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SN74LVC1T45-Q1: Output voltage error when input floating

Jingyuan Zhao1
Prodigy 440 points
Part Number: SN74LVC1T45-Q1
Other Parts Discussed in Thread: SN74LVC1T45, SN74LVC1G126-Q1

Dear team,

My customer is using SN74LVC1T45-Q1 as level-shifter for SPI signal between high-side driver and MCU in body control module as illustrated below:

The problem is, when high-side driver 1 & high-side driver 2 both powered, SN74LVC1T45-Q1  can output 3.3V signal correctly; when only high-side driver 2 powered, the SPI SO signal level drop to ~2V.

Based on my understanding, input of CMOS LVC series can't float. So when high-side driver 1 shutdown, input of SN74LVC1T45 floats which results in CMOS output stage in SN74LVC1T45 conducts at certain level and interfere with the 3.3V voltage. 

So my questions are as follow:

  • Are my understanding correct? Is this the root cause of wrong output voltage?
  • What my customer wants is high-Z output when input floats. Can we achieve this with SN74LVC1T45?
  • If not, what's the alternative auto-grade part?

Thanks a lot!

Best Regards,

Charles

  • 0
    TI__Mastermind 45915 points

    Hey Jingyuan,

    If the input is floating then the voltage can drift pass the switch threshold causing the output to switch. You can use the SN74LVC1G126-Q1 which has an output enable. If you connect that OE input through a pull-up to the supply of driver 1, it should disable when that supply is powered down.

    With its over-voltage tolerant inputs, it can handle the 5 V input when Vcc is 3.3 V allowing you to level shift down.