LSF0108-Q1: Vref_A is open

Tomoaki Yoshida91

Genius 5255 points

Part Number: LSF0108-Q1
Other Parts Discussed in Thread: SN74LVC1G07

Hi team,

If the low-side A power supply terminal is open, is the internal reference MOS gate fixed to the high-side B voltage?

The general connection is as follows.

I think that Vccb voltage appears at the Vref_A pin when Vref_A is open.

In that case, the switch gate voltage is Vccb, SW is always ON, and A1 and B1 are short-circuited.

If there is a internal pull-down resistor on the EN pin, this can be prevented.

Is there a internal pull-down resistor on the EN pin?

Best regards,

Tomoaki Yoshida

over 5 years ago

0 Clemens Ladisch over 5 years ago

Guru 285550 points

There is no pulldown on EN; this would conflict with the external pullup.

The switch is open only if A1 or B1 is below the gate voltage. Does this actually happen?

The LSF is not designed for partial power-down applications.

0 Emrys Maier over 5 years ago

TI__Guru* 96862 points

Hello Yoshida-san,

You are correct that if Vref_A is left open that the EN pin will be at 3.3V (in the above image). This will allow signals to pass from A1 to B1 or from B1 to A1, however the voltage will be clamped to ~2.5V (3.3V - 0.8V).

Clemens is correct - there is no internal pull-down resistor. You can setup an external circuit to disable the LSF device when the VrefA supply is disconnected:

* Screenshot from Using the Enable Pin with the LSF Family video.

An open-drain buffer such as SN74LVC1G07 can be used to force the EN pin to 0V if the VCCA pin is disconnected. Basically the input to the 1G07 would be coming directly from the Vref_A pin, and a pull-down resistor would have to be added at Vref_A (2.2kohm should work well).

0 Tomoaki Yoshida91 over 5 years ago in reply to Emrys Maier

Genius 5255 points

Hi Emrys-san,

Thank you for your support.

I understand well.

I will consider countermeasure.

Best regards,

Tomaki Yoshida

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LSF0108-Q1: Vref_A is open