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SN74LV221APWRE4 Datasheet(PDF) 10 Page - Texas Instruments |
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SN74LV221APWRE4 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 22 page SN54LV221A, SN74LV221A DUAL MONOSTABLE MULTIVIBRATORS WITH SCHMITT TRIGGER INPUTS SCLS450G − DECEMBER 1999 − REVISED APRIL 2005 10 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 can cause problems when powering down the ’LV221A 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 ’LV221A can 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 7 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 or 4 can be used to determine values for pulse duration, external resistance, and external capacitance. (1) |
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