74HC423 Datasheet, PDF (2/14 Pages) NXP Semiconductors – Dual retriggerable monostable multivibrator with reset

Philips Semiconductors
Dual retriggerable monostable
multivibrator with reset
Product specification
74HC/HCT423
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/HCT423 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/HCT423 are dual retriggerable monostable
multivibrators with output pulse width control by two
methods. The basic pulse time is programmed by
selection of an external resistor (R_EXT) and capacitor
QUICK REFERENCE DATA
GND = 0 V; T_amb = 25 °C; t_r = t_f = 6 ns
(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.
When nR_D is LOW, it forces the nQ output LOW, the
nQ output HIGH and also inhibits the triggering.
Figures 7 and 8 illustrate pulse control by 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 “423” is identical to the “123” but cannot be triggered
via the reset input.
SYMBOL
PARAMETER
CONDITIONS
TYPICAL
HC
HCT
UNIT
t_PHL/ t_PLH
propagation delay
nA, nB to nQ, nQ
C_L = 15 pF; V_CC = 5 V;
R_EXT = 5 kΩ; C_EXT = 0 pF
25
26
ns
nR_D to nQ, nQ
20
22
ns
C_I
input capacitance
3.5
3.5
pF
t_W
minimum output pulse width nQ, nQ notes 1 and 2
75
75
ns
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 × V_CC2 × f_o) = sum of outputs
C_L = output load capacitance in pF
V_CC = supply voltage in V
C_EXT = timing capacitance in pF
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