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LSF0204: Low logic level is pulling up

Hyomin Ahn
Prodigy 10 points
Part Number: LSF0204

Tool/software:

Hello,

I am using the LSF0204 component to convert 5V logic levels to 1.8V logic levels. 

I am driving the VrefB pin with the 5V output of an Arduino Nano board, and the VrefA pin with the 1.8V output of an ADP151-1.8 regulator.

I am using the same 1.8V to drive the enable pin as well. 

I noticed that when the LSF0204 is not connected to the ADP151, the 1.8V output is at the correct value.

However, when the LSF0204 is indeed connected, the VrefA pin is set to 2.3V. 

Could it be because I only have 200Ohm pull up resistors on the 5V side, and no pull-up resistors on the 1.8V side?

If that is the case, would using the nominal values of 310Ohms for the 5V side, and 97Ohms (or 100Ohms) for the 1.8v side be able to solve this?

Thank you in advance!

  • +1
    Guru 285410 points

    How are you supplying VrefA without the ADP151? I guess you are using a lab power supply that is able to sink current. In that case, a resistor between VrefA and GND with at most 100 kΩ would help. (During normal operation, a small current flows into VrefB and out of VrefA. If the power supply or other devices connected to VrefA are not able to sink this current, then the voltage at VrefA will float up.)

    You need pull-up resistors only at I/O pins that are used as outputs. Their value depends on the desired speed and the total capacitance; 200 Ω might be too low.

  • +1 in reply to Clemens Ladisch
    Prodigy 10 points

    Yes, I am able to bypass the ADP151 with jumper pins and couple a lab power supply!

    I guess it might also be a better idea to drive the VrefA with an LDO that has better current sinking capabilities then.

    Thank you so much!!!

  • 0 in reply to Hyomin Ahn
    Prodigy 10 points

    The ADP151 is able to supply 200mA of current. Would you know how much the current that flows from VrefA to VrefB would be?

  • 0 in reply to Hyomin Ahn
    TI__Mastermind 27951 points

    Hi Hyomin,

    It can be calculated through VREFB- (VREFA + VTH) / 200kohms. With the LSF0204, this 200kohm is internal to the device. 

    This set of videos also help to explain further in detail: www.ti.com/.../5614318177001

    Regards,

    Jack

  • 0 in reply to Hyomin Ahn
    Guru 285410 points

    Most power supplies are not capable of sinking current. In general, adding a resistor is much easier than finding a supply with a bidirectional output.