Understand and apply supervisory ics to avoid low-voltage power-up glitch headaches
Achieving this result requires a proprietary circuit and IC such as the MAX16162, a nanopower supply supervisor with glitch- free power-up. With this tiny IC— available in four-bump WLP and four-pin SOT23 packages—the reset output is held low whenever VDD is lower than the threshold voltage, preventing a voltage glitch on the reset line. Once the voltage threshold is reached and the delay period is completed, the reset output de-asserts and enables the microcontroller ( Figure 5 ).
N-channel pass gates. Additional supervisory functions include undervoltage and overvoltage monitoring and reporting, as well as microprocessor reset generation. The type and source of faults are 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 an unlimited number of power supplies. Conclusion Glitches are present in every application, but they have not posed a significant issue for higher voltage applications which dominated until recently. Now, power supply voltages are moving lower, making system turn-on less reliable due to 0.9-volt glitches. As shown, designers can improve reliability using newer supervisory ICs that offer glitch-free operation to provide the highest degree of system protection for low-power/ low-voltage applications.
Figure 6: The MAX16161 and MAX16162 are similar but with a small functional and pinout difference: the MAX16161 has an MR input that asserts a reset when it receives an appropriate input signal, while the MAX16162 has separate VCC and VIN pins. (Image source: Analog Devices)
Figure 7: A circuit using the MAX16161 can be configured so the device not only ensures glitch-free power-up but also manages power-rail sequencing between two rails. (Image source: Analog Devices)
There are also many designs that have multiple rails and more complex sequencing needs. In these situations, the Analog Devices LTC2928 Multichannel Power Supply Sequencer and Supervisor offers a solution ( Figure 8 ). This four-channel cascadable power supply sequencer and high-accuracy supervisor allows designers to configure power-
management sequencing thresholds, order, and timing using just a few external components. 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
Unlike conventional supervisory ICs that 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 pin assignments ( Figure 6 ). 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 volts.
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 resets and “power OK” assertions among two or more rails. The MAX16161 and MAX16162 can be used as simple power supply sequencers ( Figure 7 ). After the output voltage of the first regulator becomes valid, the MAX16161/ MAX16162 insert a delay and generate the enable signal for the second regulator after the reset timeout period. Because the MAX16161/MAX16162 never de- assert reset until the supply voltage is correct, the controlled supply is never incorrectly enabled.
Figure 5: The MAX16162 holds the reset output low whenever VDD is lower than the threshold voltage, preventing a voltage glitch on the reset line. (Image source: Analog Devices)
Figure 8: The LTC2928 power sequencer manages power-up and power-down sequencing among four independent rails, and enables user control over key parameters. (Image source: Analog Devices)
we get technical
8
9
Powered by FlippingBook