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MC14538B データシート(PDF) 7 Page - Motorola, Inc |
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MC14538B データシート(HTML) 7 Page - Motorola, Inc |
7 / 10 page MOTOROLA CMOS LOGIC DATA 7 MC14538B RETRIGGER OPERATION The MC14538B is retriggered if a valid trigger occurs fol- lowed by another valid trigger before the Q output has returned to the quiescent (zero) state. Any retrigger, after the timing node voltage at pin 2 or 14 has begun to rise from Vref 1, but has not yet reached Vref 2, will cause an increase in output pulse width T. When a valid retrigger is initiated , the voltage at CX/RX will again drop to Vref 1 before progressing along the RC charging curve toward VDD. The Q output will remain high until time T, after the last valid retrig- ger. RESET OPERATION The MC14538B may be reset during the generation of the output pulse. In the reset mode of operation, an input pulse on Reset sets the reset latch and causes the capacitor to be fast charged to VDD by turning on transistor P1 . When the voltage on the capacitor reaches Vref 2, the reset latch will clear, and will then be ready to accept another pulse. It the Reset input is held low, any trigger inputs that occur will be inhibited and the Q and Q outputs of the output latch will not change. Since the Q output is reset when an input low level is detected on the Reset input, the output pulse T can be made significantly shorter than the minimum pulse width specifi- cation. POWER–DOWN CONSIDERATIONS Large capacitance values can cause problems due to the large amount of energy stored. When a system containing the MC14538B is powered down, the capacitor voltage may discharge from VDD through the standard protection diodes at pin 2 or 14. Current through the protection diodes should be limited to 10 mA and therefore the discharge time of the VDD supply must not be faster than (VDD). (C)/(10 mA). For example, if VDD = 10 V and CX = 10 µF, the VDD supply should discharge no faster than (10 V) x (10 µF)/(10 mA) = 10 ms. This is normally not a problem since power supplies are heavily filtered and cannot discharge at this rate. When a more rapid decrease of VDD to zero volts occurs, the MC14538B can sustain damage. To avoid this possibility use an external clamping diode, DX, connected as shown in Fig. 11. Figure 11. Use of a Diode to Limit Power Down Current Surge PIN ASSIGNMENT 13 14 15 16 9 10 11 12 5 4 3 2 1 8 7 6 AB RESET B CX/RXB VSS VDD QB QB BB AA RESET A CX/RXA VSS VSS QA QA BA VSS Dx VDD VDD Rx Cx Q Q RESET |
同様の部品番号 - MC14538B |
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同様の説明 - MC14538B |
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