Knowing power-on-reset requirements in regulators, bandgaps

Self-starting is an important but sometimes difficult task, whether it is motivating humans, starting cold automobiles, or providing power-on-reset (POR) for ICs. PORs ensure systems start in known and safe states. We will discuss several misapplications of this technology and how designers can avoid startup problems.

Many integrated circuits (IC) require special handling at power-up. Analog and digital circuits may need to be placed in predictable conditions at startup. To do that, we use circuits that are commonly called power-on-reset (POR) circuits.

POR makes sure that there is an orderly and predictable sequence of events during power-up. For example, circuits that supply bias currents in the circuit need to be available and stable to assure proper control of the circuit. One case that consumers notice this is in a stereo system. The audio can be delayed by 10 seconds or so after the power is applied, to protect the amplifier and speakers from big pops (transients). An amplifier feeding a speaker typically is biased at half the power-supply level. That silence out of the speaker is a quiescent DC voltage. While the quiescent bias level is being established during power-up, there can be sudden jumps that would cause ugly pops and destroy the speakers. That is why savvy audiophiles would never risk damage by plugging and unplugging equipment with power on.

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Originally published by Maxim Integrated Products at www.maxim-ic.com as "Self-Starting Can Be Hard to Do—Understanding Power-on-Reset Requirements in Regulators and Bandgaps".