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Output pins were shorted with ground in SN74LVCC3245APWR (Octal Transceiver)

Dhinesh Mahadevan
Prodigy 40 points

We are facing problem with the IC SN74LVCC3245APWR (Octal Transceiver)

VCCA=3.3V; Decoupling Capacitor - 0.1uF VCCB=5V; Decoupling Capacitor - 0.1uF DIR=3.3V with 4.7K ohm Pull up resistor OE#=GND with 4.7K ohm Pull down resistor Temp : Ambient 30 deg C.

Input pins was controlled by FPGA and '0' is driven by default. B port pins have 1Kohm pull down resistor.

B port pins were connected to SN74HCT14D (Inverter) through 26AWG cable.

B port pins were connected to SN74HCT14D through a cable of 26AWG.

Output pins were not toggling as per the input voltage. B port pins were showing 0 ohms with respect to ground.

Please guide us.

  • 0
    TI__Mastermind 23156 points
    Hello Dhinesh,
    This post has been moved to the appropriate forum so that we can support your needs best.
    -Francis Houde
  • 0 in reply to fhoude
    TI__Guru 62050 points
    Hi Dhinesh ,

    With the DIR bit high , the operation is from A bus to B bus . The FPGA is connected to A bus with default to zero and B port has pull down resistor to gnd . Is the input A pins toggling while the measurement is happening and what is the Vih and Vil of it?
  • 0 in reply to ShreyasRao
    Prodigy 40 points

    Hi Shreyas,

    VIH=3.28V, VIL=0.2V.

    Yes, Input pins were toggling.

    Whenever high is driven for one input, B port Current is increasing to 0.3A(5V) and voltage is floating between 0.1 to 0.4V.

  • 0 in reply to Dhinesh Mahadevan
    TI__Guru 62050 points
    Hi Dhinesh ,

    Is there a requirement to have the pulldown resistor to gnd at the B port ? 0.3A is way beyond the driving capabilities of the device in which case you could be causing damage to it . I would suggest to remove the 1k resistor .
  • 0 in reply to ShreyasRao
    Prodigy 40 points

    Shreyas,

    You misunderstood I think,

    After failure of IC, current from 5V is increased to 0.3A during low to high transition at input.

    Before failure of IC, current from 5V is increasing to 0.05A (5V/1Kohm) during low to high transition at input.

    So I dont think that removing 1Kohm pull down resistor gives the solution for this problem.

  • 0 in reply to Dhinesh Mahadevan
    TI__Guru 62050 points
    Hi Dhinesh ,

    I believe your setup is having 1Kohm on your output to Gnd and the current consumption from the Vcc supply is increasing to 0.3A during low to high transition . The current to be supplied on output (IOH or IOL ) will come from the supply which is mimicking the load which the part will be connected to.
    If this is 50mA, it is violating the recommended conditions and normal operating characteristics of the device. The abs max table has 50mA for the continuous output current which will be pushing the limits on the device , however , 5V/1Kohm is 5mA within operating limits .
    Again , increasing to 300mA would permanently damage the device beyond recovery .