ST M74HC4538 User Manual

Features

■ High speed:

t

= 25 ns (typ.) at VCC = 6 V

PD

■ Low power dissipation

standby state:
I

= 4 μA (max.) at TA=25°C

CC

active state:
I

= 200 μA (max.) at VCC=6V

CC

■ High noise immunity:

V

= V

NIH

■ Symmetrical output impedance:

|I

| = IOL = 4 mA (min.)

OH

■ Balanced propagation delays:

t

≅ t

PLH

■ Wide operating voltage range:

V

(opr) = 2 to 6 V

CC

■ Wide output pulse width range:

t

WOUT

■ Pin and function compatible with

74 series 4538

Table 1. Device summary

NIL

PHL

= 28 % V

CC

(min.)

= 120 ns ~ 60 s over at VCC = 4.5 V

M74HC4538

Dual retriggerable monostable multivibrator

SO-16

DIP-16

TSSOP16

Description

The M74HC4538 is a high speed CMOS monostable
multivibrator fabricated with silicon gate C2MOS
technology.

Each multivibrator features both a negative A, and a
positive B, edge 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
monostable multivibrators are retriggerable. That is,
they 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 in Omhs and C is in Farads.

All the inputs are equipped with protection circuits
against static discharge and transient excess voltage.

Order code Package Packaging

M74HC4538B1R DIP-16 Tube

M74HC4538RM13TR SO-16 Tape and reel

M74HC4538TTR TSSOP16 Tape and reel

May 2008 Rev 2 1/18

www.st.com

18

Pin connection and IEC logic symbols M74HC4538

1 Pin connection and IEC logic symbols

Figure 1. Pin connections and IEC logic symbols

V

16

CC

15

2T1

14

2T2

13

2CD

Inputs

12

2A

11

2B

10

9

Outputs

Q2
Q2

Inputs

Outputs

1T1

1T2

1CD

1A

1B

Q1
Q1

GND

1

2

3

4

5

6

7

8

Table 2. Pin description

Pin number Symbol Name and function

1, 15 1T1, 2T1 External capacitor connections
2, 14 1T2, 2T2 External resistor, capacitor connections
3, 13 1CD,
4, 12 1A, 2A Trigger inputs (low to high, edge-triggered)
5, 11 1B, 2B Trigger inputs (high to low, edge-triggered)
6, 10 Q1, Q2 Pulse outputs

2CD Direct reset inputs (active low)

7, 9 Q1, Q2 Complementary pulse outputs

8 GND Ground (0 V)

16 V

CC

Positive supply voltage

2/18

M74HC4538 Pin connection and IEC logic symbols

Figure 2. Input and output equivalent circuit

GND

V

CC

Output

Input

Table 3. Truth table

V

CC

GND

Inputs Outputs

AB

CD QQ

H H Output enable
X L H L H Inhibit
H X H L H Inhibit

L H Output enable

X X L L H Inhibit

Note

Figure 3. System diagram

3/18

Pin connection and IEC logic symbols M74HC4538

Figure 4. Timing chart

Figure 5. Block diagram

1. Cx, Rx, Dx are external components.

2. Dx is a clamping diode.
The external capacitor is charged to V
turned off Cx is discharged mainly through an internal parasitic diode (see figures). If Cx is sufficiently large
and V
up. If the voltage supply filter capacitor is large enough and V
automatically limited and damage to the IC is avoided. The maximum forward current of the parasitic diode
is approximately 20 mA. In cases where Cx is large the time taken for the supply voltage to fall to 0.4 V
can be calculated as follows: t
In cases where t
current.

decreases rapidly, there will be some possibility of damaging the IC with a surge current or latch-

CC

> (VCC - 0.7) x Cx/20 mA.

is too short an external clamping diode is required to protect the IC from the surge

f

f

in the standby state, i.e. no trigger. When the supply voltage is

CC

decreases slowly, the surge current is

CC

CC

4/18

M74HC4538 Functional description

2 Functional description

Standby state

The external capacitor Cx, is fully charged to VCC in the standby state. Hence, before
triggering, transistor Qp and Qn (connected to th e Rx/Cx node) are both turned-off . The two
comparators that control the timing and the two reference voltage sources stop operating.
The total supply current is therefore only leakage current.

