MC74HC4538A Motorola, MC74HC4538A Datasheet - Page 10

• Manufacturer Part Number: MC74HC4538A
• Description: Dual Precision Monostable Multivibrator
• Manufacturer: Motorola

MOTOROLA
reference circuit (#17), the output of the upper reference cir-
cuit goes low (#18). This causes the output latch to toggle,
taking the Q output of the HC4538A to a low state (#19), and
completing the time–out cycle.
POWER–DOWN CONSIDERATIONS
down the HC4538A because of the amount of energy stored
in the capacitor. When a system containing this device is
powered down, the capacitor may discharge from V CC
through the input protection diodes at pin 2 or pin 14. Current
through the protection diodes must be limited to 30 mA;
therefore, the turn–off time of the V CC power supply must not
be faster than t = V C C
V CC = 5.0 V and C x = 15 F, the V CC supply must turn off no
faster than t = (5.0 V)
not a problem because power supplies are heavily filtered
and cannot discharge at this rate.
the HC4538A may sustain damage. To avoid this possibility,
use an external damping diode, D x , connected as shown in
Figure 11. Best results can be achieved if diode D x is chosen
to be a germanium or Schottky type diode able to withstand
large current surges.
RESET AND POWER ON RESET OPERATION
output of the HC4538A to a low state.
curs (#20) while C x is charging up toward the reference volt-
age of the upper reference circuit (#21). When a reset
MC54/74HC4538A
When C x charges up to the reference voltage of the upper
Large values of C x may cause problems when powering
When a more rapid decrease of V CC to zero volts occurs,
A low voltage applied to the Reset pin always forces the Q
The timing diagram illustrates the case in which reset oc-

(15 F)/30 mA = 2.5 ms. This is usually

C x / (30 mA). For example, if
Figure 11. Discharge Protection During Power Down
A
B
RESET
C X
3–10
occurs, the output of the reset latch goes low (#22), turning
on transistor M1. Thus C x is allowed to quickly charge up to
V CC (#23) to await the next trigger signal.
will be high causing a reset condition. This will prevent the
trigger–control circuit from accepting a trigger input during
this state. The HC4538A’s Q outputs are low and the Q not
outputs are high.
RETRIGGER OPERATION
HC4538A may be retriggered during timing out of the output
pulse at any time after the trigger–control circuit flip–flop has
been reset (#24), and the voltage across C x is above the low-
er reference voltage. As long as the C x voltage is below the
lower reference voltage, the reset of the flip–flop is high, dis-
abling any trigger pulse. This prevents M3 from turning on
during this period resulting in an output pulse width that is
predictable.
trigger mode is a function of loop delay, M3 conductivity, and
V DD . Minimum retrigger time, trr (Figure 7), is a function of
1) time to discharge R x C x from V DD to lower reference
voltage (T discharge ); 2) loop delay (T delay ); 3) time to charge
R x C x from the undershoot voltage back to the lower refer-
ence voltage (T charge ).
gerable mode.
Retrigger Time in the HC4538A Dual Precision Monstable
Multivibrator is being prepared. Please consult the factory for
its availability.
D X
R X
On power up of the HC4538A the power–on reset circuit
When used in the retriggerable mode (Figure 12), the
The amount of undershoot voltage on R x C x during the
Figure 13 shows the device configured in the non–retrig-
An Application Note (AN1558/D) titled Characterization of
Q
Q
V CC
High–Speed CMOS Logic Data
DL129 — Rev 6