how the MAX16161 / MAX16162, Maxim Integrated's family of glitch-free supervisors, can be utilized.

Introduction

Supervisor ICs play a crucial role and act as a first line of defense in most systems, increasing reliability by protecting against overvoltage transients and power failure conditions. The purpose of a voltage supervisor is to ensure proper system power-up; provide immunity against a blackout, voltage transient, or brownout conditions; and monitor system voltage rail for undervoltage conditions. This ensures there is no failure or performance degradations that compromise the underlying systems.

The role of the power-on-reset (POR) feature in most supervisors is to guarantee a valid output signal with a correct logic level during power-on. Having a near-zero POR voltage (V_POR) is most desirable to ensure no indeterminate logic output is fed to the subsequent system. However, it is a great challenge for designers to make V_POR lower, as the output MOSFET driver needs a minimum supply voltage to maintain a correct logic output. This causes an indeterminate logic state at the active-low RESET output of the supervisor, as shown in Figure 1 and Figure 2. Figure 3 shows a typical input voltage specification of a conventional supervisor, defining the minimum V_CC required to guarantee a valid RESET logic.

This application note explains various scenarios where the power-up glitch and some other important aspects of conventional supervisors can cause severe safety concerns and result in system reliability issues and failures.

Brownout and Blackout Conditions

Blackout is a condition in circuits when there is an unintentional fall in supply voltage to zero voltage for a short period, whereas in brownout conditions, the voltage falls to a non-zero voltage level. Some sensitive electronic systems that require precisely regulated supply voltages may be unable to function in these circumstances. Long-term brownouts can cause premature wear and performance degradation to a system, which would be catastrophic in some mission-critical or safety applications. If brownout occurs, the supervisor IC asserts a processor reset until the supply voltage rises to a safe level. Conventional supervisors tend to create reset output glitches or indeterminate logic states during a blackout or brownout condition as shown in Figure 2.

The MAX16161/MAX16162 are designed in such a way that no indeterminate reset output occurs during a blackout or brownout condition. The reset output stays low, even when the supply voltage falls below the UVLO or V_POR. Figure 4 and Figure 5 demonstrate how glitch-free supervisors behave in such conditions.

Example Application 1: Interfacing with FPGA and Low-Voltage Processor Cores

As technology advances, lower supply rails with tighter tolerances pose challenges for system designers, requiring them to protect data and move the system to a safe mode when key components like field-programmable gate arrays (FPGAs), application-specific integrated circuits (ASICs), or memory banks are exposed to high-voltage spikes. In low-voltage processors, the I/O logic levels offer little margin and the maximum voltage of logic low (VIL) can be as little as 0.4V. Above this is when it reads an invalid logic as shown in Figure 5.

During power-up of these spike-sensitive devices, processors need to be in a valid reset logic state until all supply rails are stable. Otherwise, any spikes at the logic inputs, from the supervisor's RESET output, can trigger an accidental reset release of the FPGA or ASIC. In other words, a reliable supervisor should never introduce a high impedance or undefined logic level in the system at any point in time. Figure 6 demonstrates a simplified diagram of the MAX16161/MAX16162 in such applications.

Example Application 2: Power Path Monitor and Control

Figure 7 shows a load switch associated with a current sensor network and the MAX16161 as a load switch controller. During conditions such as power-up, short circuit to ground, battery or supply removal, and blackout or brownout conditions, the MAX16161 ensures that the power path load switch or the high-side MOSFET is in the off state.

Example Application 3: Reliable Candidate in Intrinsic Safety Applications

Intrinsic safety (IS) is an approach to the design of equipment used in hazardous areas. The idea is to avoid any spark-producing condition or to reduce the available energy to a level where it will not cause ignition. This means preventing sparks and keeping temperatures low. Glitches or surges during any power-up or blackout condition can be fatal in systems used in critical applications such as explosives. Other important points to consider in such applications include the following: immediate shutoff of power if any issue is developed; no false power-on; and minimizing mechanical points of failure that can violate the standards of approval agencies such as FM/UL in the United States and CELENAC in Europe.

Now let us discuss how the MAX16161/MAX16162 can be a good solution to address the concerns listed above. Figure 9 shows the typical application circuits and expected behavior of the MAX16161/MAX16162. Since the MAX16161/MAX16162 offer the fewest number of functional pins and require a minimum number of external components, the failure rate or malfunction is kept to a minimum, which help them to comply with reliability and safety standards. Since the timing parameters and threshold voltage settings of the MAX16161/MAX16162 are all factory-trimmed, the issues related to a short/open of the external passive components, often associated with traditional supervisors, are avoided. As the MAX16161/MAX16162 create no glitch or invalid state, accidental power-on or ambiguous logic states can be easily avoided.

Novel and Integrated Solution for a Glitch-Free Supervisor

Maxim Integrated has introduced the MAX16161/MAX16162 nanoPower, true glitch-free voltage supervisors. The MAX16161/MAX16162 can sink the current through the RESET pin even if V_CC is 0V. This assures the valid state of RESET at a zero-supply voltage and provides glitch-free power-up/-down operations.

The MAX16161/MAX16162 do not require any external components for its glitch-free operation, which makes them tiny and cost-effective. Figure 9 demonstrates the power-up and power-down of the MAX16162. The main features and benefits of the MAX16161/MAX16162 are as follows:

• No power-up glitch
• 825nA (typical) quiescent current extends battery life
• Positive and negative level-triggered MR input options (MAX16161)
• MR debounce circuitry (MAX16161)
• Separate VCC and VIN inputs (MAX16162)
• Multiple available reset timeout periods
• Threshold voltage options
  
  
  • 1.7V to 4.85V (MAX16161)
  • 0.6V to 4.85V (MAX16162)
• 4-bump WLP and 4-pin SOT23 packages

-40°C to +125°C Operating Temperature Range

Conclusion

The MAX16161/MAX16162 nanoPower glitch-free supervisors are great candidates for applications where safety and reliability are the priority. This application note described various applications where this family of devices can provide multiple advantages over the existing solutions available on the market. Notable benefits of this family include: reliable power-up and assured levels of RESET output even when the V_CC is equal to zero; the fewest number of functional pins for intrinsic safety applications; and compact solution size with tiny wafer-level packages as well as industry-standard SOT23 package options; and 825nA of quiescent current to help extend system battery life.