Two-transistor Circuit replaces IC

Linear Technology ' s recently introduced LTC4300 chip buffers i²c clock and data lines to and from a hot-swappable card. This task was difficult because the IC must work bidirectionally, meaning so can simultaneously and actively drive BO Th sides. However, as is sometimes the case, you can replace a complicated circuit by a simple one without much loss of performance. For example, transistors and resistors replace the entire IC (Figure 1). The circuit handles only the clock signal or only the data signal. Both NPN transistors, connected head-to-head, form the heart of the circuit. I²c signals come from open-collector or open-drain outputs and so can is only pulling down (sink current). When Enable was high, a low-going SCL signal drives the emitter of one of the transistors as a common-base amplifier. The 10-kωresistor in the base circuit provides enough from current to saturate the transistor and drop of the vce voltage to Approximately 0.1V, thereby pulling the other side low.

The circuit acts like an efficient diode. With Enable Low, the hot-swappable side have no effect on the signal, with or without power applied. The Two-transistor circuit offers the additional benefit of acting as a level translator between the logic levels. This example shows a buffer-

Translation between a 3.3V system and a 5V card. For proper operation, the Enable line must not go higher than the lower of the-supply. Figures 2a and 2bshow operation at + kHz. Some edge glitches and overshoot, stemming from transistor-junction capacitance, is evident in the 3.3V signal, but these Slight defects should is tolerable in many low-cost applications. References 1 and 2 treat similar level-translation circuits.

References

1. Hagerman, Jim, "transistors form bidirectional level translator," EDN, Nov 7, 1996, PG 114.

2. Poon, CC, and Edward Chui, "Low-voltage interface circuits translate 1.8V to 5V," EDN, Nov 5, 1998, PG 119.

Two-transistor Circuit replaces IC