MC54/74HC4538A Просмотр технического описания (PDF) - Motorola => Freescale

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MC54/74HC4538A

Dual precision Monostable Multivibrator (Retriggerable, Resettable)

Motorola => Freescale 

MC54/74HC4538A

When Cx charges up to the reference voltage of the upper

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

Large values of Cx may cause problems when powering

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 VCC

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 VCC power supply must not

be faster than t = VCCCx/(30 mA). For example, if

VCC = 5.0 V and Cx = 15 µF, the VCC supply must turn off no

faster than t = (5.0 V)(15 µF)/30 mA = 2.5 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 HC4538A may sustain damage. To avoid this possibility,

use an external damping diode, Dx, connected as shown in

Figure 11. Best results can be achieved if diode Dx is chosen

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 oc-

curs (#20) while Cx is charging up toward the reference volt-

age of the upper reference circuit (#21). 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.

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 flip–flop has

been reset (#24), and the voltage across Cx is above the low-

er reference voltage. As long as the Cx 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.

The amount of undershoot voltage on RxCx during the

trigger mode is a function of loop delay, M3 conductivity, and

VDD. Minimum retrigger time, trr (Figure 7), is a function of

1) time to discharge RxCx from VDD to lower reference

voltage (Tdischarge); 2) loop delay (Tdelay); 3) time to charge

RxCx from the undershoot voltage back to the lower refer-

ence voltage (Tcharge).

Figure 13 shows the device configured in the non–retrig-

gerable mode.

An Application Note (AN1558/D) titled Characterization of

Retrigger Time in the HC4538A Dual Precision Monstable

Multivibrator is being prepared. Please consult the factory for

its availability.

DX

CX

VCC

RX

A

Q

B

Q

RESET

Figure 11. Discharge Protection During Power Down

MOTOROLA

3–10

High–Speed CMOS Logic Data

DL129 — Rev 6

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