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APPLICATION INFORMATION
Caution in Use
Powerdown Considerations
Output Pulse Duration
V
CC
R
T
C
T
ToR /C
ext ext
Terminal
ToC
ext
Terminal
t
w
+ K R
T
C
T
(1)
SN74AHC123A-EP
DUAL RETRIGGERABLE MONOSTABLE MULTIVIBRATOR
SCLS703A – JULY 2006 – REVISED MARCH 2007
To prevent malfunctions due to noise, connect a high-frequency capacitor between V
CC
and GND, and keep the
wiring between the external components and C
ext
and R
ext
/C
ext
terminals as short as possible.
Large values of C
ext
can cause problems when powering down the SN74AHC123A devices because of the
amount of energy stored in the capacitor. When a system containing this device is powered down, the capacitor
can discharge from V
CC
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 V
CC
power supply must not be faster than
t = V
CC
× C
ext
/30 mA. For example, if V
CC
= 5 V and C
ext
= 15 pF, the V
CC
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 V
CC
to zero occurs, the SN74AHC123A can
sustain damage. To avoid this possibility, use external clamping diodes.
The output pulse duration (t
w
) is determined primarily by the values of the external capacitance (C
T
) and timing
resistance (R
T
). The timing components are connected as shown in Figure 2 .
Figure 2. Timing-Component Connections
The pulse duration is given by:
if C
T
is ≥ 1000 pF, K = 1 or
if C
T
is < 1000 pF, K can be determined from Figure 9
where:
t
w
= pulse duration in ns
R
T
= external timing resistance in k Ω
C
T
= 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.
8
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