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SN74AHCT123ADGVR Fiches technique(PDF) 7 Page - Texas Instruments |
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SN74AHCT123ADGVR Fiches technique(HTML) 7 Page - Texas Instruments |
7 / 24 page SN54AHCT123A, SN74AHCT123A DUAL RETRIGGERABLE MONOSTABLE MULTIVIBRATORS SCLS420G – JUNE 1998 – REVISED APRIL 2003 7 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 RT CT To Rext/Cext Terminal To Cext Terminal Figure 2. Timing-Component Connections The pulse duration is given by: tw + K R T C T 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 K = multiplier factor Equation 1 and Figure 3 can be used to determine values for pulse duration, external resistance, and external capacitance. (1) |
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