SN74LVC1G240: Vcc =5V power supply, power down the current is bigger, about 5mA

Cooper Zhang1

Part Number: SN74LVC1G240
Other Parts Discussed in Thread: SN74LXC2T45, SN74LV1T125, SN74LV1T04

Hi ,

control signal pin1 & pin2 is 3.3V， when Vcc =5V, pin1 is pulled 3.3V （off ) ,the current is high, about 5mA； did some test, i found that it is normal when Vcc is lower than 3.6V , when Vcc is more than 3.6V ， the issue comes out； the schematic is as below,

over 2 years ago

0 Clemens Ladisch over 2 years ago

Guru 243220 points

With a 5 V supply, 3.3 V is not a valid input voltage (V_IH is higher).

When the input voltage is not near the power rails, you get cross currents. See the ΔI_CC specification, and [FAQ] How does a slow or floating input affect a CMOS device?

Add a level shifter like the SN74LXC2T45, or use logic gates with lower input thresholds like the SN74LV1T125/SN74LV1T04.

0 Cooper Zhang1 over 2 years ago in reply to Clemens Ladisch

Expert 2190 points

Hi Clemens:

Could you kindly help me check whether the schematic is ok ?

0 Clemens Ladisch over 2 years ago in reply to Cooper Zhang1

Guru 243220 points

Yes, this is a way to level-shift the signals to 5 V.

0 Emrys Maier over 2 years ago in reply to Cooper Zhang1

TI__Guru* 96591 points

One thing to keep an eye on here would be the transition rates, especially around Vcc/2 (2.5V). 47k may not be sufficiently low to produce a fast enough edge -- depending on several system factors. This is a common issue with this type of voltage translation circuit.

The customer may need to reduce that pull-up value to ensure a fast input transition (needs to be 5 ns/V) I would recommend that the transition rate be fast between VIL and VIH, so from 1.5V to 3.5V should take a maximum of approximately 15ns to transition.

[FAQ] How does a slow or floating input affect a CMOS device?

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SN74LVC1G240: Vcc =5V power supply, power down the current is bigger, about 5mA