74HC123 Datasheet, PDF (2/17 Pages) NXP Semiconductors – Dual retriggerable monostable multivibrator with reset

Philips Semiconductors
Dual retriggerable monostable
multivibrator with reset
Product specification
74HC/HCT123
FEATURES
• DC triggered from active HIGH or
active LOW inputs
• Retriggerable for very long pulses
up to 100% duty factor
• Direct reset terminates output
pulse
• Schmitt-trigger action on all inputs
except for the reset input
• Output capability: standard (except
for nR_EXT/C_EXT)
• I_CC category: MSI
GENERAL DESCRIPTION
The 74HC/HCT123 are high-speed
Si-gate CMOS devices and are pin
compatible with low power Schottky
TTL (LSTTL). They are specified in
compliance with JEDEC standard no.
7A.
The 74HC/HCT123 are dual
retriggerable monostable
multivibrators with output pulse width
control by three methods. The basic
pulse time is programmed by
selection of an external resistor
(R_EXT) and capacitor (C_EXT). The
external resistor and capacitor are
normally connected as shown in
Fig.6.
Once triggered, the basic output
pulse width may be extended by
retriggering the gated active
LOW-going edge input (nA) or the
active HIGH-going edge input (nB).
By repeating this process, the output
pulse period (nQ = HIGH, nQ = LOW)
can be made as long as desired.
Alternatively an output delay can be
terminated at any time by a
LOW-going edge on input nR_D, which
also inhibits the triggering.
An internal connection from nR_D to
the input gates makes it possible to
trigger the circuit by a positive-going
signal at input nR_D as shown in the
function table. Figures 7 and 8
illustrate pulse control by retriggering
and early reset. The basic output
pulse width is essentially determined
by the values of the external timing
components R_EXT and C_EXT. For
pulse widths, when C_EXT < 10 000 pF,
see Fig.9.
When C_EXT > 10 000 pF, the typical
output pulse width is defined as:
t_W = 0.45 × R_EXT × C_EXT (typ.),
where:
t_W = pulse width in ns;
R_EXT = external resistor in kΩ;
C_EXT = external capacitor in pF.
Schmitt-trigger action in the nA and
nB inputs, makes the circuit highly
tolerant to slower input rise and fall
times.
The ‘123’ is identical to the ‘423’ but
can be triggered via the reset input.
QUICK REFERENCE DATA
GND = 0 V; T_amb = 25 °C; t_r = t_f = 6 ns
SYMBOL PARAMETER
CONDITIONS
TYPICAL
UNIT
HC HCT
t_PHL/ t_PLH propagation delay
C_L = 15 pF;
nA, nB to nQ, nQ ^{VCC = 5 V;}
26 26 ns
nR_D to nQ, nQ
R_EXT = 5 kΩ;
C_EXT = 0 pF
20
23
ns
C_I
input capacitance
3.5 3.5 pF
C_PD
power dissipation
capacitance per
notes 1 and 2 54 56 pF
monostable
Notes
1. C_PD is used to determine the dynamic power dissipation (P_D in µW):
P_D = C_PD × V_CC2 × f_i + ∑(C_L × V_CC2 × f_o) + 0.75 × C_EXT
× V_CC2 × f_o + D × 16 × V_CC where:
f_i = input frequency in MHz
f_o = output frequency in MHz
D = duty factor in %
C_L = output load capacitance in pF
V_CC = supply voltage in V
C_EXT = timing capacitance in pF
∑ (C_L × V_CC2 × f_o) sum of outputs
2. For HC the condition is V_I = GND to V_CC
For HCT the condition is V_I = GND to V_CC − 1.5 V
1998 Jul 08
2