Datasheet

 
   
SCLS352H − JULY 1997 − REVISED OCTOBER 2005
9
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
caution in use
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.
power-down considerations
Large values of C
ext
can cause problems when powering down the ’AHC123A 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 ’AHC123A devices
can sustain damage. To avoid this possibility, use external clamping diodes.
output pulse duration
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.
V
CC
R
T
C
T
To R
ext
/C
ext
Terminal
To C
ext
Terminal
Figure 2. Timing-Component Connections
The pulse duration is given by:
t
w
+ K R
T
C
T
if C
T
is ≥1000 pF, K = 1.0 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 kΩ
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.
(1)