SN74LV1T125: testing issues

Anna

Part Number: SN74LV1T125

Hi Dear,

The customer used SN74LV1T125, and the circuit diagram is as follows.

1-During the testing process, the IN pin was not connected to any signal. It was pulled up to 5V through a 5K resistor, but the measured IN pin voltage was as low as 0.5V. May I ask why the IN pin was not pulled up to a high level?

2-What is the input impedance of this chip?

3-As shown in the figure below, what does this parameter mean and how can it be understood?

over 2 years ago

0 Clemens Ladisch over 2 years ago

Guru 317370 points

1. The input leakage current is never more than 1 µA. A voltage drop of 4.5 V over the resistor would imply a current of 900 µA. Either the device is damaged, or there is a solder error. Does this still happen when replacing the chip or board?

3. Supply current flows into V_CC and out of GND, so this is not related to your problem. This specification describes shoot-through current, see [FAQ] How does a slow or floating input affect a CMOS device?

0 Anna over 2 years ago in reply to Clemens Ladisch

Intellectual 995 points

Hi Dear,

After the customer replaced the chip, the IN pin still cannot be pulled up to 5V. What is the problem?

0 Clemens Ladisch over 2 years ago in reply to Anna

Guru 317370 points

This might be a board error; check for continuity between input and ground, without the chip. If pin 2, outside the board, has a connection to GND, then it is not a SN74LV1T125.

0 Joshua Salinas over 2 years ago in reply to Anna

TI__Mastermind 23600 points

Hello Anna,

2) The inputs are generally high impedance. We can calculate the worst case impedance by using max input voltage (5.5 V) and max input leakage current (1 µA) which results in 5.5 MΩ.

Similar to what Clemens mentioned, please verify there are no shorts in Pin 2. If the chip was changed and the same results occurred, then this could be a board issue.

Regards,

Josh

0 Anna over 2 years ago in reply to Joshua Salinas

Intellectual 995 points

Hi Josh,

Is there a reference design? The customer wants to use it to control the power-on timing. Does the 2-pin need to be connected to a pull-down resistor to GND? Or is the 2-pin directly connected to the open drain output of the MCU?

0 Clemens Ladisch over 2 years ago in reply to Anna

Guru 317370 points

The input (pin 2) can be connected directly to the MCU.

The connection between pin 2 and ground should not be there. We are asking you to determine whether this error is in the chip itself, or in the board.

0 Joshua Salinas over 2 years ago in reply to Anna

TI__Mastermind 23600 points

Hello Anna,

Unfortunately, we don't have any reference designs or evaluation modules for this device.

Regards,

Josh

Logic

Logic forum

SN74LV1T125: testing issues