Are you still worried about glitches when powering on at low voltage? This IC can do it

• Spikes caused by noise on signal or power lines

• Sudden power rail drops caused by load transients

• The microsecond period when the upper and lower MOSFETs in the bridge accidentally turn on at the same time due to different gate driver on/off times (this is a very bad situation)

• Momentary uncertain signals and race conditions due to timing tolerances and differences between components

  
  

As more and more low-power devices operate at lower and lower voltages, this glitch becomes a major problem. Let's consider a system with three logic levels: 3.3 V, 2.5 V, and 1.8 V (Figure 3). For a 3.3 V system, the output low voltage threshold (Vol) and input low voltage threshold (Vil) are between 0.4 V and 0.8 V. If a glitch occurs at 0.9 V, there is a risk that the processor may become unstable due to power-on and power-off operations.

Figure 3: The logic level decreases from 3.3 V to 1.8 V, as does the associated voltage threshold. (Image source: Analog Devices)

The situation is more sensitive for nominal 1.8 V systems. Now, Vol and Vil are much lower, 0.45 V and 0.63 V respectively. In this system, the 0.9 V glitch represents a larger percentage, giving it a greater potential for error.

A glitch affects system operation, how will this situation develop? Let's consider that the supply voltage VDD slowly rises to 0.9 V and remains at that value for a short period of time (Figure 4). Although this voltage is not sufficient to turn on the monitoring IC, it may still turn on the microcontroller and cause it to operate in an unstable state. Because it is in an indeterminate state at 0.9 V, the microcontroller RESET input interprets the glitch as a logic 1 or 0, incorrectly enabling or disabling it.

This will cause the microcontroller to only execute part of the instruction or not write the memory completely. These are just two things that can happen, but can lead to system failure and catastrophic consequences.

Overcoming this problem does not require reverting to a higher voltage rail, nor does it require complex system-level architecture to eliminate the glitch or minimize its impact. Instead, we need a new generation of monitoring ICs that can identify unique aspects of a problem and prevent glitches, regardless of voltage levels during power-on or power-down conditions.

Achieving this requires the use of proprietary circuits and ICs, such as the MAX16162 , a nanopower power supply monitor with glitch-free power-up. With this small IC in a four-bump WLP and four-lead SOT23 package, the reset output remains low as long as VDD is below the threshold voltage, preventing voltage glitches on the reset line. Once the voltage threshold is reached and the delay time expires, the reset output deasserts and enables the microcontroller (Figure 5).

Another pair of terms that has some overlap and ambiguity is sequencer and monitor. The supervisor is used to monitor a single supply voltage and assert/release reset under specified conditions. In contrast, a sequencer is used to coordinate relative reset and "power good" assertions between two or more power rails.

The MAX16161 and MAX16162 can be used as simple power supply sequencers (Figure 7). The MAX16161/MAX16162 insert a delay after the first regulator's output voltage becomes valid and generate an enable signal for the second regulator after a reset timeout. Because the MAX16161/MAX16162 never cancel reset until the supply voltage reaches the correct value, the controlled power supply can never be incorrectly enabled.

There are also many designs with multiple power rails and more complex sequencing requirements. At this point, Analog Devices' LTC2928 multi-channel power supply sequencer and monitor is a solution (Figure 8).

Using this four-channel cascaded power sequencer and high-precision monitor, designers can configure power management sequencing thresholds, sequencing and timing with just a few external components. The device ensures that the power rails are enabled in the required sequence. In addition to power-on sequencing, the device can also manage complementary and often equally critical power-down sequencing.

Sequencing outputs are used to control power enable pins or N-channel transmission gates. Other monitoring functions include undervoltage and overvoltage monitoring and reporting, and generation of microprocessor resets. Reports the type and source of the fault for diagnostic purposes. Provides individual channel control functionality to enable output and monitoring functions to be performed independently. For systems with more than four power rails, multiple LTC2928s can be easily connected to sequence an unlimited number of power supplies.

There are glitches in every application, but so far they have not caused serious problems in the dominant high-voltage applications. Now that the supply voltage is going lower, system turn-on reliability will be reduced due to the 0.9 V voltage glitch.

As shown, designers can take advantage of newer supervisory ICs to improve reliability. This IC enables glitch-free operation and provides maximum system protection for low-power/low-voltage applications.