Level shifter: clamping (input+VccA) when input and VccA are powered from same signal

Question

I'm currently testing a PCB I designed, and found out the following circuit don't work properly:

the SYS_EN signal is connected to a 1.8V rail through a 10k pull-up resistor.

TXU0101DRYRU5 is a level shifter. Not having my 1.8V rail yet (this circuit is part of the logic for activating the 3.3V rail, which in turns generates the 1.8V rail), I had the clever (or rather bad) idea of using my SYS_EN signal also to power the low side Vcc (VccA) of the level shifter.

When I turn everything on at once, everything works fine (SYS_EN reaches about 1.78V, BY goes high, all my voltage rails go up). The current on SYS_EN is about 20µA (in circuit, about 2µA for U5, the rest goes in a 100k pull-down resistor not represented)

However, if I first turn on power, and only afterwards turn on SYS_EN, then SYS_EN never reaches 1.8V: it stays stuck at 0.7 - 0.8V, and the current is far bigger (about 100µA).

To make sure, I bought a TXU0101 in SOT-23 package to test it out of circuit, and observed the same behavior (excepted currents that are now a bit lower: 2µA when powering simultaneously, 80µA when powering first the 5V then SYS_EN from 1.8V (lab supply) through a 10k pull-up: this is because now there is nothing else on the SYS_EN signal (in particular not the 100k pull-down).

Tomorrow, we will cut the track for VccA and power it with a 1.8V obtained from 5V with a zener + resistor.

However, I'm curious why my circuit don't work as it? Did I miss something in the datasheet? Is it "normal" that ICs don't work properly if you power-up Vcc and inputs at the same time? Is the problem that SYS_EN isn't powered directly by 1.8V but through a 10k resistor?

PS: this is a follow up on this question (that didn't got any answer nor comment)

EDIT: at @Reinderien request: I added the complete schematics (for the breadboard test, which is enough to reproduce the behavior)

^{simulate this circuit – Schematic created using CircuitLab}

Note that in the 0-85°C range, the max currents are:

• -0.1 to +1.5µA for the input pin (A)
• -1 to +2.5µA for the supply pin (VccA)

So a total of 4µA max, which is only 40mV drop trough the 10k resistor: so at least in steady state, it shouldn't mater. So why is it a problem during power-up? Big inrush current while the voltage is too low, and the voltage drop being to high to ever reach a voltage that allow nominal current consumption?

Answer 1

If there is any inrush current, then the resistor will prevent the voltage from rising.

A power supply must have a low-impedance connection to the rail (10 kΩ is certainly too much), and needs a decoupling capacitor.

You could use a level shifter that needs only a single supply, i.e., a comparator like the TLV4041.