74HC_HCT4538_15 Datasheet, PDF (15/24 Pages) NXP Semiconductors – Dual retriggerable precision monostable multivibrator

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
DEXT
GND
CEXT
REXT
V_CC
nCEXT
nREXT/CEXT
001aak193
Fig 10. Power-down protection circuit
12.2 Graphs
1.6
normalized
K factor
(K)
1.2
1.0
0.8
0.6
10^−10
10^−9
1 kΩ
2 kΩ
5.6 kΩ
51 kΩ
1 MΩ
10^−8
10^−7
10^−6
V_CC = 4.5 V; T_amb = 25 °C.
Fig 11. Typical pulse width accuracy versus external capacitance
001aak196 _1.12
R_EXT ×
C_EXT
(t_W)
0.84
0.70
0.56
10^−5
0.42
10^−4
C_EXT (F)
74HC_HCT4538_3
Product data sheet
Rev. 03 — 8 June 2009
© NXP B.V. 2009. All rights reserved.
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