ALD7555/ALD7556

Advanced Linear Devices

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APPLICATION NOTES

GENERAL INFORMATION

The ALD7555 and the ALD7556 devices are analog timers

that are, in most situations, direct replacements or direct same

pin-out upgrades for the ICM7555, ICM7556, NE/SE555 and

NE/SE556 devices. Significantly improved performances for

the ALD7555 and the ALD7556 include precision in timing,

reduced leakage currents at all the pin terminals, faster

switching speeds, reduced switching current spikes, en-

hanced discharge output drive currents, better temperature

stability, and better timing stability as a function of power

supply.

These improvements not only improve on the timer function,

but also improve on many of the thousands of circuits that

depend on this timer architecture, such as modulation cir-

cuits, Schmitt triggers, astable circuits, and myriads of mea-

surement and control circuits where the user may have

reached performance limits with their ICM7555, ICM7556,

NE/SE555 and NE/SE556 devices. For a given design, one

or more specification of the timer device may become the

circuit performance limiting factor.

The ALD7555 and the

ALD7556 devices are designed to address such limitations

and in many cases offer a solution that is simpler and lower

cost for a given design challenge than by other solutions by

using other circuit means and techniques.

ARCHITECTURE

The ALD7555 and the ALD7556 are analog timers that oper-

ate based on the RC timing principle, using an external tim-

ing resistor R and an external timing capacitor C. The C is

charged by the R and then discharged via one of the two

output pin connections provided by the ALD7555 and the

ALD7556. The control of the two outputs are provided by

one of the 4 input pins. The inputs are named THRESH-

OLD, CONTROL, RESET and TRIGGER. THRESHOLD and

TRIGGER are connected to two separate voltage compara-

tors with their respective comparator control levels set by an

internal resistor string, consisting of three equal-valued and

matched resistors. The output of the two comparators set an

internal RS flip-flop circuit, which in turn controls an OUT-

PUT and a DISCHARGE output. See Block Diagram for a

simplified equivalent circuit. The OUTPUT swings from rail

to rail of the supply voltage, whereas the DISCHARGE only

sinks current when it is active.

The ALD7555 and the ALD7556 operates by charging and

discharging the RC timing between 1/3 and 2/3 V+, and by a

feedback function provided by the user through the OUT-

PUT and/or DISCHAGE pins. This feedback is provided by

the application circuit external connections, which determines

the mode of the circuit operation. The architecture of this

timer takes advantage of the fact that all charging and dis-

charging of C are referenced by the reference resistor string

that provide reference voltages proportional to supply volt-

age V+ (Vdd). As the charging and the discharging of the

capacitor C is also proportional to V+, the frequency of os-

cillation is independent of V+ voltage levels.

The three most basic modes of these external connections

are shown as Astable Mode (Free Running Mode), 50% Duty

Cycle Mode, and the Monostable Mode. See Typical Appli-

cations. There are thousands of application circuits devel-

oped that allow the user to manipulate this feedback func-

tion, and which then produces many unique functions that is

beyond a basic timer function. The application versatility of

The ALD7555 and the ALD7556 is only limited by the imagi-

nation of the circuit designer.

LOW POWER SUPPLY REQUIREMENTS

The CMOS process and the design of the ALD7555 and the

ALD7556 devices utilize three well-matched on-chip high

impedance resistors to build the internal reference resistor

string to provide very low power supply operation. Another

technique to achieve low power supply requirement is by

using low power MOSFET circuits on-chip, and by allowing

the user to use a combination of off-chip timing resistor and

capacitor that would reduce their power consumption as well.

Generally, this is accomplished by using higher values for R

and lower values for C in a combination that would still pro-

vide the timing required. Furthermore, reduced on-chip leak-

age currents improve on not only timing precision, but also

greatly increased ranges of usable R and C values to gener-

ate the same RC time constant.

ENHANCED DISCHARGE OUTPUT DRIVE

The Discharge Output Drive Currents of the ALD7555 and

the ALD7556 devices are increased significantly (80mA)

when compared to other timers so that in many applications

where the user may use this output as an output driver in-

stead of having to add another output driver or buffer cir-

cuitry. For example, in many situations, the Discharge Out-

put Drive Current is sufficient to drive a relay or a Power

MOSFET directly.

ASTABLE OPERATION

ALD7555 and the ALD7556 devices are designed to func-

tion as astable oscillators. These timers can be connected

to self trigger and run as a free running mutivibrator. In the

free running oscillator mode, the external capacitor C is

charged through Ra and Rb, and it is discharged through Rb

only. By adjusting the values of Ra and Rb in combination

with the value of C, both the frequency and the duty cycle of

the oscillator pulse can be adjusted. In the 50% astable mode,

the charging and the discharging of C are performed by the

same R between the same voltage levels set by the refer-

ence resistor string, and therefore timer provide a true 50%

duty cycle square wave that is symmetrical. (See typical

applications)