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MC74HC4538AN Datasheet(PDF) 11 Page - ON Semiconductor |
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MC74HC4538AN Datasheet(HTML) 11 Page - ON Semiconductor |
11 / 16 page MC74HC4538A http://onsemi.com 11 When Cx charges up to the reference voltage of the upper reference circuit (#17), the output of the upper reference circuit 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 = VCC Cx/(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 occurs (#20) while Cx is charging up toward the reference voltage 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 lower reference voltage. As long as the Cx voltage is below the lower reference voltage, the reset of the flip–flop is high, disabling 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 reference voltage (Tcharge). Figure 13 shows the device configured in the non–retriggerable mode. For additional information, please see Application Note (AN1558/D) titled Characterization of Retrigger Time in the HC4538A Dual Precision Monostable Multivibrator. DX CX VCC Q Q RESET A B Figure 13. Discharge Protection During Power Down RX |
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