• State
  • Locked Locked
  • Replies 1 reply
  • Subscribers 16 subscribers
  • Views 596 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

SN74CBT3245C：供电和电流问题

tao yin
Prodigy 10 points
Part Number: SN74CBT3245C
Other Parts Discussed in Thread: SN74CB3T3245

Tool/software:

背景：使用SN74CBT3245C作为电平转换电路，一端要求5V，另外一端要求3.3V，支持双向通信，5V侧通过4.7K电阻上拉到5V，3.3V侧接入FPGA控制芯片。

1、SN74CBT3245C供电VCC看规格书输出侧会比VCC小1V，因此通常使用5V接二极管后给VCC供电，目前我验证VCC=4.3V，但输出侧在3.5V左右并不是3.3V，而且输出侧悬空和接入FPGA两者值不一样，且4.7K上拉电阻上接近0.5V压降，应该是存在电流通路，请问这个电流怎么评估？VCC供电为什么要使用二极管压降减少的形式？VCC供电可使用电阻分压或者431电路吗？
2、5V输出侧悬空，为什么上拉电阻4.7K上还会存在0.5V压降，是通过输入进到了FPGA里面形成电流通路？而且我把4.7K电阻改成1k，压降会相应的减少到0.1V左右，感觉电流是差不多恒定的，这个压降不可避免？

  • 0
    TI__Mastermind 35648 points

    Hi tao,

    I'm assuming you mean the SN74CB3T3245 not the SN74CBT3245.

    You're violating the datasheet. The max supply is 3.6V for this device. Your extra supply voltage may be causing extra current to leak through so the clamping isn't happening where it's expected, so you're seeing the extra voltages. 
    The output being different when it's connected to a load or when it's floating makes sense. The load changed so the current changed so the voltage should be different. For example look at figure 5-1 through 5-3 and how VOH changes by output current. 
    As for your 0.5V drop across the 4.7k resistor, let's try reducing the supply and see if that helps. You may be having extra leakage through some diodes. I'm assuming the question here is where the extra current is going. You are correct that the current seems consistent. It's higher leakage than expected but once again, let's try using the device within the datasheet recommended range and see how the effects look.

    Thanks,
    Rami