MM74HC4538N

MM74HC4538N Datasheet

MM74HC4538 Dual Retriggerable Monostable Multivibrator

Part	Datasheet
MM74HC4538N	MM74HC4538N (pdf)

Related Parts	Information
MM74HC4538SJX	MM74HC4538SJX
MM74HC4538SJ	MM74HC4538SJ
MM74HC4538MX	MM74HC4538MX
MM74HC4538M	MM74HC4538M

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MM74HC4538 Dual Retriggerable Monostable Multivibrator MM74HC4538 Dual Retriggerable Monostable Multivibrator The MM74HC4538 high speed monostable multivibrator one shots is implemented in advanced silicon-gate CMOS technology. They feature speeds comparable to low power Schottky TTL circuitry while retaining the low power and high noise immunity characteristic of CMOS circuits. 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 MM74HC4538 is retriggerable. That is, it may be triggered repeatedly while their outputs are generating a pulse and the pulse will be extended. Pulse width stability over a wide range of temperature and supply is achieved using linear CMOS techniques. The output pulse equation is simply PW = 0.7 R C where PW is in seconds, R is in ohms, and C is in farads. This device is pin compatible with the CD4528, and the CD4538 one shots. All inputs are protected from damage due to static discharge by diodes to VCC and ground. s Schmitt trigger on A and B inputs s Wide power supply range s Typical trigger propagation delay 32 ns s Fanout of 10 LS-TTL loads s Low input current 1 µA max Ordering Code: Order Number Package Number Package Description MM74HC4538M M16A 16-Lead Small Outline Integrated Circuit SOIC , JEDEC MS-012, Narrow MM74HC4538SJ M16D 16-Lead Small Outline Package SOP , EIAJ TYPE II, 5.3mm Wide MM74HC4538N N16E 16-Lead Plastic Dual-In-Line Package PDIP , JEDEC MS-001, Wide Devices also available in Tape and Reel. Specify by appending the suffix letter “X” to the ordering code. Connection Diagram Truth Table Inputs Clear 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 Outputs L H 2000 Fairchild Semiconductor Corporation DS005217 MM74HC4538 Block Diagrams Note Pin 1 and Pin 15 must be hard-wired to GND. Logic Diagram MM74HC4538 Timing Diagram Circuit Operation The MM74HC4538 operates as follows refer to logic diagram . In the quiescent state, the external timing capacitor, CX, is charged to VCC. When a trigger occurs, the Q output goes HIGH and CX discharges quickly to the lower reference voltage VREF Lower = 1/3 VCC . CX then charges, through RX, back up to the upper reference voltage VREF Upper = 2/3 VCC , at which point the one-shot has timed out and the Q output goes LOW. The following, more detailed description of the circuit operation refers to both the logic diagram and the timing diagram. QUIESCENT STATE In the quiescent state, before an input trigger appears, the output latch is HIGH and the reset latch is HIGH #1 in logic diagram . Thus the Q output pin 6 or 10 of the monostable multivibrator is LOW #2, timing diagram . The output of the trigger-control circuit is LOW #3 , and transistors M1, M2, and M3 are turned off. The external timing capacitor, CX, is charged to VCC #4 , and the upper reference circuit has a LOW output Transistor M4 is turned ON and transmission gate T1 is turned OFF. Thus the lower reference circuit has VCC at the noninverting input and a resulting LOW output In addition, the output of the trigger-control reset circuit is LOW. TRIGGER OPERATION The MM74HC4538 is triggered by either a rising-edge signal at input A #7 or a falling-edge signal at input B #8 , with the unused trigger input and the Reset input held at the voltage levels shown in the Truth Table. Either trigger signal will cause the output of the trigger-control circuit to go HIGH The trigger-control circuit going HIGH simultaneously initiates three events. First, the output latch goes LOW, thus taking the Q output of the HC4538 to a HIGH State Second, transistor M3 is turned on, which allows the external timing capacitor, CX, to rapidly discharge toward ground Note that the voltage across CXappears at the input of the upper reference circuit comparator. Third, transistor M4 is turned off and transmission gate T1 is turned ON, thus allowing the voltage across CX to also appear at the input of the lower reference circuit comparator. When CX discharges to the reference voltage of the lower reference circuit #12 , the outputs of both reference circuits will be HIGH The trigger-control reset circuit goes HIGH, resetting the trigger-control circuit flip-flop to a LOW State This turns transistor M3 OFF again, allowing CX to begin to charge back up toward VCC, with a time constant t = RXCX In addition, transistor M4 is turned ON and transmission gate T1 is turned OFF. Thus a high voltage level is applied to the input of the lower reference circuit comparator, causing its output to go LOW The monostable multivibrator may be retriggered at any time after the trigger-control circuit goes LOW. When CX charges up to the reference voltage of the upper reference circuit #17 , the output of the upper reference circuit goes LOW This causes the output latch to MM74HC4538 Circuit Operation Continued toggle, taking the Q output of the HC4538 to a LOW State #19 , and completing the time-out cycle. RESET OPERATION A low voltage applied to the Reset pin always forces the Q output of the HC4538 to a LOW State. The timing diagram illustrates the case in which reset occurs #20 while CX is charging up toward the reference voltage of the upper reference circuit When a reset occurs, the output of the reset latch goes LOW #22 , turning ON transistor M1. Thus CX is allowed to quickly charge up to VCC #23 to await the next trigger signal. Recovery time is the required delay after reset goes inactive to a new trigger rising edge. On the diagram it is shown as #26 to RETRIGGER OPERATION In the retriggerable mode, the HC4538 may be retriggered during timing out of the output pulse at any time after the trigger-control circuit flip-flop has been reset Because the trigger-control circuit flip-flop resets shortly after CX has discharged to the reference voltage of the lower reference circuit #25 , the minimum retrigger time, trr is a function of internal propagation delays and the discharge time of CX: device is powered down, the capacitor may discharge from VCC through the input protection diodes at pin 2 or pin Current through the protection diodes must be limited to 30 mA therefore, the turn-off time of the VCC power supply must not be faster than t = VCC•CX/ 30 mA . For example, if VCC = 5V and CX = 15 µF, the VCC supply must turn OFF no faster than t = 15V • 15 µF /30 mA = ms. This is usually not a problem because power supplies are heavily filtered and cannot discharge at this rate. When a more rapid decrease of VCC to zero volts occurs, the MM74HC4538 may sustain damage. To avoid this possibility, use an external clamping diode, DX, connected from VCC to the CX pin. SET UP RECOMMENDATIONS

