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I like the notion of two 74ACT04 gates in push pull.
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But wonder what kind of trouble you could get into using that to drive a transformer. Whenever I see a full description of the design process for a circuit with magnetics, it seems I always learn something new.? To use inverters in push-pull, the logical thing would be to invert the original signal with one gate, then feed the original and the inverted signal into two other inverter gates. However, prop delay through one gate is spec'd to be somewhere between 1 and 9 ns. A better choice might be 74ACT86 ex-or gates both driven from the same signal, the second input held high on one gate and shorted to ground on the other. Supply sequencing can definitely cause trouble. Some parts are designed to survive indefinitely, regardless of supply sequencing. Many parts will live through a millisecond or so of poorly thought out sequencing. At work, we often had digital designs with a half dozen different supplies. A single FPGA might have that many. It was something to worry about, but we generally had plenty of other problems to solve that were more pressing. Jerry, KE7ER On Wed, Dec 18, 2019 at 04:55 AM, Steven Dick wrote:
Something to keep in mind in circuits that use multiple power supplies is power sequencing:? It is important that the 5V comes on before the 3.3V and turns off after the 3.3V when the 3.3V turns off.? Otherwise you can drive the 5V unpowered device from the 3.3V driving chip.? If the 3.3V driving chip can supply a hefty amount of current, you can? possibly damage the input circuitry of the 5V powered device by forward biasing its protection diode if it has one.? That protection diode may see more current than it is designed to handle. Depending on the particular 5V device, it may not immediately fail but can ultimately fail with repeated power sequencing at turn on or turn off.? A possible safety measure is to put a small Schottky diode between the +5V and +3.3V rails in the direction such that the 3.3V supply pulls up the 5V rail if unpowered, until the 5V supply kicks in and turns off that diode.? If the 3.3V supply is derived from the 5V supply, that is normally not a problem. But if the 5V and 3.3V supplies are totally independent, that problem can easily happen. The input clamp current for the 74ACT family has a clamp diode which is pretty hefty (current rating of 20mA) so it¡¯s probably not a problem for the 74ACT chip, but you are momentarily shorting the output of the SI5351 at power up if there is a power sequence problem. |
