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SN74ALVC244: Current consumption of the chip and decoupling capacitors

Justin Kiln
Prodigy 130 points
Part Number: SN74ALVC244
Other Parts Discussed in Thread: TXB0108, , SN74AXC8T245

Hi,

To give a brief introduction, we are testing an ADC chip which outputs data at 1.8V. We are using KC705 board with its FMC XM105 Debug Card to gather the data from ADC's EVAL board. KC705 has IOs operating at 2.5V so we had to deploy a level shifter (TXB0108) in the design before the interfacing connector. Unfortunately the interface altogether generated capacitance >70pF which caused ringing on data lines. Now we are in the process of revising our design to cater the ringing and decided to add a line driver(SN74ALVC244) between the interface connector and the level shifter. 

We want to know :

1. What is the total current consumption of SN74ALVC244 operating at 2.5V?

2. What value(s) of decoupling capacitors should we place on TXB0108 and SN74ALVC244 with data lines operating at 3MHz and with interface creating capacitance >70pF?

Thanks,

Justin

  • 0
    Guru 317280 points

    Are the signal lines actually bidirectional? Using a line driver implies they're not, in which case it would be a better idea to use a unidirectional or direction-controlled level shifter.

    How many signals are there in each direction?

    1. The datasheet says that the power dissipation capacitance is 23 pF per channel. (See CMOS Power Consumption and CPD Calculation.)

    2. For decoupling capacitors, more important than the capacitance is the smallest possible package size (and short traces), to reduce ESL. Both 100 nF and 10 nF are commonly used values (but small MLCCs are nowhere near their nominal value under DC bias).

  • 0 in reply to Clemens Ladisch
    Prodigy 130 points

    Thanks Clemens for responding quickly.

    Signals are unidirectional. Can you elaborate on the downside of using a bidirectional level shifter? We did see ringing on both edges, is that what you're pointing towards?

  • 0 in reply to Justin Kiln
    Guru 317280 points

    The TXB transmits signals in both directions. If the ringing is large enough so that it looks like the beginning of a signal edge, the TXB will transmit a wrong edge on the wrong pin. A unidirectional shifter like the SN74AXC8T245 will ignore ringing at its outputs.

    Ringing is caused by parasitic capacitances and inductances of the line (and the oscilloscope probe). If you know the characteristic impedance of the line, you can match it by reducing the driver's output impedance with a series resistor.

  • 0 in reply to Clemens Ladisch
    TI__Genius 16735 points

    Hi,

    I agree with Clemen's response on this. Using a unidirectional device will simplify your design work as there are less things to worry about. This is because as Clemens mentioned, if there are any large reflections on the transmission line then it may trigger a false direction change on the TXB devices. However, if your design is robust with matched lines and not too much parasitics on them then there is nothing to worry about.

    Thanks!

    -Karan