Good Bidirectional Level Translator for a wide voltage range - Page 1
Forum section: electronics / projects

author: grimreeper132
author_rank: Newbie
timestamp: November 27, 2023, 10:59:10 pm
content: I am wanting to connect to a I2C signal, which could be operating up to 1 MHz using a 3V3 MCU. However, the I2C voltage could either running as a 5V, 3V3 or 1V2 signal. What would be the easist way to translate the I2C signal across from the input to the MCU. As all of the ICs with an auto-direction functionality I can find only work when VCCA <= VCCB, however, in my case VCCA could be both bigger or smaller than VCCB.I am guessing there is an IC which I can't find or another clever way to do this, that I haven't found, but currently my two best methods I can currently find/think of is either1) Using 2 level translators, one to translate the I2C signal to 5V, then one to translate it to 3V3. Which doubles the cost and size of this section of the design.2) Using a level translator which can go both up and down, however requires a direction control. Which again is not ideal from a control point of view.Thank you

author: Berni
author_rank: Super Contributor
timestamp: November 29, 2023, 07:08:05 am
content: Where did you find 1.2V logic level I2C peripherals? I never seen one before.Id say the easiest way to do it is use the mosfet based I2C level shifter circuit and use a microcontroller with 5V tolerant IO pins.

author: ArdWar
author_rank: Frequent Contributor
timestamp: November 29, 2023, 09:07:40 am
content: And for that matter do you really need 5V?Most newer 5V component either can be powered down to 3v3 by design, compatible with 5V TTL signaling which implicitly compatible with 3v3 CMOS signaling, or outright compatible with SMBus which is compatible with 3v3 CMOS.

author: mikerj
author_rank: Super Contributor
timestamp: November 29, 2023, 11:59:54 am
content: https://www.nxp.com/products/interfaces/ic-spi-i3c-interface-devices/voltage-level-translators/dual-bidirectional-ic-bus-and-smbus-voltage-level-translator:PCA9306

author: colorado.rob
author_rank: Frequent Contributor
timestamp: November 29, 2023, 01:20:07 pm
content: Quote from: Berni on November 29, 2023, 07:08:05 amThis is a common IO voltage used on FPGAs.

author: Zero999
author_rank: Super Contributor
timestamp: November 29, 2023, 01:58:09 pm
content: You could try a BJT. 1MHz might be pushing it a bit though.EDIT:I had a play with LTSpice.

author: Peabody
author_rank: Super Contributor
timestamp: November 29, 2023, 04:02:47 pm
content: Wouldn't something like the TCA9548A I2C multiplexer/switch do the level translating automatically? Each device would have pullup resistors to its own Vcc. I don't know about 1.2V though. Never seen that. But you would have to power the TCA9548A with the highest voltage you will be dealing with.

author: Sacodepatatas
author_rank: Regular Contributor
timestamp: November 30, 2023, 03:48:10 am
content: Quote from: Zero999 on November 29, 2023, 01:58:09 pmTry the SS8050D. It has a reverse HFE of about 50. The PNP version, SS8550D, has a reverse HFE of more than 150 (reverse HFE is the gain when the Collector acts as Emitter and vice-versa). Transistors with both high direct and reverse current gains are good candidates for bidireccional translators.https://youtu.be/h2xOw7Lc2Vw?si=9VNoJOJP1HtX7HI7

author: Berni
author_rank: Super Contributor
timestamp: November 30, 2023, 07:50:42 am
content: Ah neat trick.I didn't know they made BJTs specifically for this kind of naughty reverse operation.

author: Zero999
author_rank: Super Contributor
timestamp: November 30, 2023, 05:49:37 pm
content: Quote from: Sacodepatatas on November 30, 2023, 03:48:10 amGood find regarding the SS8050D, but this circuit doesn't need a high reverse hFE. R2 allows more than sufficient base current to turn on Q1 in reverse. When IO2 is low, IB= (1.2 - 0.7)/4k7 = 110µA and IE= 1.2/4k7 = 260µA. Most jellybean transistors have a reverse hFEof more than 2.6. A larger transistor might not be able to switch fast enough and will have higher parasitic capacitances which will take longer to charge, than the small signal BC547.