Abstract

As the technologies in MCUs, µPs, DSPs, and FPGAs move towards lower geometries and lower power, operational voltages become significantly low for these devices. Reducing the core voltage pose challenges in the use of high-accuracy power supply and voltage supervisors to avoid any catastrophic failure in the system. If the input supply voltage drifts are near the operational boundary of the microprocessor, a voltage supervisor should gracefully shut down or reset the microprocessor before presumably losing its data or not performing in some way. This application note discusses the critical parameters associated with the MAX16132–MAX16135 supervisor family and presents a reasonable approach in choosing the right reset threshold and hysteresis for voltage supervisor ICs.

Introduction

As the industry moves toward low-voltage microprocessors, DSPs, and FPGAs, the operating voltage ranges of power supplies narrows. This increases the demand for high-accuracy power supply and voltage supervisors such as the MAX16132–MAX16135, which are high-accuracy, multichannel window voltage supervisors with factory-programmed undervoltage and overvoltage thresholds.

In low operating voltage systems, if the input supply voltage drifts near the operational boundary then the microprocessor, DSP, or FPGA figure will respond to these changes by switching to a known shutdown or reset stage before losing its data. Voltage supervisors play a critical role by sensing the voltage drift and generating a clean reset signal to the microprocessor.

Typical Application Circuit

Figure 1 shows a typical application system where the MAX16132 voltage supervisor monitors a 1.2V rail that powers the core of the microprocessor. The low-dropout regulator (LDO) generates a 1.2V V_CORE voltage from a typical 3.3V system rail. When the LDO output voltage drifts outside the tolerance window of a microprocessor, the MAX16132 holds the microprocessor in reset state so that its operations are halted outside the specified voltage range

Device Operation and Features

The MAX16132–MAX16135 are low-voltage, ±1% accurate, single/dual/triple/quad-voltage window supervisors that monitor up to four system-supply voltages for undervoltage and overvoltage faults. To design a robust solution of voltage monitoring, the following key parameters should be carefully considered:

• Overvoltage and Undervoltage Thresholds
• Threshold Accuracy
• Hysteresis
• Reset Timeout Period
• Transient Immunity

The following sections will describe each parameter in detail.

Overvoltage and Undervoltage Thresholds

Most supervisory IC thresholds are determined by either internal or external resistive-divider networks with a certain threshold tolerance specified in the datasheet The MAX16132–MAX16135 family is designed differently, in that the selection of nominal input voltage does not determine the threshold level. In fact, the nominal voltage (V_INNOM) is the voltage point where no reset occurs. This is because the MAX16132–MAX16135 monitor inputs for overvoltage and undervoltage faults, a window threshold that is specified by the selection of the tolerance level. Reset occurs only with an input voltage that falls outside this window threshold. The input tolerance (TOL) can be set from ±4% to ±11% with respect to nominal input voltage with 1% resolution. The example below shows threshold calculations for overvoltage and undervoltage for a 1.2V nominal power supply.

V_INNOM = 1.2V
V_UVTH = V_INNOM(1 -5%) = 1.2V(1-0.05) = 1.2V - 0.06V = 1.140
V_OVTH = V_INNOM(1+5%) = 1.2V(1 + 0.05) = 1.2V + 0.06V = 1.260V

Threshold Accuracy

In an ideal world, we would expect that when VIN_NOM goes down from 1.2V and crosses 1.14V exactly, a reset should occur to indicate an undervoltage condition or when VIN_NOM moves upward from 1.2V and crosses 1.26V, a reset should occur to indicate an overvoltage condition. In the real world, however, there are always some deviations. In the MAX16132–MAX16135, this deviation is specified as a threshold accuracy of ±1% over the −40°C to +125°C temperature range. Therefore, in the above calculation, this inaccuracy in the threshold should be included to account for an excursion of the threshold as shown in Figure 3.

V_{UVTH_EXC} = V_UVTH(±1%) = 1.14V(±0.0114) = 1.1514V to 1.1286

V_{OVTH_EXC} = V_oVTH(±1%) = 1.26V(±0.0126) = 1.2726V to 1.2474V

For a given part, therefore, the overvoltage threshold could be anywhere within the shaded orange region and the undervoltage threshold could be anywhere within the shaded blue region.

Hysteresis

The MAX16132–MAX16135 also feature input hysteresis that is factory-programmable to either 0.25% or 0.50%. The hysteresis is calculated with respect to the nominal input voltage. For the 1.2V nominal input voltage and 0.5% hysteresis, there is the following:

V_INNOM = 1.2V
HYS = 0.5%
V_HYS = V_INNOM (0.5%) = 1.2V(0.005) = 6mV

If the reset is due to an overvoltage event at the input, 6mV must be subtracted from the voltage point where the reset was triggered to bring the device out of reset at the end of the reset timeout period. See Figure 4 for a graphical description.

If the reset is due to an undervoltage event at the input, 6mV must be added from the voltage point where the reset was triggered to bring the part out of reset. See Figure 5.

RESET Timeout Period (t_RP)

The reset timeout period in voltage supervisors adds an advantage over discrete voltage detector circuits. The MAX16132–MAX16135 provide flexibility for a designer to choose from 23 different RESET timeout period options. This can be useful for the proper sequencing of multiple power supplies, such as in FPGA applications or to prevent system glitches during power-up. When the system voltage rises above the undervoltage threshold, the supervisory IC releases the reset signal and allows the system to return from the RESET state. However, the supply must be above the threshold voltage for at least the RESET timeout period before the RESET signal is released. This ensures the system voltage is stable and avoids any power-up faults in the system. Figure 6 shows the timing diagram of the MAX16132 during power-up and during a system undervoltage fault.

Transient Immunity

While we are working in low operating voltages, one must be especially careful to avoid coupled noise problems. Noise can be coupled into PCB traces from adjacent transformers, AC lines, DC-DC switching regulators or RF circuits, or from other external sources. High-frequency noise rejection in voltage supervisory ICs can facilitate safe and reliable system operation. Figure 7 shows system noise coupled to supply voltage and Figure 8 shows the response of MAX16132 to this type of high-frequency noise. The x-axis shows the transient duration and the y-axis shows the percentage of overdrive for which the supply voltage is less than the undervoltage threshold voltage.

Summary

The MAX16132–MAX16135 allows designers the flexibility to choose a wide range of nominal voltage from 1V to 5V and input tolerance from ±4% to ±11% using factory-trimmed OTP options. With a combination of multiple OTP trim options available, it is important to choose the correct OTP setting to meet system requirements. This application note discussed the critical parameters associated with the MAX16132–MAX16135 supervisor family when designing a reliable system and it introduced a reasonable approach for choosing the right reset threshold and hysteresis of voltage supervisor ICs.