Good designs need the right translation


Translators are becoming indispensable devices on any board to connect two systems working at different voltage levels. The translation solution’s input threshold (Vil, Vih) should be within the driver’s output level (Vol, Voh) and similarly, the output from the solution (Vol, Voh) should be within the valid input threshold of the receiver (Vil, Vih).

Designers can achieve voltage translation with single-supply translators or dual-supply translators.

Single-supply translator examples include the SN74AUP1T and SN74LV1T.

Figure 1: Functional block diagram for single-supply translators

These devices have single-supply voltage, which basically tracks the peripheral supply voltage as shown in Figure 1. The input ports are overvoltage-tolerant, which allows them to act as down-translators. Up-translation happens with the compatible Vih - Vil levels appropriate to Vcc. The swing of the input signal must pass above Vih and below Vil for up-translation to occur.

The advanced ultra-low-power CMOS (AUP) family is optimized for low power consumption in the order of 0.5uA, while the SN74LVT family of devices has a wide operating range from 1.8V to 5.5V. Open-drain devices allow flexibility of translating to a voltage determined by pull-up resistor but have issues of constant current flow when output voltage is low.

There is always an issue of higher current consumption while translating using the single-supply translators, which becomes critical while operating in power-sensitive applications, as shown in Figure 2.

Figure 2: Power-consumption analysis

The delta Icc spec in the data sheet is the difference in the current consumption for each input, which is not at one of the rails. The delta Icc spec is highest, in order of mA, when Vin is approximately at midpoint of Vcc.  

Dual-supply translator devices basically have two separate supplies: one that tracks the input port and one that tracks the peripheral with the desired voltage translation, as shown in Figure 3.

Figure 3: Dual-supply translators with VCCA and VCCB

Dual-supply translators are available in different classes: bidirectional (LSF, Gunning Transistor Logic [GTL]), auto-direction sensing (TXB, TXS) and direction-configurable (AVCT, LVCT).

A bidirectional translator allows translation on either of its two ports acting as inputs or outputs. At the heart of the LSF is a passive field-effect transistor (FET) switch, which uses external pull-up resistors to translate between any two voltage levels. LSF is suited for both high-speed open-drain and push-pull applications, with an overall range from 1V to 5V without the need for a direction terminal. The GTL family of devices is suited for low-voltage applications, translating from 3.6V low-voltage transistor-transistor logic (LVTTL) to GTL logic.

Auto-direction-sensing translators eliminate the need to have a separate input for direction control, easing software development costs and related synchronization issues.

While TXB010x, and TXB030x devices are intended for push-pull applications with fast edge rates, the TXS010x/-E is intended for open-drain applications. Both the TXB010x and TXS010x/-E families of devices have partial-power-down Ioff feature. Figure 4 demonstrates TXB010x /TXS010x device family translation capability against the bit width offered in TI’s portfolio

Figure 4: Auto-direction-sensing translator analysis

Direction-controlled translators come with a separate direction-control pin that will determine the direction of communication. Whenever the data transmission is bidirectional, there could be potential bus contention, which is eliminated using the direction control of these devices. Figure 5 compares the bit width offered vs the translation voltages. AVC1T45 devices operate in the 1.2V to 3.6V range, whereas LVC1T45 devices operate in a wider range from 1.65V to 5.5V. Since Ioff is a common feature of these devices, they offer Vcc isolation wherein the ports are in High-impedance mode when either of the Vcc is at ground potential.

Figure 5: Configurable direction-control translator analysis

Unidirectional translators like the AVC2T245 have dual supplies tracking two separate voltages configurable from 0.9V to 3.6V, suitable for low-powered battery usage. There are TI’s cross-bar technology (CBT) and translation voltage clamp (TVC) families that are used in specific switching applications given their down-translation capabilities.

A mismatched voltage levels between devices can create issues that can be mitigated by the use of single-supply or dual-supply translators. Have you had to use translation recently? If so, how did you use voltage translation in your design? Will you be using the same translation methodology in your next application after reading all the choices offered?

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