AN2433 Freescale Semiconductor / Motorola, AN2433 Datasheet - Page 7

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

Interfacing to
Microcontroller
Inputs
MOTOROLA
The p-channel transistor of the output driver forms an inherent diode to V
and the n-channel device forms an inherent diode to V
the inherent diode of the thick-field protection device.
Because the protections are now both to V
clamped to within a diode drop of the power supply rails, i.e. V
V
input-only pin is that the n-channel output device starts to conduct before the
thick-field protection device, thus the clamp level for voltages above V
typically be lower than for a digital input-only pin.
Therefore, in the 5V V
to the input buffer will increase, until it reaches 5.6V. At this level of voltage, the
protection diode to V
be no current entering the input buffer because of the very high impedance of
its gate. The same scenario will happen in the 3V V
clamping value will be approximately 3.6V.
As the technology moves from 5V to 3V, input level protection will still be
available, although the level of current injection will be reduced. In extreme
situations, series resistors or some other form of protection will be required to
limit the currents to safe levels.
The main objective for interfacing circuits is to avoid over voltages on 3V inputs
(and outputs for bi-directional pins), which are connected to 5V outputs and
limit the current flowing from 5V parts.
Input pin structures usually contain ESD protection diodes, which clamp input
voltages when they go below V
For a 5V output being applied to a 3V input, when the maximum input voltage
is exceeded on the 3V input, the ESD protection diode on the input of the
device can be forward biased and current will flow into the 3V V
take the voltage level of the 3V power supply up to the input voltage minus the
voltage drop across the protection diode. Therefore, this difference in voltage
will cause a high current to be drawn (almost a short-circuit), which will lead to
heat and possible long-term reliability problems.
To avoid such problems, the voltage on inputs should not exceed V
in order not to conduct the clamping diode. This means that for 3V minimum
(V
cases the easiest work around is to use simple series resistors that limit
currents flowing into 3V inputs, but usually these can add delays and board
space.
SS
DD3min
Freescale Semiconductor, Inc.
– 0.6V respectively. The main difference for an I/O pin compared to an
For More Information On This Product,
) voltage the maximum input voltage should be V
5V to 3V Design Considerations
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DD
DD
will switch on and current will flow to V
case, if the input voltage rises above 5V, the voltage
SS
(GND) or beyond V
DD
and V
DD
SS
SS
DD
, input signals are
case, except that the
, which is in parallel to
levels.
Imax
DD
= 3.5V. In many
Microcontroller Inputs
DD
DD
but there will
+ 0.6V and
. This could
DD3
DD
AN2433/D
+ 0.5V
DD
will
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