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MM74HC4538 Dual Retriggerable Monostable Multivibrator

MM74HC4538 Dual Retriggerable Monostable Multivibrator

The MM74HC4538 high speed monostable multivibrator one shots is implemented in advanced silicon-gate CMOS technology. They feature speeds comparable to low power Schottky TTL circuitry while retaining the low power and high noise immunity characteristic of CMOS circuits.

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 MM74HC4538 is retriggerable. That is, it may be triggered repeatedly while their outputs are generating a pulse and the pulse will be extended.

Pulse width stability over a wide range of temperature and supply is achieved using linear CMOS techniques. The output pulse equation is simply PW = 0.7 R C where PW is in seconds, R is in ohms, and C is in farads. This device is pin compatible with the CD4528, and the CD4538 one shots. All inputs are protected from damage due to static discharge by diodes to VCC and ground.
s Schmitt trigger on A and B inputs s Wide power supply range s Typical trigger propagation delay 32 ns s Fanout of 10 LS-TTL loads s Low input current 1 µA max
Ordering Code:

Order Number Package Number

Package Description

MM74HC4538M

M16A
16-Lead Small Outline Integrated Circuit SOIC , JEDEC MS-012, Narrow

MM74HC4538SJ

M16D
16-Lead Small Outline Package SOP , EIAJ TYPE II, 5.3mm Wide

MM74HC4538N

N16E
16-Lead Plastic Dual-In-Line Package PDIP , JEDEC MS-001, Wide
Devices also available in Tape and Reel. Specify by appending the suffix letter “X” to the ordering code.

Connection Diagram

Truth Table

Inputs

Clear

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

Outputs
L H
2000 Fairchild Semiconductor Corporation DS005217

MM74HC4538

Block Diagrams

Note Pin 1 and Pin 15 must be hard-wired to GND.

