LSF0204: LT not able to translate from 1.8V to 5V at frequencies above 5MHz

Elbin Thomas

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

Tool/software:

We are using LSF0204RUTR for our design we are able to communicate to CAN IC upto 5MHz via SPI if i go higher than 5Mhz(to10Mhz), i am not getting any response from the CAN IC.

When we probed the SPI lines the 5V side is reaching up to 2.4V only.

the max current sink capability of my processor is 16mA so i can't reduce the pull up values very low, current value is 2.2Kohm.

Could anyone help us to proceed further?

2 months ago

+1 Clemens Ladisch 2 months ago

Guru 295220 points

The LSF itself is just a passive switch; the speed of rising edges at the output is limited by the low-pass filter formed by the pull-up resistor and the circuit's capacitance (device and trace capacitances).

The LSF is optimized for bidirectional open-drain signals. For SPI signals, which are unidirectional, the TXU0304 would be a better solution.

0 Jack Guan 2 months ago

TI__Mastermind 30596 points

Hi Elbin,

I agree with Clemens' feedback here. Please see app note for additional recommendations across various serial interfaces.

Regards,

Jack

0 Elbin Thomas 2 months ago in reply to Clemens Ladisch

Prodigy 10 points

Hi Clemens, is there any way to make it work, we need SPI running at 16MHz.

0 Clemens Ladisch 2 months ago in reply to Elbin Thomas

Guru 295220 points

To make it work, use the TXU0304.

With the LSF0204, you'd have to use stronger pull-up resistors. But I doubt that your processor can sink 16 mA with a 1.8 V supply.

0 Elbin Thomas 2 months ago in reply to Clemens Ladisch

Prodigy 10 points

Hi Clemens, we have changed the pull up to 1K (attached the waveform)- -Frequency is now 10MHz.

In LT datasheet it's mentioning the SPI interface also, what will be the max supported SPI speed.

0 Clemens Ladisch 2 months ago in reply to Elbin Thomas

Guru 295220 points

The maximum speed depends on the R and C. The C is a property of your board; the R is limited by the drive strength of your devices.

You would not have this problem with a buffered translator like the TXU0304.

0 Jack Guan 2 months ago in reply to Elbin Thomas

TI__Mastermind 30596 points

Hi Elbin,

The LSF relies on external RC components to make up the transition edges which will further determine max data rate. It seems in your application that the Cload is too high and overall limiting the bandwidth of your SPI signals. I would recommend using a buffered translator (TXU0304), similar to what Clemens' has suggested.

Regards,

Jack

0 Elbin Thomas 2 months ago in reply to Clemens Ladisch

Prodigy 10 points

Hi Clemens, got response from CAN after changing to 1k (for 16MHz)

Thanks.

0 Elbin Thomas 2 months ago in reply to Jack Guan

Prodigy 10 points

Hi Jack, next time on will choose active LT for SPI applications,

Thanks

+1 Jack Guan 2 months ago in reply to Elbin Thomas

TI__Mastermind 30596 points

Hi Elbin,

Please let us know if any additional questions arise. I will close this thread now.

Regards,

Jack

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LSF0204: LT not able to translate from 1.8V to 5V at frequencies above 5MHz