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Work Principle of 555 Monostable Model

2015-02-04 08:11
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The Monostable multi-vibrator composed of 555 has been shown in the picture 2-1.This chapter takes the bipolar 555 as an example; functional terminals marked in the picture are relevant to each other. This monostable circuit has jointed a time network that consists of a resistance Ra and a capacitor C. In the picture 2-1, the coercive resetion terminal pin 4 connects VDD, threshold terminal pin 6 VTH and discharge terminal DIS pin7 connect to the midpoint of RaC time network. Picture 2-2 is the simplified circuit, the right one is the waveform of monostable circuit. When in a stable state, the output voltage V0 is in low level; when the circuit in a temporary stable state, voltage on the capacitor ascends exponentially: Vc(t)=VDD(1-e-1/RaC) When voltage on the capacitor reaches to 2/3VDD, the circuit backs to be stable. When the circuit is in a temporary stable state, the time that V0(t) takes to ascend from 0 to 2/3VDD is td: 2/3VDD-VDD(1-e-td/RaC) From the formulas above, we can conclude that time of outputting high level: td=-RaCln(1/3) td≈1.1RaC(s) The prolonged time of monostable model td is irrelevant to outer voltage VDD but is determined by time constant of time network.

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