AN883 Datasheet, PDF (2/4 Pages) Silicon Laboratories – LOW-COST IC LEVEL TRANSLATOR

AN883
For the SCL line, a Shottky diode is used to prevent the SCL line from pulling all the way high on the lower voltage
device while allowing it to pull all the way high on the higher voltage device. The voltage drop across the Schottky
diode must be low enough so that when the higher-voltage device pulls low, there is still a logic low on the lower-
voltage device. For the Si7034 used in this case, the logic low level is 0.54 V (0.3 x VDD), and the Schottky diode
drop is about 0.35 V, so there is no issue. If the lower voltage device was on a 3.3 V rail, an ordinary diode could be
used.
The SDA line is bidirectional, and Q2 is added so that when the lower voltage device pulls down on SDA, SDA of
the higher voltage device will pull down. In a case where clock stretching needs to be supported, SCL can be made
bidirectional by adding the same transistor circuit across D1.
The emitter follower circuit is a low-cost method for providing a 1.8 V rail from the higher 5 V supply. Alternatively,
this circuit can be replaced with an LDO for a simpler layout or if additional voltage regulation is required.
3. Waveforms
The circuit was tested for the case of 3.3 to 1.8 V level translation. Figure 2 shows the waveforms on the SDA line
with a clean translation from 3.3 V (blue trace) to 1.8 V (yellow trace).
Figure 2. SDA Line at 1 V per Division
Zooming in to 200 mV per division, the circuit operation can be clearly seen in Figure 3.
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Rev 0.1