LSF0108: EN pin usage

Ariel Pickholz26

Intellectual 1295 points

Part Number: LSF0108

Hi Team ,

My AA large customer is in a progressive layout phase right now with the LSF0108.

The design team wishes to control the EN pin dynamically by setting it to GND using a pull

down resistor and while they need to use it they will enable it.

The application notes , D/S and EVM are always showing

a 200K PU to the higher voltage rail (Rail B).

1. Can they use this EN pin dynamically ?

2. Do they need the PU at all if they are driving this pin from another source when

it is required ? (any special voltage limits required or slew rate or else)

3. What should be the typical PD resistor if the above use case is OK ?

Please advise urgently ,

Arik.P.

over 5 years ago

0 Karan Kotadia over 5 years ago

TI__Genius 16725 points

Hello,

Please watch the following LSF training series and let me know if you still have any questions:
training.ti.com/TLM-LSF-Enable

1. Yes it is possible to use an open drain driver without a pullup to drive the LSF enable pin. Please watch the video for proper use.

2. Yes a pull up of 200k on VCCB is required as it will limit the current flowing from VCCB to VCCA. If you watch the training bias circuit video it will explain the circuit of the LSF and the need for the pullup.

3. I am not clearly understanding the dynamic use case with a pull down resistor. If you can provide a schematic I will be able to help.

Thanks!
-Karan

0 Ariel Pickholz26 over 5 years ago in reply to Karan Kotadia

TI__Intellectual 1295 points

Hi Karan ,

Thank you for the usfull data. Still teh csutoemr asks me the following :

Is the EN pin can be controlled with full drive and not open drain?
(We would want the LSF01 to be disabled at power on if we implement the EN as open drain it will be enabled at power on)

Please advise ,

Arik.P.

0 Emrys Maier over 5 years ago in reply to Ariel Pickholz26

TI__Guru* 96591 points

No, the EN pin cannot be driven high. That will ruin the bias of the LSF and the device will no longer translate.

The videos Karan referred to explain this in detail.

0 Thanh Dang Nguyen over 5 years ago in reply to Emrys Maier

Prodigy 60 points

Hello,

I have the same problem with VREB and EN pin.

Are the VREB and EN pin can be controlled with MCU's output pin?

Below figure is my shematic, the MCU side (B side) is 3.3V and the A side is 1.8V

0 Thanh Dang Nguyen over 5 years ago in reply to Emrys Maier

Prodigy 60 points

Hello,

I have the same problem with VREB and EN pin.

Are the VREB and EN pin can be controlled with MCU's output pin?

Below figure is my shematic, the MCU side (B side) is 3.3V and the A side is 1.8V

0 Karan Kotadia over 5 years ago in reply to Thanh Dang Nguyen

TI__Genius 16725 points

Hi Thanh,

Please watch the following training series on the LSF family to understand why it is not possible to drive the EN pin with a push pull device.
training.ti.com/TLM-LSF-Enable

For more questions please start a new E2E post.

Thanks!
-Karan

Logic

Logic forum

LSF0108: EN pin usage