AN2433 Freescale Semiconductor / Motorola, AN2433 Datasheet - Page 4

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

Freescale Semiconductor, Inc.
AN2433/D
both be slightly on. Indeed, they will be in their respective saturation regions
and their resistance to ground will be much lower than compared to their off
states, see
Figure 2
, thus I
will reach quite a high value; the maximum
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could be as high as 4 mA. Typically I
is around 100 – 200 A per input pin.
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This added current consumption will only occur when a logic high is applied to
the pin and, normally, not all the inputs will be at ‘1’ at the same time.
Nevertheless, even with these allowances, the extra current consumption is a
major disadvantage in a mixed voltage system.
Furthermore, a mixed voltage application requires several voltage supplies.
Single chip solutions providing 3V, 5V, plus a third variable supply voltage (
12V required, for example, for RS-232 communications) are already available
from some power supply manufacturers. This helps in reducing the surplus of
components required by a mixed voltage system.
Figure 2. Typical CMOS input structure and I
current example
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While the penalties of a hybrid system in comparison to a ‘pure’ 3V system
emerge quite clearly, this does not mean that a system will be better off running
at 5V. In fact a 5V/3V mixed system still requires less power than a 5V version;
therefore it is advantageous to replace 5V parts with 3V ones wherever
possible, especially in portable applications where long battery life is important.
4
5V to 3V Design Considerations
MOTOROLA
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