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Need help selecting a level translator

Lawrence Suto
Prodigy 10 points
Other Parts Discussed in Thread: SN74LVC2T45, SN74CB3T3125

Im trying to interface an MCU driven at 5V with a SPI IC that at the max can only take 3.9V on any pin.

We are going to need the VOH, VOL, VIH, and VIL (as well as the I/O voltage tolerances) for both the SPI IC and the PIC

To successfully interface the PIC with the SPI IC, two requirements must be met.
The driver needs to be able to cover the full range of the receiver so:
1.  The high and low voltage range of the receiver (SPI IC) must be a subset of the driver's (PIC's) high and low voltage range.
2.  The driver's output voltage must not exceed the I/O voltage tolerance of the receiver (SPI IC).

In other words, the PIC's high voltage out must be higher than the SPI IC's high voltage in, yet remain lower than its I/O voltage tolerance.  That is, it must lie between them, to wit:

PIC V0L < SPI IC VIL
                 SPI IC VIH < PIC V0H < I/O tolerance



Second:

We may want to look at
1. Dual-supply devices:(Bi-directional, low power
2. Devices with over-voltage-tolerant inputs: (Our SPI IC DOES have an over-voltage-tolerant PIC input.)
3. CB3T (5 - 3.3 down translation)
4. CBT/CBTD

We may want to look at
1. Dual-supply devices:(Bi-directional, low power
2. Devices with over-voltage-tolerant inputs: (Our SPI IC DOES have an over-voltage-tolerant PIC input.)
3. CB3T (5 - 3.3 down translation)
4. CBT/CBTD

 

(5 - 3.3 down translation)
5.

Using dual-supply level translators usually is the best option for most level-translation applications. 

In situations where these devices might not be the most optimal solution, other solutions should be considered. 

Bus switches and overvoltage-tolerant devices should be considered for down-translation applications

Thanks

Lawrence

 

  • 0
    TI__Prodigy 170 points

    Assuming that you are using a typical 4 channel SPI bus, the best fit here would be SN74LVC2T45.

    http://focus.ti.com/lit/ds/symlink/sn74lvc2t45.pdf

    You would need to use (2) pieces with each IC driving in opposite directions.

    This will give you 24 – 32 mA drive and faster output slew rates.

     

    Another option is to use a passive FET switch for down-translation.

    The SN74CB3T3125 is a 4-channel bus switch that would work well here.

    http://focus.ti.com/lit/ds/symlink/sn74cb3t3125.pdf

    The only tradeoff in using this device is that you will need to add pullup resistors on the output side of the device to achieve up-translation.

    In addition, it does not provide any active drive so the output slew rates achieved through the bus switch will depend on the chosen pullup resistor values as well as the current sinking capability of the downstream and upstream drivers (PIC and SPI ICs).

    An example of CB3T implementation for both down and up translation is shown on the following document on pg. 28.http://focus.ti.com/lit/an/scda008/scda008.pdf

    Assuming that you are driving short trace lengths and there are no connectors or cables in the transmission path, an auto-sensing, bidirectional translator would be a final option.

    http://focus.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=txb0104&fileType=pdf

    This translator does not require a direction control so you would be able to translate in different directions on separate channels simultaneously, meaning you only need to use (1) IC.

    It is worth noting, however, that this translator can only drive up to a max output frequency of 50MHz and is much more sensitive to capacitive loading than the aforementioned devices, so take those details into account in selecting the correct device.

  • 0
    Intellectual 410 points
    (Late, but it needs saying...)

    There's absolutely no reason to go to all this trouble. If power consumption is not a big problem, all you need are a couple of series resistors.

    It's not made clear in all data sheets, but maximum input voltage and current specs are an either/or thing; if you limit the voltage or the current to reasonable levels, nothing bad will happen.

    CMOS is prone to destructive latch-up if large currents flow into input pins. Most CMOS inputs solve this problem with protection diodes to the power supply rails that shunt such currents. If you add enough series resistance to limit the current to something non-destructive, and if the resultant power dissipation won't be a problem, you can just hook up the wires directly.

    E.g. from a 5V source, we will drop 1.1V to 3.9V: The 3.3V supply plus one diode drop. If we add 1.1K or more of resistance, the input current will be limited to 1 mA and the power dissipated to 0.6 mW. On the other hand, you need to avoid slowing down your SPI bus. Assuming 25 pF of input capacitance (probably generous), 4K of resistance will give you a time constant of 100 ns, fast enough to run a 2 MHz SPI bus.

    So try something in the 2.2–4.7K range.