rev 0.1 1/15 copyright ? 2015 by silicon laboratories AN883 AN883 l ow -c ost i 2 c l evel t ranslator 1. introduction this applications note discusses a low-cost circuit for i 2 c level translation. this ci rcuit was developed for the si701x, si702x, and si703x humidity sens ors but will work in many applications. this circuit provides i 2 c level translation from a higher voltage supply, such as 5 v, to a lower voltage, supply such as 1.8 or 3.3 v. in addition, the option al emitter follower circuit provides a low-voltage power supply rail from the higher 5 v supply. note that some devices allow for higher voltage tolerance on i 2 c inputs. for example, the si7034 has a 3.3 v tolerant i 2 c interface, so the level transl ation is only required for 5 v i 2 c designs. 2. circuit the basic circuit is shown in figure 1. figure 1. low-cost i 2 c level translator circuit in this case, we are translating from 5 v to 1.8 v, and the i 2 c speed is assumed to be sufficiently slow for clock stretching to not occur (i.e. less th an 300 khz for the si701x, si702x, and si703x humidity sensors). in this case, the scl line is unidirectional. optionally replace with ldo 5v 1.8v 1.8v u3 gnd 1 vcc 4 scl 3 sda 2 r4 4.7k r7 6.65k d2 bas40 q3 mmbt3904 d1 bas40 r2 4.7k r1 4.7k q2 mmbt3904 r8 10k d3 bas16x c1 0.1uf r5 10k u1 si7034 sda 4 vss 5 nc 3 nc 6 vdd 1 scl 2 r6 10k r3 4.7k
AN883 2 rev 0.1 for the scl line, a shottky diode is used to prevent th e scl line from pulling all the way high on the lower voltage device while allowing it to pull all th e 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 logi c 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 provid ing a 1.8 v rail from the higher 5 v supply. alternatively, this circuit can be replaced with an ldo for a simpler la yout or if additional voltage regulation is required. 3. waveforms the circuit was tested for the case of 3.3 to 1.8 v leve l 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.
AN883 rev 0.1 3 figure 3. sda line at 200 mv per division when the 3.3 v side (blue line) goes all the way low, the 1.8 v side (yellow line) goes to about 350 mv, which is the schottky diode drop. when the 1.8 v side (yellow line) go es all the way low, the 3.3 v side goes to about 150 mv, which is the saturation voltage of the transistor. 4. conclusion the low-cost translator circuit allows translation from hi gher voltages, such as 3.3 or 5 v, to lower voltages, such as 1.8 or 3.3 v. for the case where scl is unidirectional , only 2 diodes, 3 resistors, and one transistor are needed. in high volume manufacture, the cost of these components would be well under $0.05.
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