Design Ideas

Two transistors form bidirectional level translator

Jim Hagerman, Nokia Mobile Phones, San Diego, CA

 You often need to convert a logic signal from one power-supply voltage to another. This conversion is a relatively simple task, unless the signal happens to be bidirectional. Serial buses such as Access.Bus, I₂C, and SBI use bidirectional data lines. Some buses may require translating logic from one voltage to another; for example, from 3 to 5V. Figure 1 shows a simple solution to the conversion problem. The input signal can come from either side. In fact, you can drive both inputs low simultaneously (as in the I₂C bus) without incurring latchup.

  This example illustrates a translation from 5 to 3V, but it can accommodate almost any other voltage levels, provided the logic-low levels are equal (usually 0V). Translations from 1 to 100V are possible, albeit rather slow. The key to this circuit is the unusual cross-coupling of the emitters to the collectors. When you drive one input terminal low, the opposite transistor acts as a common-base amplifier that saturates, pulling the other terminal low to within one V^CE(SAT). This action also pulls the emitter of the input transistor low, to within one V^CE(SAT) of its collector, thereby cutting off collector current (although base current flows) and preventing latchup.

  It is important that the base resistors R² both connect to the lower voltage supply. Tests show that the circuit operates easily up to 300 kHz. The major impediment to fast operation is the delay that occurs when Q¹ comes out of saturation on a rising edge. The PSpice simulation in Figure 2 shows the glitch this delay causes. (DI #1944)

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