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Voltage translation: It doesn’t get any easier than this
Today, modern design houses are trying to find smaller devices that use less power, and they want to do this with lower cost for applications like industrial automation, PCs, servers and telecommunications equipment. One stumbling block to achieving these goals is when designers run into processors that operate at one voltage that need to connect to peripherals or other subsystems that operate at many different voltages. This drives the need to easily translate voltages up and down. Traditionally, this translation was done using multiple discrete components. Let’s discuss why a single logic component that uses only a single power rail can simplify your design while effectively and efficiently translating voltages. We’ll also teach you how to translate up and down easily.
TI’s SN74LV1T family can perform up or down voltage translation with only one power rail. The devices have over-voltage tolerant inputs that allow down translation up to 5.5V to the Vcc level, which can be as low as 1.8V. The family also has a lowered switching threshold that allows it to translate up to the Vcc level, which can be as high as 5.5V. (See figure 1) This solves the design house’s problem of overcoming the multiple voltage levels needed in a single application.
Figure 1: SN74LV1T replaces many discrete components
How to translate down
The SN74LV1T family makes it very easy to translate down. Since the inputs are tolerant to 5.5V at any valid Vcc, they can be used to down translate. The input can be any level above Vcc up to 5.5V, and the output will equal the Vcc level which can be as low as 1.8V. The SN74LV1T is unique in that the ICC current remains less than or equal to the specified value while translating down. The current draw when translating can be seen in figure 3 below.
Down translation possibilities with SN74LV1T family:
How to translate up
Translating up is also really simple. The input switching threshold is lowered so the high level of the input voltage can be much lower than a typical CMOS Vih. For instance, if the Vcc is 3.3V then the typical CMOS switching threshold would be VCC/2 or 1.65V. This means the input high level must be at least Vcc*.7 or 2.31V. On the SN74LV1T devices, the input threshold for 3.3V Vcc is approximately 1V. This allows a signal with a 1.8V Vih to be translated up to the Vcc level of 3.3V. See example in figure 3.
Up translation possibilities with SN74LV1T family:
Figure 2: Switching threshold with 3.3V Vcc
In battery-powered devices where power consumption is critical, the power consumption can be higher when the input is lower than Vcc. Figure 4 shows an example of the power consumption. It’s important to consult the datasheets if power consumption is a key concern.
Figure 3: Power consumption when translating
The SN74LV1T family of devices provides a very simple way to perform a function and translate to another voltage level whether you need to translate up or down. Available in small packages, one device can replace multiple discrete components with no pull-ups required. The SN74LV1T family ultimately simplifies design, using less board space and reducing cost.
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Thank you for your post. For 1.8V to 5V translate or vise versa. Could we use two SN74LV1T in series?
You could use 2 LV1T parts to get from 1.8V to 5V but it might be easier to use an open drain part like SN74LVC1G07.
For 5V to 1.8V, one LV1t device is all you need.
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