LSF0108: Logic forum

Oliver Ou

Intellectual 2845 points

Part Number: LSF0108

Hi Expert,

Customer found an issue when using LSF0108 to shift 3.3V to 1.85V. Circuit:

Circuit:

When system under S5, 1.85V will be shut down, but 3.3V won't.

So we observe that there is leak current on 1.85V when S5

According to LSF0108 design note, enable pin should tie to Vref_B which is high side.

But Can we connect Vref_B with low voltage(1.85V)? so the level shifter can be disconnected when 1.85V is shutted down.

Thanks,

over 5 years ago

0 Clemens Ladisch over 5 years ago

Guru 317280 points

Normal operation of the LSF requires that a small current flows into Vref_B and out of Vref_A.

To disable the LSF, you should pull EN low; see the Using the Enable Pin with the LSF Family video. But you could also disconnect Vref_B and EN.

0 Emrys Maier over 5 years ago

TI__Guru* 97046 points

Hi Oliver,

This isn't leakage current - it is normal operation for the device. I cannot explain the full operation of the device here, but there is a great video series that explains in detail how this device works located here:

https://training.ti.com/node/1139264?context=1139264

One of these videos explains the exact correct operation of the enable pin.

Please let me know if I can clarify anything in regards to this.

0 Servantes Lee over 5 years ago in reply to Clemens Ladisch

Prodigy 10 points

Clemens Ladisch said:

Normal operation of the LSF requires that a small current flows into Vref_B and out of Vref_A.

To disable the LSF, you should pull EN low; see the Using the Enable Pin with the LSF Family video. But you could also disconnect Vref_B and EN.

Hi Clemens,

So, it is OK to apart enable signal from Vref_B?

One more question, may I connect EN to Vref_A?

Thanks.

0 Clemens Ladisch over 5 years ago in reply to Servantes Lee

Guru 317280 points

No, the EN pin must always be connected to Vref_B.

To pull EN low, you need to use an open-drain/open-collector component. (See the video.)