• State TI Thinks Resolved
  • Locked Locked
  • Replies 9 replies
  • Answers 4 answers
  • Subscribers 18 subscribers
  • Views 2543 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LSF0102: Translator power consumption

jordan hill
Prodigy 65 points
Part Number: LSF0102
Other Parts Discussed in Thread: SN74AUP1G34, SN74AUP1T34, SN74AVC2T244

Hi,

I'm trying to figure out the power consumption of the LSF0102DQER level translator. The picture below states ICC is 1uA, is this for both power supplies? Does this number increase with the number of data channels being used?i'm using 2( RX and TX). Does this change if VCCA or VCCB change? im using 1.8v to 3.3.  what is the quiescent current?

Thank you!

  • 0
    TI__Guru* 96591 points
    Hi Jordan,
    The LSF family of translators are pretty unique in how they operate. They are completely passive - ie there aren't any CMOS gates inside the device drawing power.

    The majority of power consumption from the use of this device will be from the pull-up resistors, but there is a small current sourcing from the B supply and sinking into the A supply. This current is determine by the supplies and external resistor value (which should be 200kohm). here's the equation to determine the mentioned supply current:

    Icc = (VB - 0.85 - VA)/200000

    For your case, the value turns out to be ~3.25uA
  • 0 in reply to Emrys Maier
    Prodigy 65 points

    Hi,

    I still have a couple questions. Since i'm using this on UART lines which are push-pull outputs do i still need this 200k pull up? where is the .85 in that equation come from? and is this the working current or quiescent current? also a question on the EN pin, the datasheet says it must be low during power up and power down, is a transistor with a pull down resistor recommended to achieve this? 

    Thank you,

    Jordan

  • 0 in reply to jordan hill
    TI__Guru* 96591 points
    Hi Jordan,
    I have a video series that should be coming out any day now ( just waiting on some red tape ) that explains the LSF family in detail.

    For now, I can answer these two questions directly - the 0.85 comes from the turn-on voltage of the diode-connected FET inside the device.

    The 200kohm resistor is not a pull-up - it is part of the bias circuit, and is essential to how the device operates. Check out the application diagram in any of the LSF datasheets to see how it's supposed to be connected.

    The EN pin must be left floating during operation, but can be driven low to force a high-impedance state on the device. To do this during power-up, an n-channel MOSFET can be connected from the Vref_B / EN node to ground, and then the input to that FET can be held high with a large pull-up.

    These devices are rather hard to explain via text - which is why we've created the aforementioned video series. I will make a note to post the links as soon as they are available.
  • 0 in reply to Emrys Maier
    Prodigy 65 points

    Hi,

    thank you for the explanation and i will wait for the video to get a better understanding. but  for now here is the basic circuit im going to use it in, can you confirm if this is the right configuration please.

  • 0 in reply to jordan hill
    TI__Guru* 96591 points
    Thanks for the schematic, that helps to clear things up.

    The bias circuit looks correct, and the enable pin is being used properly.

    I'm assuming you are just labeling the lines for me with RX/TX and they aren't actually tied together

    The up-translation will need to have a pull-up resistor on the B side pin (B2, if I understand this schematic correctly).
  • 0 in reply to Emrys Maier
    Prodigy 65 points

    HI,

    you are correct on the RX/TX lines, so only one pull up resistor is needed? my only concern is i need ultra low power and the data sheet recommends sinking 3mA lowest? if im 115200 baud rate can i use a 1 or 2MegaOhm pullup?

  • 0 in reply to jordan hill
    TI__Guru* 96591 points
    If you want to do voltage translation with low power, I would recommend a completely different setup.

    You can use two active translation devices, one for up-translation, and one for down-translation.

    Down translation is easy - just use an SN74AUP1G34 and power it from 1.8V. The inputs are over-voltage tolerant and will support the 3.3V signals without any problem.

    Up translation can be done with the SN74AVC2T244 (which has an output enable but is dual channel), or the SN74AUP1T34, which is single channel but has no output enable.
  • 0
    TI__Guru* 96591 points

    I'm happy to announce that we have officially released a series of 8 videos that explain how to use the LSF family of translators in detail. These videos are part of a new training series called "The Logic Minute" and are available here:

    The Logic Minute

    Please let me know if I can be of further assistance.

  • 0 in reply to Emrys Maier
    TI__Guru* 96591 points
    I updated the link above. It turns out that the link I originally posted was for internal use only - sorry about that!