LSF0108: How to use

Hidekazu Someno

Part Number: LSF0108

Hi Tean,

My customer has the following questions and they requests the answers.

Would you please check them from you?

Customer is considering the signal interface between 3.3V CPU and 1.8V communication device and checking Figure-6 on datasheet.

SDIO interface/ 25MHz CLK

Q1: On the Figure-6 on datasheet, Rpu(pullup resisters) has illustrated on both 3.3V and 1.8V sides. Are the pull-up resisters needed on both sides?

Q2: There is the description that the pull-up resisters must limit the current flowing through PASS-SW to <15mA. But according to Table-5, the pull-up resister for three current values(15m, 5m and 3m) is been showing.

-a): To keep pass current <15mA is only limitation. Is this understanding correct?

-b): Is there any limitation about minimum current flowing through PASS-SW? If so, How much current is it?

-c): If the pull up resisters on both 3.3V and 1.8V sides are needed, is it OK to use 10k for all pulup resisters on both sides?

Q3: Is it OK to connect the 200k resister on VREF_B pin directly to Vpu(3.3v)?

Q4: How is the I/O pins(Ax & Bx) of unused channels handled?

Q5: How is the bidirectional signal operation achieved on this Figure-6 configuration?

Q6: Is there any concern when used at 25MHz signal speed?

Regards,

H.Someno

over 5 years ago

0 Clemens Ladisch over 5 years ago

Guru 326380 points

See [FAQ] How do the LSF translators work? and Translate Voltages for SDIO.

1. Pull-up resistors are not needed on pins that are always inputs driven by push/pull outputs. See the FAQ in Voltage-Level Translation With the LSF Family.

2. Keeping the current below 15 mA limits the voltage drop over the pass transistor. But the external device must be able to sink the current through both pull-up resistors, and you might want to save power. Making the pull-resistor higher lowers the speed that you can achieve. (The actual speed limit depends on the parasitic capacitances on your board.) Also see [FAQ] How do I size pull-up or pull-down resistors?

3. The resistor must be connected to a voltage that is higher than Vref_A. This might or might not be the same voltage used by the signals.

4. See [FAQ] What should be done with unused I/O pins of the level translator devices?

5. The LSF is a passive switch. The transistor switches on when the I/O pin on either side is pulled low by an external device.

6. The LSF can support 25 MHz, if the pull-up resistors are sized right.

0 Emrys Maier over 5 years ago in reply to Clemens Ladisch

TI__Guru* 97676 points

Hi Someno-san,

I would recommend to start with this short video series: The Logic Minute: Understanding the LSF Family of Bidirectional, Multi-Voltage Level Translators

It's also linked in the FAQ Clemens provided, but there's so many sources of information, I thought that having a starting point would be helpful for you.

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LSF0108: How to use