LSF0204: LSF0204RUT Pulling 1.8V rail to 2.3V

Camilo Garcia Trujillo

Part Number: LSF0204
Other Parts Discussed in Thread: TXU0304, TXU0104, TXU0204

Hi,

I have the circuit in the following schematic. With an FT4232 connected to the 3.3V side of the LSF0204RUT and headers on the 1.8V side. The goal for this board is to go from 3.3V logic to 1.8V. There is nothing connected to the 1.8V rail aside from the LSF0204RUT and the TLV70218.

When I don't have the LSF0204RUT in the board I measure about 1.9V out of the TLV70218.

However when I place the LSF0204RUT in the board I measure around 2.3V on the 1.8V rail and can see 2.3V logic. Is there a reason why the LSF0204RUT is pulling the 1.8V rail up?

Best

Camilo

over 2 years ago

0 Jack Guan over 2 years ago

TI__Mastermind 41146 points

Hey Camilo,

Are you able to remove the resistor R3? The EN pin is internally shorted to VREF_B through a 200k pullup, so there is no need for this additional connection. I suspect that this pullup is allowing that excess voltage to leak into A-side.

Regards,

Jack

0 Camilo Garcia Trujillo over 2 years ago in reply to Jack Guan

TI__Expert 5045 points

Hi Jack,

I have removed R3 but I still see 2.3V at the output of the LDO. Any other ideas?

Best,

Camilo

0 Jack Guan over 2 years ago in reply to Camilo Garcia Trujillo

TI__Mastermind 41146 points

Hey Camilo,

It is generally not a good idea to have LDOs as the power supply of this device as they are unable to sink much current. During normal operation, there will be a leakage current from VREFB to VREFA- if the LDO cannot sink this current, then the current will charge up (as there is nowhere else to go) which increases the voltage seen on A-side.

For more information on this leakage current path, see https://www.ti.com/video/5614318177001?cu=1134826

Regards,

Jack

0 Camilo Garcia Trujillo over 2 years ago in reply to Jack Guan

TI__Expert 5045 points

Hi Jack,

Just a couple of questions then:

So what would you recommend to power this device?
What would need to change so that the LSF doesn't pull up the VrefA supply?
Is there any other part you would recommend to use for bidirectional translation? I saw the TXU0304, which is not bidirectional but it has one channel inverted for the readback.

Best,

Camilo

0 Clemens Ladisch over 2 years ago in reply to Camilo Garcia Trujillo

Guru 317370 points

The LSF can be used with an LDO if you ensure that the lower power supply is able to sink the current flowing out of VREF_A. It is no more than 5 µA in this circuit, so a 220 kΩ resistor between VREF_A and GND will work.

As far as I know, the FT4232 does not have bidirectional signals, so the TXU0104/TXU0204/TXU0304 would be appropriate. (What are the four signals?)

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LSF0204: LSF0204RUT Pulling 1.8V rail to 2.3V