LSF0108: Power Sequence and Enable

Onur Kusakoglu

Part Number: LSF0108

Good morning,

I am using LSF0108 chips in my design mainly for receiving 5V signals and converting it to 3V3.

I did the schematic entry based on the datasheet info. VREFB is 5V and VREFA is 3.3 and 200 ohm resistors are connected to VREB input and enable signal.

My question here is, 3V3 is enabled first and 5V is enabled after 3V3. Before the chip is enabled VREFA is applied to component.

I couldn't find anything opposed to this in data sheet but I wanted to make sure if this approach is true.

I attached the screenshot from my circuitry

Best Regards

over 4 years ago

0 Clemens Ladisch over 4 years ago

Guru 316690 points

Powering up Vref_A first is safe. Currents will flow only when Vref_B > Vref_A.

You must not use a pull-down resistor on Vref_B.

0 Emrys Maier over 4 years ago

TI__Guru* 97046 points

Hi Onur,

This short video explains the bias circuit in detail: Understanding the Bias Circuit for the LSF Family

And this one covers the use of the enable pin: Using the Enable Pin with the LSF Family

Please review both (total it's only about 5 minutes), then let me know if you have any questions.

0 Onur Kusakoglu over 4 years ago in reply to Clemens Ladisch

Intellectual 965 points

Good morning,

Thanks for the information,

In datasheet when I check the application operating condition(Table-3), I see VrefB can go up to 5.5V. In my application it is going to be a constant 5.1V which is in the spec.

On the other hand, on chapter 6.3- For both VIO Input/Output Voltage and VREF_A/B/EN, max V range is stated as 5V.

In my application I will have VrefB at 5.1V level and VrefA at 3.3V level. I expect the IO voltages are also at same voltage level with Vref voltages. VrefB IOs are all Input to the levelshifter.

Would 5.1V be a problem?

Best Regards

Onur

0 Clemens Ladisch over 4 years ago in reply to Onur Kusakoglu

Guru 316690 points

The electrical characteristics are tested with 5.5 V. The 5 V specification in section 6.3 appears to be a mistake.

This device was designed for 5 V, but 5.5 V is the worst case of a nominal 5 V supply with ±10 % tolerances, so that range must be included. Your 5.1 V supply is safe as long as its actual voltage stays below 5.5 V.

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LSF0108: Power Sequence and Enable