MC74HC4538A

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reference circuit (#17), the output of the upper reference

circuit 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

through the input protection diodes at pin 2 or pin 14.

Current through the protection diodes must be limited to 30

no faster than t = (5.0 V)

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

usually not a problem because power supplies are heavily

filtered and cannot discharge at this rate.

the HC4538A may sustain damage. To avoid this possibility,

to be a germanium or Schottky type diode able to withstand

large current surges.

RESET AND POWER ON RESET OPERATION

A low voltage applied to the Reset pin always forces the

Q output of the HC4538A to a low state.

The timing diagram illustrates the case in which reset

voltage of the upper reference circuit (#21). When a reset

occurs, the output of the reset latch goes low (#22), turning

On power up of the HC4538A the power−on reset circuit

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

When used in the retriggerable mode (Figure 12), the

HC4538A may be retriggered during timing out of the

output pulse at any time after the trigger−control circuit

is below the lower reference voltage, the reset of the

flip−flop is high, disabling 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,

Figure 13 shows the device configured in the

non−retriggerable mode.

For additional information, please see Application Note

(AN1558/D) titled Characterization of Retrigger Time in

the HC4538A Dual Precision Monostable Multivibrator.

Q

Q

RESET

A

B

Figure 13. Discharge Protection During Power Down