TS555 datasheet(8/12 Pages) STMICROELECTRONICS

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TS555

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TYPICAL CHARACTERISTICS

Figure 1 : Supply Current (each timer) versus

Supply Voltage

APPLICATION INFORMATION

MONOSTABLE OPERATION

In the monostable mode,the timer operates like a

one-shot generator. Referring to figure 2, the ex-

ternal capacitor is initially held discharged by a

transistor inside the timer.

Figure 2 :

The circuit triggers on a negative-going input sig-

nal when the level reaches 1/3 VCC. Once trig-

gered, the circuit remains in this state until the set

time has elapsed, even if it is triggered again dur-

ing this interval. The duration of the output HIGH

state is given by t = 1.1 R x C.

It can be noticed that since the charge rate and the

threshold level of the comparator are both directly

proportional to the supply voltage, the timing inter-

val is independent of the supply. Applying a nega-

tive pulse simultaneously to the Reset terminal

(pin 4) and the Trigger terminal (pin 2) during the

timing cycle discharges the external capacitor and

causes the cycle to start over. The timing cycle

now starts on the positive edge of the reset pulse.

While the reset pulse is applied, the output is driv-

en to the LOW state.

When a negative trigger pulse is applied to pin 2,

the flip-flop is set, releasing the short circuit across

the external capacitor and driving the output

HIGH. The voltage across the capacitor increases

exponentially with the time constant

τ = R x C.

When the voltage across the capacitor equals 2/3

VCC, the comparator resets the flip-flop which then

discharges the capacitor rapidly and drives the

output to its LOW state.

Figure 3 shows the actual waveforms generated in

this mode of operation.

When Reset is not used, it should be tied high to

avoid any possible or false triggering.

Figure 3 :

SUPPLY VOLTAGE, V

(V)

300

200

100

VCC

Reset

Trigger

Out

TS555

Control Voltage

0.01 F

CAPACITOR VOLTAGE = 2.0V/div

Ω

t = 0.1 ms / div

INPUT = 2.0V/div

OUTPUT VOLTAGE = 5.0V/div

R = 9.1k , C = 0.01 F , R = 1.0k

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