• State Resolved
  • Locked Locked
  • Replies 7 replies
  • Subscribers 19 subscribers
  • Views 2367 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

SN74HC595: several question from datasheet

TS Yang
Genius 4800 points
Part Number: SN74HC595
Other Parts Discussed in Thread: SN74AHC595, TPIC6C595, SN74LV595A, CD74ACT164, CD74ACT299

Hello,

I'm considering TI's SN74HC595 for my new application, and have some questions when I saw the SN74HC595 datasheet as below.

1. below text means that SNx4HC595 use Low-drive CMOS device, so there would be no ringing problem due to overshoot or undershoot? Am I right?

2.As for the below, the max source(driving) current per port is 35mA and total 70mA at VCC 5V condition.
How about at VCC 3.3V condition?
And does TI recommend using sink current? If there's no problem to use, please let me know the current limitation for sink current. 

thanks,

TS

  • 0
    Guru 261020 points

    The HC logic family has weaker output drivers than those in the other CMOS families, which implies that ringing is also lower.

    However, the AHC family is faster and has a little bit higher drive strength, and manages to do this with even lower ringing (see pages 3/4 of the AHC Designer's Guide (SCLA013)):

    It also specifies the output current for 3.3 V (See the VOL specification in the datasheet). So consider using the SN74AHC595 instead.

    CMOS device typically have symmetrical drive strength, i.e., source and sink currents have the same limits.

    The 35 mA/70 mA values quoted in the datasheet are absolute maximum ratings; they do not yet permanently damage the device, but correct operation is not guaranteed. You should stay inside the recommended operating conditions.

    If you want to use more than 4 mA at 3.3 V, then you need to use another logic family. The 595 does not exist in LVC, so you have to use some Power Logic device (for example, TPIC6C595).

  • 0 in reply to Clemens Ladisch
    Genius 4800 points

    Hello!

    thanks for your kind answer. I have addiotional questions as below.

    1. Can you recommend the device which has same control type as SN74HC595 and 3.3V Input(include control line) and VCC(3.3V or 5V) of higher driving current?

    A. 5mA ~ 20mA various driving current are using now.

    2. AC/ACT family has no shift register of same spec? I ask this because the AC/ACT family is suitable due to higher driving current.

    3. As for the HC/HCT, AC/ACT families, is it possible to design VCC=5V, 3.3V Input(include control line), 5V output as below picture? if yes, what is driving current?

         I ask this because I'm not sure I can use this as a level shifter such as Figure 2 below. I cannot found the clarify explanation at datasheet.

    4. As for the TPIC6C595, the recommended VCC is 4.5V to 5.5V, so I'm afraid I cannot use this for VCC 3.3V. Please check again.

    (for 5V, Driving current is not that high.)

    5. HC/HCT family does not saying about IOH for VCC=3.3V means TI does not guarantee the constant current for this condition?

    thanks,

    TS

  • 0 in reply to TS Yang
    Guru 261020 points

    1. How much drive current do you actually need? If it is no more than 8 mA, you could use the SN74LV595A.

    2. AC(T) shift registers

    3. Devices with TTL-compatble inputs (HCT, ACT) run only at 5 V and can use 3.3 V input signals.

    4. The TPIC devices require 5 V input signals.

    5. When 3.3 V is not specified, use the value specified for the next lower voltage (here: 2 V).

  • 0 in reply to Clemens Ladisch
    Genius 4800 points

    thanks for quick answer.

    1. I already mentioned that I need 5mA ~ 20mA for various models. Please recommend for more than 8mA case.

    thanks,

  • 0 in reply to TS Yang
    Guru 261020 points
    Do you need 5 mA or 20 mA?
  • 0 in reply to Clemens Ladisch
    Genius 4800 points
    From 5mA to 20mA are used in many other projects.
  • 0 in reply to TS Yang
    Guru 261020 points
    To get more than 8 mA, use multiple outputs in parallel. Or devices like CD74ACT164 or CD74ACT299, if you can live with the different logic.