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參數(shù)資料
| 型號(hào)： | MM54HC4538J |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 諧振器 |
| 英文描述： | HC/UH SERIES, DUAL MONOSTABLE MULTIVIBRATOR, CDIP16 |
| 封裝： | CERAMIC, DIP-16 |
| 文件頁(yè)數(shù)： | 9/10頁(yè) |
| 文件大小： | 199K |
| 代理商： | MM54HC4538J |
Circuit Operation (Continued)
RESET OPERATION
A low voltage applied to the Reset pin always forces the Q
output of the ’HC4538 to a low state.
The timing diagram illustrates the case in which reset occurs
(#20) while C
X 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 C
X is allowed to quickly charge up to VCC (#23) to
await the next trigger signal.
Recovery time is the required delay after reset goes inactive
to a new trigger rising edge. On the diagram it is shown as
(#26) to (#27).
RETRIGGER OPERATION
In the retriggerable mode, the ’HC4538 may be retriggered
during timing out of the output pulse at any time after the
trigger-control circuit flip-flop has been reset (#24). Because
the trigger-control circuit flip-flop resets shortly after C
X has
discharged to the reference voltage of the lower reference
circuit (#25), the minimum retrigger time, t
rr is a function of
internal propagation delays and the discharge time of C
X:
at room temperature
POWER-DOWN CONSIDERATIONS
Large values of C
X may cause problems when powering
down the HC4538 because of the amount of energy stored in
the capacitor. When a system containing this device is pow-
ered down, the capacitor may discharge from V
CC 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 V
CC power supply must not be faster
than t = V
CCCX/(30 mA). For example, if VCC = 5V and CX
= 15 F, the V
CC supply must turn off no faster than t =
(15V)
(15 F)/30 mA = 2.5 ms. This is usually not a problem
because power supplies are heavily filtered and cannot dis-
charge at this rate.
When a more rapid decrease of V
CC to zero volts occurs, the
HC4538 may sustain damage. To avoid this possibility, use
an external clamping diode, D
X, connected from VCC to the
C
X pin.
SET UP RECOMMENDATIONS
Minimum
R
X = 1k
Minimum
C
X = 0pF.
DS005217-5
www.fairchildsemi.com
8
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