Explain the working principle of ICM7555MOS integrated time base circuit in detail

The time base circuit is a timing circuit, which generates a pulse with a fixed time width under the action of a trigger pulse. The time width is determined by the timing resistor and the timing capacitor. Because the time base circuit has this function, it is usually used as a monostable flip-flop and delay circuit. Although there are many types of integrated time base circuits, including bipolar time base circuits and MOS time base circuits, their working principles and circuit block diagrams are basically the same. In this section, we introduce a typical ICM7555MOS integrated time base circuit. The internal block diagram of the circuit is shown in Figure 6.3-1. It consists of two comparators, R-S flip-flops, and inverters. The working principle of the time base circuit. Inverting input terminal voltage of comparator 1,The non-inverting input voltage of comparator 2 ,  The number in the small square indicates the serial number of the circuit pin. The connection of external components and pins shown in Figure 6.3-1 is a typical connection of ICM7555 circuit as a monostable flip-flop. The external timing resistor Rr and timing capacitor Cr determine the output pulse width. Take a monostable trigger as an example to briefly introduce the working principle of the ICM7555 circuit. After the circuit is set, the output terminal Q of the RS flip-flop is at low level, the voltage at the output terminal (pin 3) of the circuit is also at low level, and the gate of M1 is at high level, which turns on M1 and discharges the capacitor Cr. The voltage across both ends is V. Approximately zero. The working principle of the time base circuit. When the inverting input of comparator 2 is added below after the negative trigger pulse of the level, the RS flip-flop flips, its output terminal Q jumps from low level to high level, the output terminal of the circuit also changes from low level to high level, and the gate of M1 changes from high level. At low level, M1 is turned off. At this time, the power supply voltage VDD charges the capacitor CT through the Rr resistor, and the voltage Vc at both ends of the capacitor gradually rises. When it rises to Vs, the state of the comparator 1 is reversed, and the R terminal of the RS flip-flop changes from low to low. The level changes to a high level, so that the output terminal Q of the RS flip-flop changes to a low level again, and the output terminal of the circuit also changes from a high level to a low level, resulting in a timing pulse width T. The working principle of the time base circuit. At the same time, the gate of M1 becomes high level, the capacitor Cr is discharged through the M1 tube, the potential of Vc gradually drops to zero, and the circuit returns to the initial state. The output pulse width T is determined by the charging equation of the voltage Vc across the capacitor Cr. , which is

When t=T,, Substituting this relational expression into the above formula, get the pulse width expression

The above formula shows that the width T of the output pulse is determined by the external resistor RT and the capacitor Cr.

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