LSF0108: PLACEMENT

KARTHIK KRISHNA JAYACHANDRAN

Intellectual 380 points

Part Number: LSF0108

1) WHY LSF DEVICE SHOULD BE PLACED AT HIGH VOLTAGE SIDE?

2) SHOULD THE PULL UP BE PLACED CLOSED TO LSF DEVICE ?

over 5 years ago

0 Karan Kotadia over 5 years ago

TI__Genius 16735 points

Hi,

I am not clear on your first question. Can you specify what you mean?

2. The LSF device is a passive translator. It doesn't have any drive strengths. The placement of the pullup resistor doesn't matter that much. It is however recommended to keep the trace lengths as short as possible. Here is a whole series on the LSF devices we have created. I recommend you watch entire series as it is very short.

https://training.ti.com/introduction-voltage-level-translation-lsf-family?context=1134826-1139264-1134789

Thanks!

-Karan

0 KARTHIK KRISHNA JAYACHANDRAN over 5 years ago in reply to Karan Kotadia

Intellectual 380 points

Actually in LSF layout guideline it was specified that "Place LSF close to high voltage side." I guess it means is that if we have a voltage transition from 1V8 to 3v3 the LSF device should be close to the 3V3 pins which requires transition. Why is that?

0 ShreyasRao over 5 years ago in reply to KARTHIK KRISHNA JAYACHANDRAN

TI__Guru 62060 points

Hi Karthik,

Placing close to the target device limits the trace length, which in turn reduces the parasitic capacitive loading on the LSF outputs. As mentioned already, the LSF is a passive translator device which depends on the external pullup resistors to achieve level translation. With higher loading, the transition time ( RC time) will be longer and will limit the max operating frequency the LSF can support. The layout section is a guideline but not a requirement.

The longer the trace length, the higher the cap loading, lower the freq support or lower resistor required for supporting the same freq.

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LSF0108: PLACEMENT