• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 19 subscribers
  • Views 258 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.

SN74LVC1G74: SN74LVC1G74 can't trigger and drops out at low VCC

Wenwei Qiao
Prodigy 120 points
Part Number: SN74LVC1G74

Hello: we have a design with SN74LVC1G74DCU as shown in the picture. It is basically a power on/off toggle. All the ICs in the schematic are powered by battery which varies in voltage.

At 2.9V and above, U2 SN74LVC1G74DCU can trigger on and off by the pushbutton.

However, at 2.8V and lower, U2 can't trigger on (i.e., Q doesn't go high). But it can trigger off (i.e., Q goes to low) when it was on.

When U2 is triggered on at 2.9V and above, it stays on (Q=high) when we move down the battery voltage until 2.42V. In other words, U2 shuts off itself at 2.42V. U3 stays on (RESET\=inactive) until the battery voltage is 2.32V. When we move the battery back to 2.42V, U3 MAX6336U24 goes back on (RESET\=inactive) immediately. But U2 SN74LVC1G74DCU can't trigger until the battery is 2.9V.

U3 MAX6336U24 is supposed to reset out when the voltage is 2.4V or low. the measured value 2.32V is normal within the tolerance. 

But U2 SN74LVC1G74DCU is supposed to work as low as 1.8V. Why does it drop out at 2.42V and can't trigger at any VCC below 2.9V?

Thanks for help.

  • 0
    Guru 277220 points

    I suspect that some voltage dips lower. Can you show an oscilloscope trace of at least VCC and CLK when this happens?

    The switch is not debounced. How do you prevent double edges?

  • 0 in reply to Clemens Ladisch
    Prodigy 120 points

    Hi, Clemens, thanks for help.

    We tested the PCB by using a desktop power supply to eliminate any battery issue (capacity, voltage dipping, high impedance, etc.) and to be easy to adjust the voltage. I am sorry that I didn't mention that.

    We didn't put in a debouncing circuit due to the PCB real estate issue. And, we kind of expected and actually didn't experience it. We don't think it is an issue related this low voltage problem because it works fine on 2.9V and above and the pushbutton signal (on D of SN74LVC1G74DCU) is fine when the flip-flop doesn't trigger at lower voltage.

    We first thought it might be a problem from U3 MAX6336US24. Its reset level 2.32V is very close to U2's black-out 2.42V. So, we monitored its RESET\ pin. It is stable. Even if it was glitched around 2.3 to 2.4V, it couldn't explain why U2 SN74LVC1G74 can't trigger on at other voltages (2.5V, 2.6V, 2.7V, ..., 2.8V) that is far away from U3's reset level (which in turn was monitored steady stable).

    The mark on U2 SN74LVC1G74 is N74Q. We are suspecting this is a 5V device with all of its behaves.

  • 0 in reply to Wenwei Qiao
    TI__Guru* 96802 points

    Hello,

    The SN74LVC1G74 works down to 1.65V - in literally millions of systems worldwide. Every unit that leaves our factory is tested for this and is funcitonal prior to shipping.

    Most likely the issue is related to your application.

    Can you share scope shots of the VCC, input, and output signals when this occurs? Preferably at a 100ns / div time scale.