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SN74LS09: Higher VOH levels

Art Mecina
Genius 16575 points
Part Number: SN74LS09
Other Parts Discussed in Thread: SN74LS07, SN74HCS09

Hello Guys,

Good day.

Our customer is using the  SN74LS09DR chip. They have some clarifications though. The datasheet  states “Open-collector devices are often used to generate higher VOH levels.”

They wanted to know how much is this "higher voltages?"  They would like to use this AND gate to convert 5V signals to 10V pullup signals (via 3.3k resistor). They are operating the AND gate at 5V.

Thanks on advance!

Art
 

  • 0
    TI__Guru* 96681 points

    Hi Art,

    The LS09 has a limit of 5.5V at the output. TTL outputs typically only can reach about 2.4V, so a pull-up resistor to 5V is a very significant improvement.

    The SN74LS07 has the highest voltage outputs in all of the logic portfolio - they support pull-up to 30V.

    Two channels could be used as an AND gate - with the outputs shorted together and pulled up to 10V through the mentioned 3.3k resistor.

  • 0 in reply to Emrys Maier
    TI__Genius 16575 points

    Hello Emrys,

    Our customer have a follow-up inquiry and comment on this.

    Originally they were using NAND gates (open drain); the SN74HC03DR to be exact. The pcb was already laid out. Then they discovered all 12 of their data bits need to be inverted so they thought it would be easy to swap to an open collector AND gate; i.e., the SN74LS09DR since that was the same pinout and footprint. Your suggestion of the SN74LS07 won’t fit on their board...

    Going back to the original NAND gate--- it has an interesting note that they wonder if it’s true for any open drain/collector gate? “NOTES: 1. The input and output voltage ratings may be exceeded if the input and output current ratings are observed”. Do you think this would hold true for the SN74LS09DR?

    Thanks!

    Art

  • 0 in reply to Art Mecina
    Guru 267510 points

    That remark about current ratings implies that there is an ESD protection diode between the pin and VCC; the current limit protects the diode from burning up.

    An input or output pin that allows voltages higher than VCC does not have such a diode, and a resistor will not protect against voltages exceeding the absolute maximum rating.

    The SN74HCS09 has the same pinout and characteristics as the '03. But it should be noted that for both the '03 and the '09, the clamping diode will not result in 10 V at the output, but pull it down to about VCC + 0.5 V. So the original design would not have worked, and the board will not work with the HCS09. The board layout must be changed.