LSF0002: LSF0002

Ian Wang

Mastermind 27755 points

Part Number: LSF0002
Other Parts Discussed in Thread: SN74AXC2T45, TXU0102

Tool/software:

Hi Sir,

customer using LSF0002 for 70MHz signal but looks waveform abnormal when f>2MHz.

Do you have idea for this? thanks.

thanks, Ian.

2 months ago

+1 Clemens Ladisch 2 months ago

Guru 282290 points

Please show the waveform.

0 Ian Wang 2 months ago in reply to Clemens Ladisch

TI__Mastermind 27755 points

Hi Neil,

could you pls help upload waveform?

thanks, Ian.

0 NEIL lee 2 months ago in reply to Ian Wang

Prodigy 40 points

When inputting 2MHz from B, A output amplitude (1.8V) is still normal, when inputting more than 2MHz from B, A output amplitude start

0 Clemens Ladisch 2 months ago in reply to NEIL lee

Guru 282290 points

Are there any long cables or traces?

For high speeds, the LSF needs strong pull-up resistors, which require a high drive strength of the other devices. What is that device that is labelled "1.1 V I/F", and what drive strength does it have?

It might be a better idea to use a buffered level shifter like the SN74AXC2T45.

0 NEIL lee 2 months ago in reply to Clemens Ladisch

Prodigy 40 points

Yes, experiment with a longer line.
How many pull-up resistors are needed?
1.1 V I/F’ is the sensor signal spi clk .
Does the SN74AXC2T45 have a faster conversion speed? We need to use one that can run 70 MHZ, can be bi-directional, and comes in a small package.

0 Clemens Ladisch 2 months ago in reply to NEIL lee

Guru 282290 points

SPI is not bidirectional. Please specify exactly what the interface is.

0 Jack Guan 2 months ago

TI__Mastermind 26781 points

Hi Neil, Ian,

The external pullups/ cap. load at the outputs of the LSF will dictate the max achievable speed with the device. It seems in your case, the cap. load is still too high for the RC time constant to fully ramp up and would further suggest using shorter cabling/ traces and retest. In any case, the signals appear to be fixed direction, which the TXU0102/ AXC2T45 is able to better support due to their buffering architecture.

See [FAQ] How can I estimate Data Rate Frequency not given for my voltage level? for estimation of data rate w/AXC2T45.

Regards,

Jack

0 NEIL lee 2 months ago in reply to Jack Guan

Prodigy 40 points

1.We need signal 1.1V conversion 1.8V, can run 70 MHz / 8bit speed. You recommend txu0102 /up to 200 mbps support and SN74AXC2T45 /up to 380 Mbps support,Can we support our needs?

2. LSF0002 , What is the length of the shortest wire it want to use? We have tried to adjust the resistance change. RA adjusted resistance 280 ohms, RB adjusted 21K ohms, can only support 5MHZ, and then adjust RA to smaller resistance RB to larger resistance.
The amplitude of the output waveform starts to change, and cannot meet the 1.8V.

0 Clemens Ladisch 2 months ago in reply to NEIL lee

Guru 282290 points

1. At 1.1 V, AXC is the fastest family. (TXV translators are faster, but require at least 1.2 V.)

2. That depends on the total capacitance. The LSF is a passive switch; I doubt that your 1.1 V device has enough drive strength to achieve 70 MHz through the LSF.

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LSF0002: LSF0002