Trigger operation

Triggering occurs when:

– A is low and B has a falling edge
– B is high and A has a rising edge

After the multivibrator has been retriggered, t he comparator C1 and C2 start operating and
Qn is turned on. Cx then discharges through Qn. The voltage at the node Rx/Cx external
falls.

When it reaches V

the output of comparator C1 becomes low. This in turn resets the

REFL

flip-flop and Qn is turned off.
At this point C1 stops functioning but C2 continues to operate.
The voltage at R/C external begins to rise with a time constant set by the external

components Rx and Cx.
Triggering the multivibrator causes Q to go high after internal delay due to the flip-flop and

the gate. Q remains high unt il the voltage at R/C e xte rnal rises again to V

. At this point

REFH

C2 output goes low and G goes low. C2 stops operating. That means that after triggering
when the voltage R/C external returns to V

the multivibrator has returned to its

REFH

monostable state . In the case where Rx · Cx are la rge enough and the d ischarge time of the
capacitor and the delay time in the IC can be ignored, the width of the output pulse t

w(out)

is

as follows:

)OUT(W

Rx•Cx72.0=t

Re-triggered operation

When a second triggered pulse follows the first, its effect will depend on the state of the
multivibrator. If the capacitor Cx is being charged, the v oltag e level of Rx/Cx e xternal falls to
V

again and Q remains high i.e. the retrigger pulse arrives in a time shorter than the

REFL

period Rx · Cx seconds, the capacitor charging time constant. If the second trigger pulse is
very close to the initial trigger pulse it is ineffective; i.e. the second trigger m ust arriv e in t he
capacitor discharge cycle to be ineffective; hence the minimu m time for a second trigger to
be effective, t

(min.) depends on VCC and Cx.

rr

Reset operation

CD is normally high. If CD is low, the trigger is not effective because Q output goes lo w an d
trigger control flip-flop is reset. Also transistor Op is turned on and Cx is charged quickly to
V

. Then, if CD input goes low the IC becomes waiting state both in operating and non

CC

operating state.

5/18

Maximum rating M74HC4538

3 Maximum rating

Stressing the device above the rating listed in the “Absolute maximum ratings” table may
cause permanent damage to the device. These are stre ss r a tings only, and operation of the
device at these or any other conditions above those indicated in the operating sections of
this specification is not implied. Exposure to absolute maximum rating conditions for
extended periods may affect device reliability. Refer also to the STMicroelectronics SURE
Program and other relevant quality documents.

Table 4. Absolute maximum ratings

Symbol Parameter Value Unit

V

V

I

I

CC

I

GND

P

T

1. 500mW at 65 ° C; derate to 300 mW by 10 mW/ ° C from 65° C to 85° C

Supply voltage -0.5 to +7 V

CC

DC input voltage -0.5 to VCC + 0.5 V

V

I

DC output voltage -0.5 to VCC + 0.5 V

O

I

DC input diode current ± 20 mA

IK

DC output diode current ± 20 mA

OK

I

DC output current ± 25 mA

O

or

DC VCC or ground current ± 50 mA

Power dissipation 500

D

Storage temperature -65 to +15 0 °C

stg

Lead temperature (10 sec) 300 °C

T

L

3.1 Recommended operating conditions

Table 5. Recommended operating conditions

Symbol Parameter Value Unit

V

V

T

t

Cx External capacitor No limitation pF

Supply voltage 2 to 6 V

CC

Input voltage 0 to V

V

I

Output voltage 0 to V

O

Operating temperature -55 to 125 °C

op

= 2.0 V 0 to 1000 ns

V

CC

, tfInput rise and fall time (CD only)

r

V

= 4.5 V 0 to 500 ns

CC

= 6.0 V 0 to 400 ns

V

CC

(1)

CC
CC

mW

V
V

Rx External resistor

6/18

VCC ≤ 3.0 V 5 K to 1 M

≥ 3.0 V 1 K to 1 M

V

CC

Ω