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Since the details regarding the handling of unused pins differ from device to device and from pin
to pin, please follow the instructions given in the relevant individual datasheets or databook.
CMOS logic IC inputs, for example, have extremely high impedance. If an input pin is left open, it
can easily pick up extraneous noise and become unstable. In this case, if the input voltage level
reaches an intermediate level, it is possible that both the P-channel and N-channel transistors
will be turned on, allowing unwanted supply current to flow. Therefore, ensure that the unused
input pins of a device are connected to the power supply (Vcc) pin or ground (GND) pin of the same
device. For details of what to do with the pins of heat sinks, refer to the relevant technical
datasheet and databook.
3.3.5
Latch-up
Latch-up is an abnormal condition inherent in CMOS devices, in which Vcc gets shorted to ground.
This happens when a parasitic PN-PN junction (thyristor structure) internal to the CMOS chip is
turned on, causing a large current of the order of several hundred mA or more to flow between Vcc
and GND, eventually causing the device to break down.
Latch-up occurs when the input or output voltage exceeds the rated value, causing a large current
to flow in the internal chip, or when the voltage on the Vcc (Vdd) pin exceeds its rated value,
forcing the internal chip into a breakdown condition. Once the chip falls into the latch-up state,
even though the excess voltage may have been applied only for an instant, the large current
continues to flow between Vcc (Vdd) and GND (Vss). This causes the device to heat up and, in
extreme cases, to emit gas fumes as well. To avoid this problem, observe the following precautions:
(1) Do not allow voltage levels on the input and output pins either to rise above Vcc (Vdd) or to
fall below GND (Vss). Also, follow any prescribed power-on sequence, so that power is applied
gradually or in steps rather than abruptly.
(2) Do not allow any abnormal noise signals to be applied to the device.
(3) Set the voltage levels of unused input pins to Vcc (Vdd) or GND (Vss).
(4) Do not connect output pins to one another.
3.3.6
Input/Output protection
Wired-AND configurations, in which outputs are connected together, cannot be used, since this
short-circuits the outputs. Outputs should, of course, never be connected to Vcc (Vdd) or GND
(Vss).
Furthermore, ICs with tri-state outputs can undergo performance degradation if a shorted output
current is allowed to flow for an extended period of time. Therefore, when designing circuits, make
sure that tri-state outputs will not be enabled simultaneously.
3.3.7
Load capacitance
Some devices display increased delay times if the load capacitance is large. Also, large charging
and discharging currents will flow in the device, causing noise. Furthermore, since outputs are
shorted for a relatively long time, wiring can become fused.
Consult the technical information for the device being used to determine the recommended load
capacitance.
3 General Safety Precautions and Usage Considerations
3-7
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