I need to drive the EN pin of a TLV76750DRVR ( https://www.ti.com/lit/ds/symlink/tlv767.pdf ) from a SAMD21G18 3.3v CPU.

Now, if I understand correctly the TLV767 has it's EN pin internally pulled up to IN (In my case, 2S li-ion cells, max 8.4V).
Because of this pullup I can't connect the EN pin to my CPU because the voltage will exceed 3.3v on that pin.

What would be the correct way to connect this? Can I do this with a mosfet like this?

 

Just drive it directly from the processor. The pull-up current is 400nA. The processor's protection network will deal with it.

 

Oh... I didn't think about it that way. You're sure it won't hurt the processor? The current might be low but the fact that there's a pullup between EN and IN (too high voltage) made me think connecting it to the processor was a bad idea

 

See section 37.10 of the datasheet "Injection current". The limit is 1mA, 2500 times what you are giving it.

 

I'm trying to understand this section. On page 876 (section 37.10) there are conditions for Vpin and they are below 4.2V.
They also state "If Vpin is greater than VDD+0.6V, a current limiting resistor is required."

So as long as this current limiting resistor ensures the current is below 1mA this is fine for the SAMD21? And because the TLV767 has a 400nA limit it's also fine to connect it to the SAMD21.

 

I'm trying to understand this section. On page 876 (section 37.10) there are conditions for Vpin and they are below 4.2V.
They also state "If Vpin is greater than VDD+0.6V, a current limiting resistor is required."

So as long as this current limiting resistor ensures the current is below 1mA this is fine for the SAMD21? And because the TLV767 has a 400nA limit it's also fine to connect it to the SAMD21.

That's exactly what I think it means.

 

Thanks. I do hope we're understanding this correctly. It still feels strange to have >3.3V on the pin and have the processor deal with it. I will try it on a breadboard first before sending it out for production.

Now, if I do want to add a mosfet. How would I do that? In the attachment in my first post I added the FET between EN and GND but since the EN pullup is only 400nA it will probably not work at all...

 

Thanks. I do hope we're understanding this correctly. It still feels strange to have >3.3V on the pin and have the processor deal with it. I will try it on a breadboard first before sending it out for production.

Now, if I do want to add a mosfet. How would I do that? In the attachment in my first post I added the FET between EN and GND but since the EN pullup is only 400nA it will probably not work at all...

Your original circuit will work.

 

What are the logic thresholds of the TLV76750DRVR at 8.4V? It is possible 3.3V would be seen as a low, or, more likely, an invalid input. If so, you can get around it by making the output open drain and only driving the output low, not high.

 

What are the logic thresholds of the TLV76750DRVR at 8.4V? It is possible 3.3V would be seen as a low, or, more likely, an invalid input. If so, you can get around it by making the output open drain and only driving the output low, not high.

The TLV767 datasheet says:

VEN (HIGH) Enable pin logic high:
Test conditions: 2.5 V ≤ VIN ≤ 16 V
Min: 1.2V

VEN (LOW) Enable pin logic low:
Test conditions: 2.5 V ≤ VIN ≤ 16 V
Max: 0.4V

So 3.3V should be seen as a high?

 

You also said I could make the output open drain, only driving it low, not high. But isn't that what I'll be doing already in the schematic in the first post?

 

So 3.3V should be seen as a high?

Yes, as the minimum Enable high (enabled) voltage is 1.2V.
No pullup needed.

 

You also said I could make the output open drain, only driving it low, not high. But isn't that what I'll be doing already in the schematic in the first post?

Yes, but you don’t need the external transistor. You simply make the micro pin an input when you want a high and an output when you want a low.

But, the logic thresholds of your chip make a 3.3V high fine, so just drive it directly with an output pin.

 

Ok. So i connect the output pin directly to the tlv767 and just toggle the output pin between input and output?
But setting it high/low will work too?

 

Ok. So i connect the output pin directly to the tlv767 and just toggle the output pin between input and output?
But setting it high/low will work too?

You can do it that way, but setting it as an output and changing it from high to low is just as effective, and more intuitive when you look at the software in 3 years' time.

 

I ran out of pins so I added an MCP23008 I2C expander to control the TLV767 (I have multiple, for multiple different voltages).
Can I connect the output pin of an MCP23008 to the EN pin of the TLV767 too?
The same way it was suggested above (connecting the SAMD21 pin to the TLV767)?

I had some PCB's made like this but when I set the MCP23008 pin to LOW, then even all serial communication with my samd21 is lost until I powercycle the whole board.

I'm not sure what's happening here...