ALD7555/ALD7556

Advanced Linear Devices

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MONOSTABLE OPERATION

In this mode of operation the ALD7555 and the ALD7556

can be connected as a one-shot circuit which produces an

output pulse with a user-adjustable pulse delay time. The

pulse delay time is set by the external R and C values, which

together produce a RC time constant that is proportional to

the time delay. (See typical applications) The pulse is started

with an external negative going Trigger pulse applied to TRIG-

GER pin. This negative going pulse set an internal flip-flop

so that the external R and C can start the RC timing while

the OUTPUT pin is in the HIGH state. The external C is

being charged by the external R. When the voltage between

the R and C is charged passed the internal threshold volt-

age at the THRESHOLD pin, which is set at 66.6% of V+,

the internal comparator of the ALD7555 and the ALD7556

resets the internal flip-flop. This then turns on the Discharge

Driver at DISCHARGE pin, and discharges the timing ca-

pacitor C. The cycle is completed when the OUTPUT is driven

to a low state and the ALD7555 and the ALD7556 are again

waiting for the next negative going trigger pulse at TRIG-

GER pin.

CONTROL Voltage and RESET pins

The CONTROL pin directly accesses one input to the upper

comparator. As the input reference resistor string has on-

chip high impedance resistors, an input voltage at the CON-

TROL input can easily change the voltage at the comparator

input. This allows the user to change the oscillation fre-

quency, or modulate the oscillation frequency of the analog

timer, with a separate user provided frequency. The CON-

TROL pin also allows a user-provided inhibit signal to stop

and start the timerís oscillation.

The RESET terminal directly resets the internal RS flip-flop

circuit, which in turn controls the OUTPUT and DISCHARGE

pins. This function is activated by a low voltage input of 0.7V

of 100 ns minimum duration. By injecting a variety of input

signals in a combination to the TRIGGER , THRESHOLD,

CONTROL and RESET pins, many interesting modulation

and demodulation signals can be manipulated and/or gen-

erated by the circuit designer.

APPLICATION NOTES (cont’d)