Fairchild 74VHC221A service manual

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74VHC221A Dual Non-Retriggerable Monostable Multivibrator

May 2007

74VHC221A Dual Non-Retriggerable Monostable Multivibrator

Features

■

High Speed: t Low Power Dissipation: I

■

Active State: I

■

High Noise Immunity: V Power down protection is provided on all inputs

■

Pin and function compatible with 74HC221A

8.1ns (Typ.) at V

PD

CC

600µA (Max.) at T

CC

NIH

5V

CC

4µA (Max.) at T

A

V

28% V

NIL

25°C

CC

25°C

A

(Min.)

General Description

The VHC221A is an advanced high speed CMOS Monostable Multivibrator fabricated with silicon gate CMOS technology. It achieves the high speed operation similar to equivalent Bipolar Schottky TTL while maintaining the CMOS low power dissipation. Each multivibrator features both a negative, A, and a positive, B, transition triggered input, either of which can be used as an inhibit input. Also included is a clear input that when taken LOW resets the one-shot. The VHC221A can be triggered on the positive transition of the clear while A is held LOW and B is held HIGH. The VHC221A is nonretriggerable, and therefore cannot be retriggered until the output pulse times out. The output pulse width is determined by the equation:

PW = (Rx)(Cx)

where,

PW is in seconds, R is in ohms, and C is in farads.

Limits for R

External capacitor, C External resistors, R

An input protection circuit ensures that 0 to 7V can be applied to the input pins without regard to the supply voltage. This device can be used to interface 5V to 3V systems and two supply systems such as battery back up. This circuit prevents device destruction due to mismatched supply and input voltages.

and C

x

are:

x

: No limit

x

:V

x

CC

V

CC

2.0V, 5k Ω Min.

3.0V, 1 k Ω Min

Ordering Information

Package

Order Number

74VHC221AM M16A 16-Lead Small Outline Integrated Circuit (SOIC), JEDEC MS-012, 0.150" Narrow 74VHC221ASJ M16D 16-Lead Small Outline Package (SOP), EIAJ TYPE II, 5.3mm Wide 74VHC221AMTC MTC16 16-Lead Thin Shrink Small Outline Package (TSSOP), JEDEC MO-153, 4.4mm

Surface mount packages are also available on Tape and Reel. Specify by appending the suffix letter “X” to the ordering code.

Number Package Description

Wide

H =

≥

74VHC221A Dual Non-Retriggerable Monostable Multivibrator

Connection Diagram

Logic Symbol

IEEE/IEC

Block Diagrams

Note A: Cx, Rx, Dx are external Capacitor, Resistor, and

Diode, respectively.

Note B: External clamping diode, Dx;

External capacitor is charged to V

level in the wait

CC

state, i.e. when no trigger is applied.

Truth Table

Inputs Outputs

FunctionA B CLR QQ

HH Output Enable

X LHLHInhibit

HX HLHInhibit

LH Output Enable LH Output Enable XX LLHReset

HIGH Voltage Level

L = LOW Voltage Level

= HIGH-to-LOW Transition = LOW-to-HIGH Transition

If the supply voltage is turned off, Cx discharges mainly through the internal (parasitic) diode. If Cx is sufficiently large and V

drops rapidly, there will be some possibil-

CC

ity of damaging the IC through in rush current or latchup. If the capacitance of the supply voltage filter is large enough and V

drops slowly, the in rush current is

CC

automatically limited and damage to the IC is avoided. The maximum value of forward current through the para-

sitic diode is ±20 mA. In the case of a large Cx, the limit of fall time of the supply voltage is determined as follows:

t

(V

f

(t supply voltage reaching 0.4 V

– 0.7) Cx / 20mA

CC

is the time between the supply voltage turn off and the

f

CC

)

In the event a system does not satisfy the above condition, an external clamping diode (Dx) is needed to protect the IC from rush current.

X = Don’t Care

System Diagram

74VHC221A Dual Non-Retriggerable Monostable Multivibrator

Timing Chart

Functional Description

1. Stand-by State The external capacitor (Cx) is fully charged to V

the Stand-by State. That means, before triggering, the Q

and Q

P

Rx/Cx node are in the off state. Two comparators that relate to the timing of the output pulse, and two reference voltage supplies turn off. The total supply current is only leakage current.

2. Trigger Operation Trigger operation is effective in any of the following

three cases. First, the condition where the A LOW, and B input has a rising signal; second, where the B input is HIGH, and the A input has a falling signal; and third, where the A input is HIGH, and the CLR

After a trigger becomes effective, comparators C1 and C2 start operating, and Q external capacitor discharges through Q age level at the Rx/Cx node drops. If the Rx/Cx voltage level falls to the internal reference voltage V the output of C1 becomes LOW. The flip-flop is then reset and Q C2 continues operating.

After Q starts rising at a rate determined by the time constant of external capacitor Cx and resistor Rx.

transistors which are connected to the

N

input is LOW and the B input has a rising signal.

is turned on. The

N

turns off. At that moment C1 stops but

N

turns off, the voltage at the Rx/Cx node

N

. The volt-

N

CC

input is

ref

Upon triggering, output Q becomes HIGH, following

in

some delay time of the internal F/F and gates. It stays HIGH even if the voltage of Rx/Cx changes from falling to rising. When Rx/Cx reaches the internal reference voltage V the output Q goes LOW and C2 stops its operation. That means, after triggering, when the voltage level of the Rx/Cx node reaches V MONOSTABLE state.

With large values of Cx and Rx, and ignoring the discharge time of the capacitor and internal delays of the IC, the width of the output pulse, t follows:

t

W

3. Reset Operation In normal operation, the CLR

CLR output is held LOW and the trigger control F/F is reset. Also, Q

L,

V

CC

This means if CLR state.

H, the output of C2 becomes LOW,

ref

H, the IC returns to its

ref

(OUT), is as

W

(OUT) = 1.0 Cx Rx

input is held HIGH. If

is LOW, a trigger has no affect because the Q

turns on and Cx is charged rapidly to

p

.

is set LOW, the IC goes into a wait

74VHC221A Dual Non-Retriggerable Monostable Multivibrator