Power-on glitches and system malfunctions are a special challenge as rail voltages drop, but the right ICs can eliminate the problem.
This glitch scenario outlined in the first part of this article becomes a major concern with the trend of low-power devices operating at lower voltages. Consider systems with three logic levels of 3.3 V, 2.5 V, and 1.8 V (Figure 1). For the 3.3-V system, the output low-voltage threshold Vol and input low-voltage threshold Vil is between 0.4 V and 0.8 V, ensuring that the logic level is low. If a glitch occurred at 0.9 V, this would potentially cause the processor to become unstable by continuously switching it off and on.
The situation for a nominal 1.8-V system is more difficult. Now, Vol and Vil are much lower at 0.45 V to 0.63 V, and the 0.9-V glitch in this system represents a larger margin of error. Therefore, this system has a higher potential for a glitch, which will cause a system error.
How does this situation play out with the glitch impacting your system operation? Consider a power-supply voltage VDD that ramps up slowly to 0.9 V and “lingers” for a short period at 0.9 V (Figure 2). Although this voltage is not enough to turn on the supervisor IC, the microcontroller could still be enabled and run in an unstable state. Since the 0.9 V value is in an indeterminant state, the glitch can be interpreted by the RESET input as either a logic 1 or 0, which would enable or disable the microcontroller erratically.
This causes the microcontroller to execute partial instructions or incomplete writes to memory, as just two examples of what might happen, likely causing system malfunction and possible catastrophic system behavior.
Addressing and solving the glitch problem
Overcoming this problem does not mean a return to higher-voltage rails or the need for complicated system-level architectures that eliminate its occurrence or minimize its impact. Instead, it requires a new generation of supervisor ICs that recognizes the problem’s unique aspects and prevents glitches from forming regardless of the voltage level during power-down or brownout conditions.
Achieving this result requires a proprietary circuit and IC, such as the MAX16162 IC, a nanopower supply supervisor with a glitch-free power-up. With this tiny IC — available in 4-bump WLP and 4-pin SOT23 packages — the reset output is held low whenever VDD is lower than the threshold voltage, which prevents a voltage glitch on the reset line. Once the voltage threshold is reached and the delay period is completed, the reset output de-asserts, which enables the microcontroller (Figure 3).
Unlike conventional supervisor ICs, which are unable to control the reset output state when VCC is very low, the MAX16162 reset output is guaranteed to remain asserted until after a valid VCC level is achieved.
The MAX16161 is a close sibling of the MAX16162 with nearly identical specifications but with one functional difference and some redefining of in-pin assignments (Figure 4). It features a manual reset (MR) input that asserts a reset when it receives an appropriate input signal, which can be either active-low or active-high, depending on the option selected. In contrast, the MAX16162 has no MR input but instead has separate VCC and VIN pins, allowing threshold voltages as low as 0.6 V.
Sequencer versus supervisor
Another pair of terms that have some overlap and ambiguity are supervisor and sequencer. A supervisor monitors a single power-supply voltage and asserts/releases reset under defined circumstances. In contrast, a sequencer coordinates the relative of resets and power-OK assertions among two or more rails.
The MAX16161 and MAX16162 can be used as a simple power-supply sequencer (Figure 5). After the output voltage of the first regulator becomes valid, the MAX16161/MAX16162 inserts a delay and generates the enable signal for the second regulator after the reset timeout period. As the MAX16161/MAX16162 never de-asserts reset until the supply voltage is correct, the controlled supply is never incorrectly enabled.
There are also many designs that have multiple rails and more complex sequencing needs. The Analog Devices LTC2928 Multichannel Power Supply Sequencer and Supervisor offers a solution (Figure 6).
This four-channel cascadeable power-supply sequencer and high-accuracy supervisor allows designers to configure power-management sequencing thresholds, order, and timing using just a few external components without software involvement and the uncertainties it brings. It ensures that power rails are enabled in the desired order. In addition to power-on sequencing, it can manage the complementary and often equally critical power-down sequencing.
The sequence outputs are used to control supply-enable pins or N-channel pass gates. Additional supervisory functions include undervoltage and overvoltage monitoring and reporting, as well as microprocessor reset generation, with the type and source of faults reported for diagnosis. Individual channel controls are available to exercise the enable outputs and supervisory functions independently. For systems with more than four rails, multiple LTC2928s can be easily connected to sequence unlimited power supplies.
Glitches are present in every application, but they have not posed a significant issue for higher-voltage applications, which dominated until recently. Supply voltages are moving lower with lower-power systems, thus making the system turn-on less reliable due to 0.9-V glitches. Fortunately, supervisory ICs offer glitch-free operation, providing the highest degree of system protection for these low-power, low-voltage applications.
Analog Devices/Maxim Integrated Products, Design Solution 7550, “Is Your Application Protected from Glitches?”