Fairchild Semiconductor CD4538BCWMX, CD4538BCWM, CD4538BCN, CD4538BCMX, CD4538BCM Datasheet

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October 1987 Revised January 1999

CD4538BC Dual Precision Monostable

General Description

The CD4538BC is a dual, precision monostable multivibrator with independent t ri gger and re set con trols. The device is retriggerable and resettable, and the contro l inputs are internally latched. Two trigger inputs are provided to allow either rising or falling edge triggering. The reset inputs are active LOW and prevent triggering while active. Precise control of output pulse-w idth has been achieved using linear CMOS techniques. The pu lse duration and accuracy are determined by exter nal components R

and CX. The

device does not allow the timing capacitor to discharge through the timing pi n on power-down condition. For this reason, no external protect ion resistor is r equire d in se ries with the timing pin. Input protection from static discharge is provided on all pins.

Features

■ Wide supply voltage range: 3.0V to 15V ■ High noise immunity: 0.45 V

(typ.)

■ Low power TTL compatibility: Fan out of 2 driving 74L

or 1 driving 74LS

■ New formula: PW

OUT

= RC (PW in seconds, R in Ohms,

C in Farads)

■ ±1.0% pulse-width variation from part to part (typ.) ■ Wide pulse-width range: 1 µs to ∞ ■ Separate latched reset inputs ■ Symmetrical output sink and source capability ■ Low standby current: 5 nA (typ.) @ 5 V

■ Pin compatible to CD4528BC

Ordering Code:

Devices also available in Tape and Reel. Specify by appending the suffix letter “X” to t he ordering code.

Connection Diagram

Pin Assignments for DIP and SOIC

Top View

Truth Table

H = HIGH Level L = LOW Level

↑ = Transition from LOW-to-HIGH ↓ = Transition from HIGH-to-LOW



= One HIGH Level Pulse



= One LOW Level Pulse

X = Irrelevant

Order Number Package Number Package Description

CD4538BCM M16A 16-Lead Small Outline Integrated Circuit (SOIC), JEDEC MS-012, 0.150” Narrow Body CD4538BCWM M16B 16-Lead Small Outline Integrated Circuit (SOIC), JEDEC MS-013, 0.300” Wide Body CD4538BCN N16E 16-Lead Plastic Dual-In-Line Package (PDIP), JEDEC MS-001, 0.300” Wide

Inputs Outputs

Clear A B Q Q

LXXLH XHXLH XXLLH HL↓



H ↑ H



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CD4538BC

Block Diagram

RX and CX are External Com ponents V

= Pin 16

= Pin 8

Logic Diagram

FIGURE 1.

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CD4538BC

Theory of Operation

FIGURE 2.

Trigger Operation

The block diagram of the CD4538B C is shown in Figure 1, with circuit operation following.

As shown in Figure 1 and Fi gure 2, before an in put tr igger occurs, the monostable is in the quiescent stat e with the Q output low, and the timing capacitor C

completely charged

to V

. When the trigger input A goes from VSS to V

(while inputs B and CD are held to VDD) a valid trigger is recognized, which turns on compara tor C1 and N-Chan nel transistor N1

(1)

. At the same time the outp ut latch is set.

With transistor N1 on, the ca pacitor C

rapidly discharges

toward V

until V

REF1

is reached. At th is point th e output

of comparator C1 changes state and transistor N1 turns off. Comparator C1 then tu rn s off w hile at the sa me ti me com parator C2 turns on. With transistor N1 off, the capacitor C

begins to charge thr ough the timing resistor, RX, toward V

. When the voltage across CX equals V

REF2

, compara-

tor C2 changes state causing the output latch to reset (Q goes low) while at the same time disabling comparator C2. This ends the timing cycle with the mono stable in the quiescent state, waiting for the next trigger.

A valid trigger is also recognized when trigger input B goes from V

to VSS (while input A is at VSS and input CD is at

)

(2)

It should be noted that in the quiescent state C

is fully

charged to V

, causing the current throu gh res istor RX to

be zero. Both comparators are “off” wi th the total device current due only to r everse junction leakages. An a dded feature of the CD4538BC is that th e output latch is set via the input trigger withou t regard to the capacitor voltage.

Thus, propagation delay from trigger to Q is independent of the value of C

, RX, or the duty cycle of the inpu t wave-

for m.

Retrigger Operation

The CD4538BC is retriggered if a valid trigger occurs

(3)

fol-

lowed by another valid t rigger

(4)

before the Q output has returned to the quiescent (zero) state. Any retrigger, after the timing node voltage at pin 2 or 14 has begun to r ise from V

REF1

, but has not yet reached V

REF2

, will cause an increase in output pulse width T. When a valid retrigger is initiated

(4)

, the voltage at T2 will again drop to V

REF1

before

progressing along the RC charg ing cur ve toward V

. The

Q output will remain high until time T, after the last valid retrigger.

Reset Operation

The CD4538BC may be reset dur ing t he generatio n of the output pulse. In the reset mode of operation, an input pulse on C

sets the reset latch an d causes the c apacitor to be

fast charged to V

by turning on transistor Q1

(5)

. When

the voltage on the capacitor r each es V

REF2

, the reset latch

will clear and then be ready to accept another pulse. If the C

input is held low, any trigger inputs that occur will be

inhibited and the Q and Q

outputs of the output la tch will not change. Since the Q output is reset when an input low level is detected on the C

input, the output pulse T can be

made significantly shor ter than the minimum pulse width specification.

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CD4538BC

FIGURE 3. Retriggerable Monostables Circuitry

FIGURE 4. Non-Retriggerable Monostables Circuitry

FIGURE 5. Connection of Unused Sections

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