HD74HC4538 Datasheet, PDF (6/14 Pages) Hitachi Semiconductor – Dual Precision Retriggerable/Resettable Monostable Multivibrators

HD74HC4538
Power-Down Considerations
Large values of C_X may cause problems when powering down the HC4538 because of the amount of energy stored in
the capacitor. When a system containing this device is powered down, the capacitor may discharge from V_CC through
the input protection diodes at pin 2 or pin 14. Current through the 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•C_X/(30 mA). For example, if V_CC =
5 V and C_X = 15 µF, the V_CC supply must turn off no faster than t = (5 V)•(15 µF)/30 mA = 2.5 ms. This is usually 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 voltage occurs, the HC4538 may sustain damage. To avoid this possibility,
use an external clamping diode.
VCC
RX
T2 ^{2, 14}
CX
VCC
M1
M2
2k
M3
4, 12
A
B
5, 11
S1
VCC
M4
Trigger-Control Circuit
CQ
C_R
Upper Reference
Circuit
+
–
Vref Upper
Lower Reference
Circuit
+
–
Vref Lower
Output Latch
Trigger-Control
Reset Circuit
3, 13
CD
6, 10
Q
7, 9
Q
Reset Latch
Figure 1. Function Diagram
Rev.2.00 Mar 30, 2006 page 6 of 13