74HC_HCT4538_3 © NXP B.V. 2009. All rights reserved.
Product data sheet Rev. 03 — 8 June 2009 15 of 24
NXP Semiconductors
74HC4538; 74HCT4538
Dual retriggerable precision monostable multivibrator
12. Application information
12.1 Power-down considerations
A large capacitor (C
EXT
) may cause problems when powering-down the monostable due
to energy stored in this capacitor. When a system containing this device is powered-down
or rapid decrease of V
CC
to zero occurs, the monostable may sustain damage, due to the
capacitor discharging through the input protection diodes. To avoid this possibility, use a
damping diode (D
EXT
) preferably a germanium or Schottky type diode able to withstand
large current surges and connect as shown in Figure 10
12.2 Graphs
Fig 10. Power-down protection circuit
001aak193
nREXT/CEXTnCEXT
GND
V
CC
R
EXT
D
EXT
C
EXT
V
CC
= 4.5 V; T
amb
= 25 °C.
Fig 11. Typical pulse width accuracy versus external capacitance
001aak196
C
EXT
(F)
10
−10
10
−5
10
−4
10
−6
10
−7
10
−9
10
−8
1.0
1.2
0.8
1.6
0.6
normalized
K factor
(K)
0.70
0.84
0.56
1.12
0.42
R
EXT
×
C
EXT
(t
W
)
1 kΩ
2 kΩ
5.6 kΩ
51 kΩ
1 MΩ
