SN74AHCT123ADE4 datasheet(7/24 Pages) TI

7 / 24 page

SN54AHCT123A, SN74AHCT123A

DUAL RETRIGGERABLE MONOSTABLE MULTIVIBRATORS

SCLS420G – JUNE 1998 – REVISED APRIL 2003

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

APPLICATION INFORMATION

caution in use

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.

power-down considerations

Large values of Cext may cause problems when powering down the ’AHCT123A devices 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 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)/30mA = 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 ’AHCT123A devices may

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

output pulse duration

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.

VCC

To Rext/Cext

Terminal

To Cext

Terminal

Figure 2. Timing-Component Connections

The pulse duration is given by:

+ K

if CT is ≥ 1000 pF, K = 1.0 or

if CT is < 1000 pF, K can be determined from Figure 5

where:

tw = pulse duration in ns

RT = external timing resistance in kΩ

CT = external capacitance in pF

= multiplier factor

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

capacitance.

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