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APPLICATION INFORMATION

Caution in Use

Powerdown Considerations

Output Pulse Duration

V

CC

R

T

C

T

ToR

/C

ext

ext

Terminal

ToC

ext

Terminal

tw + K

R

T

C

T

(1)

SN74AHC123A-EP

DUAL RETRIGGERABLE MONOSTABLE MULTIVIBRATOR

SCLS703A – JULY 2006 – REVISED MARCH 2007

To prevent malfunctions due to noise, connect a high-frequency capacitor between VCC and GND, and keep the

wiring between the external components and Cext and Rext/Cext terminals as short as possible.

Large values of Cext can cause problems when powering down the SN74AHC123A devices because of the

amount of energy stored in the capacitor. When a system containing this device is powered down, the capacitor

can discharge from VCC through the protection diodes at pin 2 or pin 14. Current through the input 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× Cext/30 mA. For example, if VCC = 5 V and Cext = 15 pF, the VCC supply must turn off no faster than

t = (5 V)

× (15 pF)/30 mA = 2.5 ns. Usually, this is not a problem because power supplies are heavily filtered and

cannot discharge at this rate. When a more rapid decrease of VCC to zero occurs, the SN74AHC123A can

sustain damage. To avoid this possibility, use external clamping diodes.

The output pulse duration (tw) is determined primarily by the values of the external capacitance (CT) and timing

resistance (RT). The timing components are connected as shown in Figure 2.

Figure 2. Timing-Component Connections

The pulse duration is given by:

if CT is ≥ 1000 pF, K = 1 or

if CT is < 1000 pF, K can be determined from Figure 9

where:

tw = pulse duration in ns

RT = external timing resistance in kΩ

CT = external capacitance in pF

K = multiplier factor

Equation 1 and Figure 3 can be used to determine values for pulse duration, external resistance, and external

capacitance.

8

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