SN74HCS86: Iq is increased when doing low temperature test

Wayne Wang20

Part Number: SN74HCS86
Other Parts Discussed in Thread: TCA6424, SN74HCS74,

Hi team,

My customer is testing SN74HCS86PWR. Customer selects this device because it has very low Iq. Customer found in room-temperature and high-temperature test, the Iq is normal. But in low-temperature test(-10℃), the Iq increased a lot. So I just want to check with you is this a normal phenomenon? What may cause the Iq increase in low temperature test? Thanks.

Best regards,

Wayne

over 3 years ago

0 Clemens Ladisch over 3 years ago

Guru 281530 points

In theory, leakage current should decrease with temperature.

What are the input voltages in this test?

0 Dylan Hubbard over 3 years ago in reply to Clemens Ladisch

TI__Mastermind 44675 points

Hey Wayne,

On top on Clemens question, could you also provide actual current numbers measured?

0 Wayne Wang20 over 3 years ago in reply to Dylan Hubbard

TI__Expert 4360 points

Hi Dylan,

Let me add more background. Customer's board has 7pcs SN74HCS86 and 14pcs SN74HCS74 and 1pc MCU and 1pc TCA6424. At first, we thought it is SN74HCS86 cause the Iq increase during low temperature test. But now have figured out actually it is caused by SN74HCS74. The test step is as below:(the board is powered by 3.3V input voltage)

1. Solder 7pcs SN74HCS86 and its external components on the board. Keep SN74HCS74, MCU , TCA6424 not soldered. In 25℃ and -40℃ test, the current on 3.3V input power is ~nA which is normal.

2. Then we keep 7pcs SN74HCS86 still soldered and solder 14pcs SN74HCS74 onto the board. MCU and TCA6424 are not soldered.

In 25℃ test, when the board is just powered on, the current on 3.3V input power can reach to 200uA. After~30 seconds, the current drops down to ~nA

In -40℃ test, the current on 3.3V input power keeps increasing as the temperature goes low. The current finally goes up to 300uA when the temperature is kept at -40℃.

SN74HCS86 schematic

SN74HCS74 schematic:

Thanks

Best regards,

Wayne

0 Wayne Wang20 over 3 years ago in reply to Clemens Ladisch

TI__Expert 4360 points

Hi Clemens，

Please see my reply to Dylan. thanks.

Best regards,

Wayne

0 Clemens Ladisch over 3 years ago in reply to Wayne Wang20

Guru 281530 points

What are the voltages of CLR, SW1_CLK, and SW2_CLK during the test?

0 Wayne Wang20 over 3 years ago in reply to Clemens Ladisch

TI__Expert 4360 points

Hi Clemens,

During the test, the MCU and TCA6424 are not soldered. SW1_CLK, and SW2_CLK are connected to SN74HCS86. CLR is floating. But it is not convenient to measure these pins in the low-temperature test.

Best regards,

Wayne

0 Clemens Ladisch over 3 years ago in reply to Wayne Wang20

Guru 281530 points

Floating CMOS inputs will have large shoot-through currents; see [FAQ] How does a slow or floating input affect a CMOS device?

0 Wayne Wang20 over 3 years ago in reply to Clemens Ladisch

TI__Expert 4360 points

Hi Clemens,

I double checked with customer. CLR signal is not floating. They are pulled up to 3.3V through a resistor and also connected to MCU. During the test, MCU is not soldered but the pull up resistor is there so the CLR signal is not floating. Sorry for the confuse.

So for SN74HCS74 there is no pin that is floating during the test. Do you have any other advice on this issue? Thanks.

Best regards,

Wayne

0 Clemens Ladisch over 3 years ago in reply to Wayne Wang20

Guru 281530 points

That ~30 s current at normal temperatures also is strange, and needs to be explained. (Or are there some capacitors that need that long for charging?)

There is no obvious explanation in what you've shown. It would be helpful if it would be possible to find out where exactly the current flows.

Logic

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SN74HCS86: Iq is increased when doing low temperature test