AN2433 Freescale Semiconductor / Motorola, AN2433 Datasheet - Page 6

• Manufacturer Part Number: AN2433
• Description: 5V to 3V Design Considerations
• Manufacturer: Freescale Semiconductor / Motorola

AN2433/D
6
In t e r n a l
O u t p u t
B u f f e r
P
N
V
V
D D
In some microcontroller families, instead of a thick-field protection device there
is a normal n-channel transistor, but the following discussion is still valid, the
only difference being the clamping voltage levels for V
The thick-field protection device forms an inherent diode junction to V
Therefore if the voltage applied becomes lower than V
will start to conduct when the pin voltage gets more than a diode drop below
V
injected, but the input will be clamped to a value which corresponds to a diode
drop below V
If the voltage at the input pin is taken above V
rises as well. The gates of the input buffers have very high impedance and
could be sufficient to provide protection when V
keeps rising, the thick-field protection device will turn on and clamp this input
voltage to protect the input buffer. The level of the clamping voltage may vary
over the operating temperature and is typically, at room temperature, up to
10–11V (5V V
Let us now examine the CMOS circuitry of an input-output pin, see
E x t e r n a l
When the pin is configured as an input, the output driver circuitry is disabled,
but its MOS transistors still affect the way the input pin reacts to illegal voltage
levels.
S S
SS
Freescale Semiconductor, Inc.
. As the negative voltage applied to the pin increases, more current will be
For More Information On This Product,
5V to 3V Design Considerations
N
SS
P in
Figure 4. Digital I/O Pin
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DD
(therefore –0.6V), irrespective of the V
) and 7–8V (3V V
V
S S
P r o t e c t io n
T h ic k
F ie ld
DD
).
B u f f e r
In p u t
DD
, the voltage at the input buffer
IN
has overtaken V
IN
SS
DD
P
N
higher than V
, this inherent diode
V
V
supply.
D D
S S
MOTOROLA
DD
Figure 4
. If V
SS
DD
.
.
IN
.