SN74LVC2T45 power sequence

Hi,

The power sequence that VCCB is set up first is fine when transition data can be treated as invalid in the system.

Best,

Lawrence

Dear Ugur bey,

I looked at the device datasheet and at first it was a little confusing as both VccA and VccB can be powered up with any voltage level between 1.65V and 5V. So then one may ask why the need for VccA powered up first instead of saying power up the lower/higher voltage supply first.

I think the issue is that the DIR pin is referenced to VccA and since DIR pin is also tied to the B side circuitry, the designers wanted to avoid any potential reverse biasing issue by connecting VccA first or at least by ramping up both voltages at the same time.

I believe you have a 5V supply coming in and the 3.3V supply comes via a regulator using this 5V supply hence the delay in the VccA(3.3V). Either case, this is just a precautionary measure and as long as these two rails are coming along very close to each other it should be fine. I would only worry if 5V (VccB) comes fully on, and only after that 3.3V(VccA) comes on line, which in this case the internal circuit components will see a full reverse bias and that may harm the IC. If this is the case then you should either use some sort of voltage sequencing via a sequencer IC such as the LM3880 or more sophisticated UCD9090. If the delay is not as much then you could also use a larger filter (shunt) capacitor to give a slower ramp to VccB and that may solve your problem altogether as well.

If you have further questions then you may call me at +90-555-669-6953 or reach me by email at murat@ti.com.

Thanks,

Murat Ilhan.

Senior Analog Field Application Engineer. 

TI-Turkey.

Hi

NXP is similar part but in their datasheet they have clearly mentioned that no particular power sequence is required as long as gnd is connected first.

My question is if TI and NXP have same part number then why one manufacturer (NXP) is not bother about the power up sequenc while other (TI) have specificed it specifically?

I am using this part is high reliability product and there is no power up seuqence but two voltage rails should not have that different ramp up time.

Any suggestions on the issue, although I personally perfer using TI parts but I am thinking to change it to NXP.

Cheers

Mujahid

Dear Mujahid,

You are correct, both parts are very similar. We added this sequencing as an extra precaution but it doesn't affect the device performance. For SN74LVC2T45, whatever the power sequence is applied, it will work well after it is powered up.

If this precautionary method of sequencing is not applied or in other words if the VccA is ramped up after the VccB then there may appear a bus contention issue due to not having an OE pin and DIR pin being referenced to VccA. 

I hope this helps,

Murat Ilhan.

Thanks Murat

It means once voltage on VCCA and VCCB are established there will be no oscillation, contention or latchup.

Is it also applies to 74LVC2T45, 74LVC4T245, 74LVCH8T245 ?

Regards,

M Mujahid