AN2433 Freescale Semiconductor / Motorola, AN2433 Datasheet - Page 8

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

AN2433/D
5V TTL Output to 3V
MCU Inputs
8
V
V
T T L
T T L
C i r c u i t
C i r c u i t
S u p p l y
S u p p l y
5 . 0 V
5 . 0 V
V
V
O u t p u t
O u t p u t
G N D
G N D
A typical bipolar output has an active high output voltage level limitation of V
– 2*V
the base-emitter and the diode voltage drops, which are both approximately
0.6V.
The maximum high output voltage value for the TTL (V
which is 5.5V. In this case, the V
at no-load. The maximum voltage on a 3V MCU input should not exceed V
3.5V (for V
maximum differential voltage between TTL output logic high and the MCU
maximum input voltage V
between the TTL output and the MCU input will limit the current flowing to the
input and thus its voltage will drop to a safe level. In the example above, the 10
k
The current flowing to the MCU input should be limited to a level which is well
below the maximum specification in order to avoid latch-up as excessive input
current can damage the internal silicon structures.
Care should be taken in choosing the correct resistor value, as too high a value
can cause significant signal propagation delay because of the RC constant
created by the resistor and the input capacitance, which is typically about 5–10
pF plus the PCB capacitance.
Also, excessive current increases the power consumption because the current
flows into the MCU input when the driving component is outputting a logic high.
Freescale Semiconductor, Inc.
Figure 5. 5V TTL output to 3.3V MCU input
resistor will limit the current to a maximum of 80 A (0.8V/10k ).
BE
For More Information On This Product,
because of internal TTL gate structure, see
DD3min
5V to 3V Design Considerations
1 0 k
1 0 k
Go to: www.freescale.com
R
R
= 3V), as stated in the previous paragraph. Thus the
Imax
I n p u t
I n p u t
is 0.8V (4.3V – 3.5V). Just placing a resistor
M C U
M C U
OH
V
V
voltage level is typically 4.3V (5.5–2*V
D D
D D
3 . 3 V
3 . 3 V
Figure 5
OH
V
V
S S
S S
) is for maximum V
where V
P
P
N
N
MOTOROLA
BE
is both
Imax
DD5
BE
DD
=
)
,