Data Sheet

V
CC
R
T
C
T
To R
ext
/C
ext
Terminal
To C
ext
Terminal
SN54LV123A, SN74LV123A
SCLS393P –APRIL 1998–REVISED OCTOBER 2013 www.ti.com
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 Cext and Rext/Cext terminals as short as possible.
Power-Down Considerations
Large values of Cext can cause problems when powering down the 'LV123A 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
y
Cext/30 mA. For example, if V
CC
= 5 V and Cext = 15 pF, the V
CC
supply must turn off no faster than t = (5 V) y
(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 'LV123A 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 12.
Figure 12. Timing-Component Connections
The pulse duration is given by:
t
w
= K × R
T
× C
T
(1)
if C
T
is .1000 pF, K = 1.0 or
if C
T
is <1000 pF, K can be determined from Figure 10
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 5 can be used to determine values for pulse duration, external resistance, and external
capacitance.
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Product Folder Links: SN54LV123A, SN74LV123A