Logic Diagram

MM74HC4538

Timing Diagram

Circuit Operation

The MM74HC4538 operates as follows refer to logic diagram . In the quiescent state, the external timing capacitor, CX, is charged to VCC. When a trigger occurs, the Q output goes HIGH and CX discharges quickly to the lower reference voltage VREF Lower = 1/3 VCC . CX then charges, through RX, back up to the upper reference voltage VREF Upper = 2/3 VCC , at which point the one-shot has timed out and the Q output goes LOW.

The following, more detailed description of the circuit operation refers to both the logic diagram and the timing diagram.

QUIESCENT STATE

In the quiescent state, before an input trigger appears, the output latch is HIGH and the reset latch is HIGH #1 in logic diagram .

Thus the Q output pin 6 or 10 of the monostable multivibrator is LOW #2, timing diagram .

The output of the trigger-control circuit is LOW #3 , and transistors M1, M2, and M3 are turned off. The external timing capacitor, CX, is charged to VCC #4 , and the upper reference circuit has a LOW output Transistor M4 is turned ON and transmission gate T1 is turned OFF. Thus the lower reference circuit has VCC at the noninverting input and a resulting LOW output

In addition, the output of the trigger-control reset circuit is LOW.

TRIGGER OPERATION

The MM74HC4538 is triggered by either a rising-edge signal at input A #7 or a falling-edge signal at input B #8 , with the unused trigger input and the Reset input held at the voltage levels shown in the Truth Table. Either trigger signal will cause the output of the trigger-control circuit to go HIGH

The trigger-control circuit going HIGH simultaneously initiates three events. First, the output latch goes LOW, thus taking the Q output of the HC4538 to a HIGH State Second, transistor M3 is turned on, which allows the external timing capacitor, CX, to rapidly discharge toward ground Note that the voltage across CXappears at the input of the upper reference circuit comparator. Third, transistor M4 is turned off and transmission gate T1 is turned ON, thus allowing the voltage across CX to also appear at the input of the lower reference circuit comparator.

When CX discharges to the reference voltage of the lower reference circuit #12 , the outputs of both reference circuits will be HIGH The trigger-control reset circuit goes HIGH, resetting the trigger-control circuit flip-flop to a LOW State This turns transistor M3 OFF again, allowing CX to begin to charge back up toward VCC, with a time constant t = RXCX In addition, transistor M4 is turned ON and transmission gate T1 is turned OFF. Thus a high voltage level is applied to the input of the lower reference circuit comparator, causing its output to go LOW The monostable multivibrator may be retriggered at any time after the trigger-control circuit goes LOW.

When CX charges up to the reference voltage of the upper reference circuit #17 , the output of the upper reference circuit goes LOW This causes the output latch to

MM74HC4538

Circuit Operation Continued
toggle, taking the Q output of the HC4538 to a LOW State #19 , and completing the time-out cycle.

RESET OPERATION

A low voltage applied to the Reset pin always forces the Q output of the HC4538 to a LOW State.

The timing diagram illustrates the case in which reset occurs #20 while CX is charging up toward the reference voltage of the upper reference circuit When a reset occurs, the output of the reset latch goes LOW #22 , turning ON transistor M1. Thus CX is allowed to quickly charge up to VCC #23 to await the next trigger signal.

Recovery time is the required delay after reset goes inactive to a new trigger rising edge. On the diagram it is shown as #26 to

RETRIGGER OPERATION

In the retriggerable mode, the HC4538 may be retriggered during timing out of the output pulse at any time after the trigger-control circuit flip-flop has been reset Because the trigger-control circuit flip-flop resets shortly after CX has discharged to the reference voltage of the lower reference circuit #25 , the minimum retrigger time, trr is a function of internal propagation delays and the discharge time of CX:
device is powered down, the capacitor may discharge from VCC through the input protection diodes at pin 2 or pin Current through the protection diodes must be limited to 30 mA therefore, the turn-off time of the VCC power supply must not be faster than t = VCC•CX/ 30 mA . For example, if VCC = 5V and CX = 15 µF, the VCC supply must turn OFF no faster than t = 15V
• 15 µF /30 mA = ms. This is usually not a problem because power supplies are heavily filtered and cannot discharge at this rate.

When a more rapid decrease of VCC to zero volts occurs, the MM74HC4538 may sustain damage. To avoid this possibility, use an external clamping diode, DX, connected from VCC to the CX pin.

SET UP RECOMMENDATIONS

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Datasheet ID: MM74HC4538N 634392